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Wang Y, Bai Y, Zeng Q, Jiang Z, Liu Y, Wang X, Liu X, Liu C, Min W. Recent advances in the metabolic engineering and physiological opportunities for microbial synthesis of L-aspartic acid family amino acids: A review. Int J Biol Macromol 2023; 253:126916. [PMID: 37716660 DOI: 10.1016/j.ijbiomac.2023.126916] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2023] [Revised: 09/10/2023] [Accepted: 09/13/2023] [Indexed: 09/18/2023]
Abstract
L-aspartic acid, L-threonine, L-isoleucine, l-lysine, and L-methionine constitute the l-aspartate amino acids (AFAAs). Except for L-aspartic acid, these are essential amino acids that cannot be synthesized by humans or animals themselves. E. coli and C. glutamicum are the main model organisms for AFAA production. It is necessary to reconstitute microbial cell factories and the physiological state of industrial fermentation cells for in-depth research into strains with higher AFAA production levels and optimal growth states. Considering that the anabolic pathways of the AFAAs and engineering modifications have rarely been reviewed in the latest progress, this work reviews the central metabolic pathways of two strains and strategies for the metabolic engineering of AFAA synthetic pathways. The challenges posed by microbial physiology in AFAA production and possible strategies to address them, as well as future research directions for constructing strains with high AFAA production levels, are discussed in this review article.
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Affiliation(s)
- Yusheng Wang
- College of Food Science and Engineering, Jilin Agricultural University, Changchun 130118, PR China
| | - Yunlong Bai
- College of Food Science and Engineering, Jilin Agricultural University, Changchun 130118, PR China
| | - Qi Zeng
- College of Food Science and Engineering, Jilin Agricultural University, Changchun 130118, PR China
| | - Zeyuan Jiang
- College of Food Science and Engineering, Jilin Agricultural University, Changchun 130118, PR China
| | - Yuzhe Liu
- College of Food Science and Engineering, Jilin Agricultural University, Changchun 130118, PR China
| | - Xiyan Wang
- College of Food Science and Engineering, Jilin Agricultural University, Changchun 130118, PR China
| | - Xiaoting Liu
- College of Food Science and Engineering, Jilin Agricultural University, Changchun 130118, PR China
| | - Chunlei Liu
- College of Food Science and Engineering, Jilin Agricultural University, Changchun 130118, PR China.
| | - Weihong Min
- College of Food Science and Engineering, Jilin Agricultural University, Changchun 130118, PR China.
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2
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Alvarado O, García-Meseguer R, Ruiz-Pernía JJ, Tuñon I, Delgado EJ. Mechanistic study of the biosynthesis of R-phenylacetylcarbinol by acetohydroxyacid synthase enzyme using hybrid quantum mechanics/molecular mechanics simulations. Arch Biochem Biophys 2021; 707:108849. [PMID: 33832752 DOI: 10.1016/j.abb.2021.108849] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
The biosynthesis of R-phenylacetylcarbinol (R-PAC) by the acetohydroxy acid synthase, (AHAS) is addressed by molecular dynamics simulations (MD), hybrid quantum mechanics/molecular mechanics (QM/MM), and QM/MM free energy calculations. The results show the reaction starts with the nucleophilic attack of the C2α atom of the HEThDP intermediate on the Cβ atom of the carbonyl group of benzaldehyde substrate via the formation of a transition state (TS1) with the HEThDP intermediate under 4'-aminopyrimidium (APH+) form. The calculated activation free energy for this step is 17.4 kcal mol-1 at 27 °C. From this point, the reaction continues with the abstraction of Hβ atom of the HEThDP intermediate by the Oβ atom of benzaldehyde to form the intermediate I. The reaction is completed with the cleavage of the bond C2α-C2 to form the product R-PAC and to regenerate the ylide intermediate under the APH+ form, allowing in this way to reinitiate to the catalytic cycle once more. The calculated activation barrier for this last step is 15.9 kcal mol-1 at 27 °C.
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Affiliation(s)
- Omar Alvarado
- Departamento de Físico-Química, Facultad de Ciencias Químicas, Universidad de Concepción, Concepción, Chile; Departamento de Química, Facultad de Ciencias, Universidad del Bío-Bío, Avenida Collao 1202, Concepción, Chile
| | - Rafael García-Meseguer
- School of Mathematics, University of Bristol, Bristol, UK; Department of Physical Chemistry, Universitat de Valencia, 46100, Burjassot, Spain
| | | | - Iñaki Tuñon
- Department of Physical Chemistry, Universitat de Valencia, 46100, Burjassot, Spain
| | - Eduardo J Delgado
- Departamento de Físico-Química, Facultad de Ciencias Químicas, Universidad de Concepción, Concepción, Chile.
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3
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Alvarado O, García-Meseguer R, Ruiz-Pernía JJ, Tuñon I, Delgado EJ. Mechanistic study of the biosynthesis of R-phenylcarbinol by acetohydroxyacid synthase enzyme using hybrid quantum mechanics/molecular mechanics simulations. Arch Biochem Biophys 2021; 701:108807. [PMID: 33587902 DOI: 10.1016/j.abb.2021.108807] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2020] [Revised: 02/06/2021] [Accepted: 02/08/2021] [Indexed: 11/26/2022]
Abstract
The biosynthesis of R-phenylacetylcarbinol (R-PAC) by the acetohydroxy acid synthase, (AHAS) is addressed by molecular dynamics simulations (MD), hybrid quantum mechanics/molecular mechanics (QM/MM), and QM/MM free energy calculations. The results show the reaction starts with the nucleophilic attack of the C2α atom of the HEThDP intermediate on the Cβ atom of the carbonyl group of benzaldehyde substrate via the formation of a transition state (TS1) with the HEThDP intermediate under 4'-aminopyrimidium (APH+) form. The calculated activation free energy for this step is 17.4kcal mol-1 at 27 °C. From this point, the reaction continues with the abstraction of Hβ atom of the HEThDP intermediate by the Oβ atom of benzaldehyde to form the intermediate I. The reaction is completed with the cleavage of the bond C2α-C2 to form the product R-PAC and to regenerate the ylide intermediate under the APH+ form, allowing in this way to reinitiate to the catalytic cycle once more. The calculated activation barrier for this last step is 15.9kcal mol-1 at 27 °C.
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Affiliation(s)
- Omar Alvarado
- Departamento de Físico-Química, Facultad de Ciencias Químicas, Universidad de Concepción, Concepción, Chile; Departamento de Química, Facultad de Ciencias, Universidad del Bío-Bío, Avenida Collao 1202, Concepción, Chile
| | - Rafael García-Meseguer
- School of Mathematics, University of Bristol, Bristol, United Kingdom; Department of Physical Chemistry, Universitat de Valencia, 46100, Burjassot, Spain
| | | | - Iñaki Tuñon
- Department of Physical Chemistry, Universitat de Valencia, 46100, Burjassot, Spain
| | - Eduardo J Delgado
- Departamento de Físico-Química, Facultad de Ciencias Químicas, Universidad de Concepción, Concepción, Chile.
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Fang J, Wan C, Wang W, Ma L, Wang X, Cheng C, Zhou J, Qiao Y, Wang X. Engineering Herbicide-Tolerance Rice Expressing an Acetohydroxyacid Synthase with a Single Amino Acid Deletion. Int J Mol Sci 2020; 21:ijms21041265. [PMID: 32070060 PMCID: PMC7072996 DOI: 10.3390/ijms21041265] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2020] [Revised: 02/10/2020] [Accepted: 02/12/2020] [Indexed: 11/16/2022] Open
Abstract
The acetohydroxyacid synthase (AHAS) is an essential enzyme involved in branched amino acids. Several herbicides wither weeds via inhibiting AHAS activity, and the AHAS mutants show tolerance to these herbicides. However, most AHAS mutations are residue substitutions but not residue deletion. Here, residue deletion was used to engineering the AHAS gene and herbicide-tolerant rice. Molecular docking analysis predicted that the W548 of the AHAS was a residue deletion to generate herbicide tolerance. The AHAS-ΔW548 protein was generated in vitro to remove the W548 residue. Interestingly, the deletion led to the tetramer dissociation of the AHAS, while this dissociation did not reduce the activity of the AHAS. Moreover, the W548 deletion contributed to multi-family herbicides tolerance. Specially, it conferred more tolerance to sulfometuron-methyl and bispyribac-sodium than the W548L substitution. Further analysis revealed that AHAS-ΔW548 had the best performance on the sulfometuron-methyl tolerance compared to the wild-type control. Over-expression of the AHAS-ΔW548 gene into rice led to the tolerance of multiple herbicides in the transgenic line. The T-DNA insertion and the herbicide treatment did not affect the agronomic traits and yields, while more branched-chain amino acids were detected in transgenic rice seeds. Residue deletion of W548 in the AHAS could be a useful strategy for engineering herbicide tolerant rice. The increase of branched-chain amino acids might improve the umami tastes of the rice.
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Affiliation(s)
- Jun Fang
- Crop Breeding and Cultivating Institute, Shanghai Academy of Agricultural Sciences, 1000 Jingqi Rd, Shanghai 201403, China; (C.W.); (X.W.); (C.C.); (J.Z.); (Y.Q.)
- Correspondence:
| | - Changzhao Wan
- Crop Breeding and Cultivating Institute, Shanghai Academy of Agricultural Sciences, 1000 Jingqi Rd, Shanghai 201403, China; (C.W.); (X.W.); (C.C.); (J.Z.); (Y.Q.)
| | - Wei Wang
- Shanghai Institute for Advanced Immunochemical Studies, ShanghaiTech University, 99 Haike Rd, Shanghai 201210, China;
| | - Liuyin Ma
- Basic Forestry and Proteomics Research Center, Haixia Institute of Science and Technology, Fujian Agriculture and Forestry University, Fuzhou 350002, China;
| | - Xinqi Wang
- Crop Breeding and Cultivating Institute, Shanghai Academy of Agricultural Sciences, 1000 Jingqi Rd, Shanghai 201403, China; (C.W.); (X.W.); (C.C.); (J.Z.); (Y.Q.)
| | - Can Cheng
- Crop Breeding and Cultivating Institute, Shanghai Academy of Agricultural Sciences, 1000 Jingqi Rd, Shanghai 201403, China; (C.W.); (X.W.); (C.C.); (J.Z.); (Y.Q.)
| | - Jihua Zhou
- Crop Breeding and Cultivating Institute, Shanghai Academy of Agricultural Sciences, 1000 Jingqi Rd, Shanghai 201403, China; (C.W.); (X.W.); (C.C.); (J.Z.); (Y.Q.)
| | - Yongjin Qiao
- Crop Breeding and Cultivating Institute, Shanghai Academy of Agricultural Sciences, 1000 Jingqi Rd, Shanghai 201403, China; (C.W.); (X.W.); (C.C.); (J.Z.); (Y.Q.)
| | - Xiao Wang
- Crop Breeding and Cultivating Institute, Shanghai Academy of Agricultural Sciences, 1000 Jingqi Rd, Shanghai 201403, China; (C.W.); (X.W.); (C.C.); (J.Z.); (Y.Q.)
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Liu Y, Li Y, Wang X. Acetohydroxyacid synthases: evolution, structure, and function. Appl Microbiol Biotechnol 2016; 100:8633-49. [DOI: 10.1007/s00253-016-7809-9] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/30/2016] [Revised: 07/28/2016] [Accepted: 08/12/2016] [Indexed: 10/21/2022]
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Petric I, Karpouzas DG, Bru D, Udikovic-Kolic N, Kandeler E, Djuric S, Martin-Laurent F. Nicosulfuron application in agricultural soils drives the selection towards NS-tolerant microorganisms harboring various levels of sensitivity to nicosulfuron. ENVIRONMENTAL SCIENCE AND POLLUTION RESEARCH INTERNATIONAL 2016; 23:4320-4333. [PMID: 26517995 DOI: 10.1007/s11356-015-5645-6] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/03/2015] [Accepted: 10/19/2015] [Indexed: 06/05/2023]
Abstract
The action mode of sulfonylurea herbicides is the inhibition of the acetohydroxyacid synthase (AHAS) required for the biosynthesis of amino acids valine and isoleucine in plants. However, this enzyme is also present in a range of non-targeted organisms, among which soil microorganisms are known for their pivotal role in ecosystem functioning. In order to assess microbial toxicity of sulfonylurea herbicide nicosulfuron (NS), a tiered microcosm (Tier I) to field (Tier II) experiment was designed. Soil bacteria harboring AHAS enzyme tolerant to the herbicide nicosulfuron were enumerated, isolated, taxonomically identified, and physiologically characterized. Results suggested that application of nicosulfuron drives the selection towards NS-tolerant bacteria, with increasing levels of exposure inducing an increase in their abundance and diversity in soil. Tolerance to nicosulfuron was shown to be widespread among the microbial community with various bacteria belonging to Firmicutes (Bacillus) and Actinobacteria (Arthrobacter) phyla representing most abundant and diverse clusters. While Arthrobacter bacterial population dominated community evolved under lower (Tier II) nicosulfuron selection pressure, it turns out that Bacillus dominated community evolved under higher (Tier I) nicosulfuron selection pressure. Different NS-tolerant bacteria likewise showed different levels of sensitivity to the nicosulfuron estimated by growth kinetics on nicosulfuron. As evident, Tier I exposure allowed selection of populations able to better cope with nicosulfuron. One could propose that sulfonylureas-tolerant bacterial community could constitute a useful bioindicator of exposure to these herbicides for assessing their ecotoxicity towards soil microorganisms.
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Affiliation(s)
- Ines Petric
- Rudjer Boskovic Institute, Division for Marine and Environmental Research, HR-10002, Zagreb, Croatia.
| | - Dimitrios G Karpouzas
- University of Thessaly, Department of Biochemistry and Biotechnology, Ploutonos 26 and Aeolou Str, 412 21, Larisa, Greece
| | - David Bru
- INRA, UMR 1347 Agroécologie, BP 86510, 21065, Dijon CEDEX, France
| | - Nikolina Udikovic-Kolic
- Rudjer Boskovic Institute, Division for Marine and Environmental Research, HR-10002, Zagreb, Croatia
| | - Ellen Kandeler
- Institute of Soil Science and Land Evaluation, Soil Biology Section, University of Hohenheim, Emil-Wolff-Str. 27, 70593, Stuttgart, Germany
| | - Simonida Djuric
- University of Novi Sad, Faculty of Agriculture, Novi Sad, Serbia
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Baig IA, Gedi V, Lee SC, Koh SH, Yoon MY. Role of a highly conserved proline-126 in ThDP binding of Mycobacterium tuberculosis acetohydroxyacid synthase. Enzyme Microb Technol 2013; 53:243-9. [PMID: 23931689 DOI: 10.1016/j.enzmictec.2013.05.006] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2013] [Revised: 05/12/2013] [Accepted: 05/14/2013] [Indexed: 11/30/2022]
Abstract
Acetohydroxyacid synthase (AHAS) of Mycobacterium tuberculosis is a promising target for the development of anti-tuberculosis agents. With the absence of an available bacterial AHAS crystal structure, that of M. tuberculosis, site-directed mutagenesis has been a useful tool for determining its structural and functional features. In this study, a highly conserved proline residue (P126 of M. tuberculosis AHAS) was selected, and the possible role was evaluated by site-directed mutagenesis. P126 was replaced by valine, threonine, alanine, and glutamate to yield P126V, P126T, P126A, and P126E, respectively. All variants were expressed in their soluble forms in Escherichia coli and purified to near homogeneity. The molecular mass (SDS-PAGE) of the purified variants was ∼68 kDa, which is similar to that of wild-type AHAS. The P126V, P126T, and P126A variants exhibited significantly lower activity than wild-type AHAS, whereas P126E was inactive under the tested assay conditions. Furthermore, the P126V and P126T variants showed a significantly decreased preference toward pyruvate and ThDP as substrate and cofactor respectively, whereas the P126A showed similar kinetics to that of wild-type AHAS. Like in AHAS from yeast Saccharomyces cerevisiae (PDB ID: 1N0H), residue P126 is located in the ThDP binding pocket of M. tuberculosis AHAS homology model. Collectively, these results suggest that the conserved P126 plays a significant role in the ThDP binding of M. tuberculosis AHAS.
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Li Y, Hu F, Wang X, Cao H, Liu D, Yao D. A rational design for trypsin-resistant improvement of Armillariella tabescens β-mannanase MAN47 based on molecular structure evaluation. J Biotechnol 2013; 163:401-7. [DOI: 10.1016/j.jbiotec.2012.12.018] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/17/2012] [Revised: 12/20/2012] [Accepted: 12/21/2012] [Indexed: 11/27/2022]
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9
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Zhao Y, Wen X, Niu C, Xi Z. Arginine 26 and Aspartic Acid 69 of the Regulatory Subunit are Key Residues of Subunits Interaction of Acetohydroxyacid Synthase Isozyme III fromE. coli. Chembiochem 2012; 13:2445-54. [DOI: 10.1002/cbic.201200362] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/31/2012] [Indexed: 11/08/2022]
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Belenky I, Steinmetz A, Vyazmensky M, Barak Z, Tittmann K, Chipman DM. Many of the functional differences between acetohydroxyacid synthase (AHAS) isozyme I and other AHASs are a result of the rapid formation and breakdown of the covalent acetolactate-thiamin diphosphate adduct in AHAS I. FEBS J 2012; 279:1967-79. [DOI: 10.1111/j.1742-4658.2012.08577.x] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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Grundman O, Khozin-Goldberg I, Raveh D, Cohen Z, Vyazmensky M, Boussiba S, Shapira M. Cloning, mutagenesis, and characterization of the microalga Parietochloris incisa acetohydroxyacid synthase, and its possible use as an endogenous selection marker. Biotechnol Bioeng 2012; 109:2340-8. [PMID: 22488216 DOI: 10.1002/bit.24515] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2011] [Revised: 03/04/2012] [Accepted: 03/23/2012] [Indexed: 11/10/2022]
Abstract
Parietochloris incisa is an oleaginous fresh water green microalga that accumulates an unusually high content of the valuable long-chain polyunsaturated fatty acid (LC-PUFA) arachidonic acid within triacylglycerols in cytoplasmic lipid bodies. Here, we describe cloning and mutagenesis of the P. incisa acetohydroxyacid synthase (PiAHAS) gene for use as an herbicide resistance selection marker for transformation. Use of an endogenous gene circumvents the risks and regulatory difficulties of cultivating antibiotic-resistant organisms. AHAS is present in plants and microorganisms where it catalyzes the first essential step in the synthesis of branched-chain amino acids. It is the target enzyme of the herbicide sulfometuron methyl (SMM), which effectively inhibits growth of bacteria and plants. Several point mutations of AHAS are known to confer herbicide resistance. We cloned the cDNA that encodes PiAHAS and introduced a W605S point mutation (PimAHAS). Catalytic activity and herbicide resistance of the wild-type and mutant proteins were characterized in the AHAS-deficient E. coli, BUM1 strain. Cloned PiAHAS wild-type and mutant genes complemented AHAS-deficient bacterial growth. Furthermore, bacteria expressing the mutant PiAHAS exhibited high resistance to SMM. Purified PiAHAS wild-type and mutant proteins were assayed for enzymatic activity and herbicide resistance. The W605S mutation was shown to cause a twofold decrease in enzymatic activity and in affinity for the Pyruvate substrate. However, the mutant exhibited 7 orders of magnitude higher resistance to the SMM herbicide than that of the wild type.
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Affiliation(s)
- Omer Grundman
- Microalgal Biotechnology Laboratory, French Associates Institute of Agriculture and Biotechnology of Drylands, The Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion, Israel
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12
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Gedi V, Yoon MY. Bacterial acetohydroxyacid synthase and its inhibitors - a summary of their structure, biological activity and current status. FEBS J 2012; 279:946-63. [DOI: 10.1111/j.1742-4658.2012.08505.x] [Citation(s) in RCA: 37] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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13
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Expression, characterization, and site-directed mutation of a multiple herbicide-resistant acetohydroxyacid synthase (rAHAS) from Pseudomonas sp. Lm10. Curr Microbiol 2011; 63:145-50. [PMID: 21638043 DOI: 10.1007/s00284-011-9953-x] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/11/2011] [Accepted: 05/02/2011] [Indexed: 10/18/2022]
Abstract
A multiple herbicide-resistant acetohydroxyacid synthase (rAHAS) gene was cloned from Pseudomonas sp. Lm10. Sequence analysis showed that the rAHAS regulatory subunit was identical to that of Pseudomonas putida KT2440 (sensitive AHAS, sAHAS), whereas six different sites [H134→N (rAHAS→sAHAS), A135→P, S136→T, I210→V, F264→Y, and S486→W] were found in the catalytic subunit. The rAHAS and sAHAS were over expressed, purified and characterized. rAHAS showed higher resistance to four kinds of AHAS-inhibitor herbicides than sAHAS. The resistance factor of rAHAS was 56.0-fold, 12.6-fold, 6.5-fold, and 9.2-fold as compared with sAHAS when metsulfuron-methyl, imazethapyr, flumetsulam, and pyriminobac-methyl used as inhibitor, respectively. The specific activity of rAHAS was lower than that of sAHAS and the K (m) value of rAHAS for pyruvate was approximately onefold higher than the corresponding value for sAHAS. Data from site-directed mutagenesis demonstrated that alteration at A135, F264, and S486 resulted in resistance reduction, while the mutation at H134, S136, and I210 has little effect on the resistance. A135 was mainly responsible for resistance to imidazolinone; F264 conferred resistance to sulfonylurea and triazolopyrimidine sulfonamide; and S486 showed multiple herbicides resistance to the four herbicides.
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Jaña G, Jiménez V, Villà-Freixa J, Prat-Resina X, Delgado E, Alderete JB. A QM/MM study on the last two steps of the catalytic cycle of acetohydroxyacid synthase. COMPUT THEOR CHEM 2011. [DOI: 10.1016/j.comptc.2011.02.030] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
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15
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Vyazmensky M, Steinmetz A, Meyer D, Golbik R, Barak Z, Tittmann K, Chipman DM. Significant Catalytic Roles for Glu47 and Gln 110 in All Four of the C−C Bond-Making and -Breaking Steps of the Reactions of Acetohydroxyacid Synthase II. Biochemistry 2011; 50:3250-60. [DOI: 10.1021/bi102051h] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Maria Vyazmensky
- Ben-Gurion University of the Negev, Department of Life Sciences, Beer-Sheva 84105, Israel
| | - Andrea Steinmetz
- Georg-August University Göttingen, Albrecht-von-Haller-Institute and Göttingen Centre for Molecular Biosciences, Ernst-Caspari-Haus, Department of Bioanalytics, Justus-von-Liebig Weg 11, 37077 Göttingen, Germany
- Martin-Luther University Halle-Wittenberg, Institute for Biochemistry and Biotechnology, Kurt-Mothes-Strasse 3, 06120 Halle/Saale, Germany
| | - Danilo Meyer
- Georg-August University Göttingen, Albrecht-von-Haller-Institute and Göttingen Centre for Molecular Biosciences, Ernst-Caspari-Haus, Department of Bioanalytics, Justus-von-Liebig Weg 11, 37077 Göttingen, Germany
- Martin-Luther University Halle-Wittenberg, Institute for Biochemistry and Biotechnology, Kurt-Mothes-Strasse 3, 06120 Halle/Saale, Germany
| | - Ralph Golbik
- Martin-Luther University Halle-Wittenberg, Institute for Biochemistry and Biotechnology, Kurt-Mothes-Strasse 3, 06120 Halle/Saale, Germany
| | - Ze'ev Barak
- Ben-Gurion University of the Negev, Department of Life Sciences, Beer-Sheva 84105, Israel
| | - Kai Tittmann
- Georg-August University Göttingen, Albrecht-von-Haller-Institute and Göttingen Centre for Molecular Biosciences, Ernst-Caspari-Haus, Department of Bioanalytics, Justus-von-Liebig Weg 11, 37077 Göttingen, Germany
- Martin-Luther University Halle-Wittenberg, Institute for Biochemistry and Biotechnology, Kurt-Mothes-Strasse 3, 06120 Halle/Saale, Germany
| | - David M. Chipman
- Ben-Gurion University of the Negev, Department of Life Sciences, Beer-Sheva 84105, Israel
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Bunik VI, Schloss JV, Pinto JT, Dudareva N, Cooper AJL. A survey of oxidative paracatalytic reactions catalyzed by enzymes that generate carbanionic intermediates: implications for ROS production, cancer etiology, and neurodegenerative diseases. ADVANCES IN ENZYMOLOGY AND RELATED AREAS OF MOLECULAR BIOLOGY 2011; 77:307-60. [PMID: 21692372 DOI: 10.1002/9780470920541.ch7] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Affiliation(s)
- Victoria I Bunik
- School of Bioinformatics and Bioengineering, and Belozersky Institute of Physico-Chemical Biology, Moscow Lomonosov State University, Moscow, Russian Federation
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Jaña G, Jiménez V, Villà-Freixa J, Prat-Resina X, Delgado E, Alderete J. Computational study on the carboligation reaction of acetohidroxyacid synthase: new approach on the role of the HEThDP- intermediate. Proteins 2010; 78:1774-88. [PMID: 20225259 DOI: 10.1002/prot.22693] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
Acetohydroxyacid synthase (AHAS) is a thiamin diphosphate dependent enzyme that catalyses the decarboxylation of pyruvate to yield the hydroxyethyl-thiamin diphosphate (ThDP) anion/enamine intermediate (HEThDP(-)). This intermediate reacts with a second ketoacid to form acetolactate or acetohydroxybutyrate as products. Whereas the mechanism involved in the formation of HEThDP(-) from pyruvate is well understood, the role of the enzyme in controlling the carboligation reaction of HEThDP(-) has not been determined yet. In this work, molecular dynamics (MD) simulations were employed to identify the aminoacids involved in the carboligation stage. These MD studies were carried out over the catalytic subunit of yeast AHAS containing the reaction intermediate (HEThDP(-)) and a second pyruvate molecule. Our results suggest that additional acid-base ionizable groups are not required to promote the catalytic cycle, in contrast with earlier proposals. This finding leads us to postulate that the formation of acetolactate relies on the acid-base properties of the HEThDP(-) intermediate itself. PM3 semiempirical calculations were employed to obtain the energy profile of the proposed mechanism on a reduced model of the active site. These calculations confirm the role of HEThDP(-) intermediate as the ionizable group that promotes the carboligation and product formation steps of the catalytic cycle.
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Affiliation(s)
- Gonzalo Jaña
- Grupo de Química Teórica y Computacional, Facultad de Ciencias Químicas, Universidad de Concepción, Casilla 160-C, Concepción, Chile
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Steinmetz A, Vyazmensky M, Meyer D, Barak Z, Golbik R, Chipman DM, Tittmann K. Valine 375 and Phenylalanine 109 Confer Affinity and Specificity for Pyruvate as Donor Substrate in Acetohydroxy Acid Synthase Isozyme II from Escherichia coli. Biochemistry 2010; 49:5188-99. [DOI: 10.1021/bi100555q] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Andrea Steinmetz
- Albrecht-von-Haller-Institute and Göttingen Centre for Molecular Biosciences, Ernst-Caspari-Haus, Department of Bioanalytics, Georg-August University Göttingen, Justus-von-Liebig Weg 11, 37077 Göttingen, Germany
| | - Maria Vyazmensky
- Department of Life Sciences, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
| | - Danilo Meyer
- Albrecht-von-Haller-Institute and Göttingen Centre for Molecular Biosciences, Ernst-Caspari-Haus, Department of Bioanalytics, Georg-August University Göttingen, Justus-von-Liebig Weg 11, 37077 Göttingen, Germany
| | - Ze′ev Barak
- Department of Life Sciences, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
| | - Ralph Golbik
- Institute for Biochemistry and Biotechnology, Martin-Luther University Halle-Wittenberg, Kurt-Mothes-Strasse 3, 06120 Halle/Saale, Germany
| | - David M. Chipman
- Department of Life Sciences, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
| | - Kai Tittmann
- Albrecht-von-Haller-Institute and Göttingen Centre for Molecular Biosciences, Ernst-Caspari-Haus, Department of Bioanalytics, Georg-August University Göttingen, Justus-von-Liebig Weg 11, 37077 Göttingen, Germany
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19
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Choi JD, Gedi V, Pham CN, Ryu KH, Lee HS, Kim GH, Yoon MY. Site-directed mutagenesis of catalytic and regulatory subunits of Mycobacterium tuberculosis acetohydroxyacid synthase. Enzyme Microb Technol 2010. [DOI: 10.1016/j.enzmictec.2009.12.003] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/20/2022]
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20
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Characterization of recombinant thiamine diphosphate-dependent phosphonopyruvate decarboxylase from Streptomyces viridochromogenes Tü494. ACTA ACUST UNITED AC 2009. [DOI: 10.1016/j.molcatb.2009.03.016] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
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21
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Chipman DM, Barak Z, Shaanan B, Vyazmensky M, Binshtein E, Belenky I, Temam V, Steinmetz A, Golbik R, Tittmann K. Origin of the specificities of acetohydroxyacid synthases and glyoxylate carboligase. ACTA ACUST UNITED AC 2009. [DOI: 10.1016/j.molcatb.2009.03.015] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022]
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22
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Huang X, He J, Sun XF, Sun JQ, Li YF, Shen JJ, Li SP. Characterization and molecular mechanism of a naturally occurring metsulfuron-methyl resistant strain of Pseudomonas aeruginosa. World J Microbiol Biotechnol 2009. [DOI: 10.1007/s11274-009-0199-8] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/20/2022]
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23
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McNaughton J, Roberts M, Smith B, Rice D, Hinds M, Sanders C, Layton R, Lamb I, Delaney B. Comparison of broiler performance when fed diets containing event DP-3O5423-1, nontransgenic near-isoline control, or commercial reference soybean meal, hulls, and oil. Poult Sci 2008; 87:2549-61. [PMID: 19038811 DOI: 10.3382/ps.2007-00467] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2023] Open
Abstract
DP-3Ø5423-1 (305423) is a genetically modified soybean that was produced by biolistic insertion of the gm-fad2-1 gene fragment and gm-hra genes into the germline of soybean seeds. Expression of gm-fad2-1 results in greater concentrations of oleic acid (18:1) by suppressing expression of the endogenous FAD2-1 gene, which encodes an n-6 fatty acid desaturase enzyme that catalyzes desaturation of 18:1 to linoleic acid (18:2). The GM-HRA protein expressed by the gm-hra gene is a modified version of the soybean acetolactate synthase enzyme that is used as a selectable marker during transformation. A 42-d feeding trial was conducted with broiler chickens to compare the nutritional performance of 305423 soybeans with nontransgenic soybeans. Diets were prepared using processed fractions (meal, hulls, and oil) from 305423 soybean plants. For comparison, additional diets were produced with soybean fractions obtained from a nontransgenic near-isoline (control) and nontransgenic commercial Pioneer brand varieties (93B86, 93B15, and 93M40). Diets were fed to Ross x Cobb broilers (n = 120/group, 50% male and 50% female) in 3 phases. Starter, grower, and finisher diets contained 26.5, 23, and 21.5% soybean meal, respectively. Soybean hulls and oil were added at 1.0 and 0.5%, respectively, across all diets in each phase. No statistically significant differences were observed in growth performance (BW, mortality, feed efficiency), organ yield (liver and kidney), or carcass yield (breast, thigh, leg, wing, and abdominal fat) variables between broilers consuming diets prepared with isolated fractions from 305423 or near-isoline control soybean. Additionally, all performance and carcass variables from control and 305423 soybean treatment groups fell within tolerance intervals constructed for each response variable using data from broilers fed diets prepared with reference soybean fractions. Based on the results from this study, it was concluded that 305423 soybeans were nutritionally equivalent to non-transgenic control soybeans with a comparable genetic background.
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Affiliation(s)
- J McNaughton
- AHPharma, 116 W. Chestnut St., Salisbury, MD 21801, USA.
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24
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Duggleby RG, McCourt JA, Guddat LW. Structure and mechanism of inhibition of plant acetohydroxyacid synthase. PLANT PHYSIOLOGY AND BIOCHEMISTRY : PPB 2008; 46:309-24. [PMID: 18234503 DOI: 10.1016/j.plaphy.2007.12.004] [Citation(s) in RCA: 184] [Impact Index Per Article: 11.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/27/2007] [Indexed: 05/04/2023]
Abstract
Plants and microorganisms synthesize valine, leucine and isoleucine via a common pathway in which the first reaction is catalysed by acetohydroxyacid synthase (AHAS, EC 2.2.1.6). This enzyme is of substantial importance because it is the target of several herbicides, including all members of the popular sulfonylurea and imidazolinone families. However, the emergence of resistant weeds due to mutations that interfere with the inhibition of AHAS is now a worldwide problem. Here we summarize recent ideas on the way in which these herbicides inhibit the enzyme, based on the 3D structure of Arabidopsis thaliana AHAS. This structure also reveals important clues for understanding how various mutations can lead to herbicide resistance.
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Affiliation(s)
- Ronald G Duggleby
- RDBiotech, 22 Parklands Boulevard, Little Mountain, Queensland 4551, Australia.
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25
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Choi KJ, Noh KM, Kim DE, Ha BH, Kim EE, Yoon MY. Identification of the catalytic subunit of acetohydroxyacid synthase in Haemophilus influenzae and its potent inhibitors. Arch Biochem Biophys 2007; 466:24-30. [PMID: 17718999 DOI: 10.1016/j.abb.2007.07.011] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2007] [Revised: 07/07/2007] [Accepted: 07/11/2007] [Indexed: 11/22/2022]
Abstract
Acetohydroxyacid synthase (AHAS; EC 2.2.1.6) is a thiamin diphosphate- (ThDP)- and FAD-dependent enzyme that catalyzes the first common step in the biosynthetic pathway of the branched-amino acids (BCAAs) leucine, isoleucine, and valine. The gene from Haemophilus influenzae that encodes the AHAS catalytic subunit was cloned, overexpressed in Escherichia coli BL21(DE3), and purified to homogeneity. The purified H. influenzae AHAS catalytic subunit (Hin-AHAS) appeared as a single band on SDS-PAGE gel, with a molecular mass of approximately 63 kDa. The enzyme catalyzes the condensation of two molecules of pyruvate to form acetolactate, with a K(m) of 9.2mM and the specific activity of 1.5 micromol/min/mg. The cofactor activation constant (K(c)=13.5 microM) and the dissociation constant (K(d)=3.3 microM) of ThDP were also determined by enzymatic assay and tryptophan fluorescence quenching studies, respectively. We screened a chemical library to discover new inhibitors of the Hin AHAS catalytic subunit. Through which, AVS-2087 (IC(50)=0.53 microM), KSW30191 (IC(50)=1.42 microM), and KHG20612 (IC(50)=4.91 microM) displayed potent inhibition as compare to sulfometuron methyl (IC(50)=276.31 microM).
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Affiliation(s)
- Kyoung-Jae Choi
- Department of Chemistry, Hanyang University, 17 Haedang-dong, Sungdong-gu, Seoul 133-791, Republic of Korea
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26
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Yu Z, Niu C, Ban S, Wen X, Xi Z. Study on structure-activity relationship of mutation-dependent herbicide resistance acetohydroxyacid synthase through 3D-QSAR and mutation. ACTA ACUST UNITED AC 2007. [DOI: 10.1007/s11434-007-0267-0] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
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27
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Xi Z, Yu Z, Niu C, Ban S, Yang G. Development of a general quantum-chemical descriptor for steric effects: density functional theory based QSAR study of herbicidal sulfonylurea analogues. J Comput Chem 2007; 27:1571-6. [PMID: 16868987 DOI: 10.1002/jcc.20464] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
Quantitative structure-activity relationship (QSAR) analysis has become one of the most effective approaches for optimizing lead compounds and designing new drugs. Although large number of quantum-chemical descriptors were defined and applied successfully, it is still a big challenge to develop a general quantum-chemical descriptor describing the bulk effects more directly and effectively. In this article, we defined a general quantum-chemical descriptor by characterizing the volume of electron cloud for specific substituent using the method of density functional theory. The application of our defined steric descriptors in the QSAR analysis of sulfonylurea analogues resulted in four QSAR models with high quality (the best model: q2 = 0.881, r2 = 0.901, n = 35, s = 0.401, F = 68.44), which indicated that this descriptor may provide an effective way for solving the problem how to directly describe steric effect in quantum chemistry-based QSAR studies.
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Affiliation(s)
- Zhen Xi
- State Key Laboratory of Elemento-Organic Chemistry and Department of Chemical Biology, Nankai University, Tianjin, People's Republic of China, 300071
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28
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McCourt JA, Duggleby RG. Acetohydroxyacid synthase and its role in the biosynthetic pathway for branched-chain amino acids. Amino Acids 2006; 31:173-210. [PMID: 16699828 DOI: 10.1007/s00726-005-0297-3] [Citation(s) in RCA: 171] [Impact Index Per Article: 9.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2005] [Accepted: 12/09/2005] [Indexed: 11/25/2022]
Abstract
The branched-chain amino acids are synthesized by plants, fungi and microorganisms, but not by animals. Therefore, the enzymes of this pathway are potential target sites for the development of antifungal agents, antimicrobials and herbicides. Most research has focused upon the first enzyme in this biosynthetic pathway, acetohydroxyacid synthase (AHAS) largely because it is the target site for many commercial herbicides. In this review we provide a brief overview of the important properties of each enzyme within the pathway and a detailed summary of the most recent AHAS research, against the perspective of work that has been carried out over the past 50 years.
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Affiliation(s)
- J A McCourt
- School of Molecular and Microbial Sciences, University of Queensland, Brisbane, Australia
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29
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The Catalytic Role of the W573 in the Mobile Loop of Recombinant Acetohydroxyacid Synthase from Tobacco. B KOREAN CHEM SOC 2006. [DOI: 10.5012/bkcs.2006.27.4.549] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
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30
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Vinogradov V, Vyazmensky M, Engel S, Belenky I, Kaplun A, Kryukov O, Barak Z, Chipman DM. Acetohydroxyacid synthase isozyme I from Escherichia coli has unique catalytic and regulatory properties. Biochim Biophys Acta Gen Subj 2006; 1760:356-63. [PMID: 16326011 DOI: 10.1016/j.bbagen.2005.10.008] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/17/2005] [Revised: 09/27/2005] [Accepted: 10/20/2005] [Indexed: 11/30/2022]
Abstract
AHAS I is an isozyme of acetohydroxyacid synthase which is apparently unique to enterobacteria. It has been known for over 20 years that it has many properties which are quite different from those of the other two enterobacterial AHASs isozymes, as well as from those of "typical" AHASs which are single enzymes in a given organism. These include a unique mechanism for regulation of expression and the absence of a preference for forming acetohydroxybutyrate. We have cloned the two subunits, ilvB and ilvN, of this Escherichia coli isoenzyme and examined the enzymatic properties of the purified holoenzyme and the enzyme reconstituted from purified subunits. Unlike other AHASs, AHAS I demonstrates cooperative feedback inhibition by valine, and the kinetics fit closely to an exclusive binding model. The formation of acetolactate by AHAS I is readily reversible and acetolactate can act as substrate for alternative AHAS I-catalyzed reactions.
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Affiliation(s)
- Valerie Vinogradov
- Department of Life Sciences, Ben-Gurion University of the Negev, POB 657, Beer-Sheva 84105, Israel
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31
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Chipman DM, Duggleby RG, Tittmann K. Mechanisms of acetohydroxyacid synthases. Curr Opin Chem Biol 2006; 9:475-81. [PMID: 16055369 DOI: 10.1016/j.cbpa.2005.07.002] [Citation(s) in RCA: 84] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/09/2005] [Accepted: 07/18/2005] [Indexed: 11/17/2022]
Abstract
Acetohydroxyacid synthases are thiamin diphosphate- (ThDP-) dependent biosynthetic enzymes found in all autotrophic organisms. Over the past 4-5 years, their mechanisms have been clarified and illuminated by protein crystallography, engineered mutagenesis and detailed single-step kinetic analysis. Pairs of catalytic subunits form an intimate dimer containing two active sites, each of which lies across a dimer interface and involves both monomers. The ThDP adducts of pyruvate, acetaldehyde and the product acetohydroxyacids can be detected quantitatively after rapid quenching. Determination of the distribution of intermediates by NMR then makes it possible to calculate individual forward unimolecular rate constants. The enzyme is the target of several herbicides and structures of inhibitor-enzyme complexes explain the herbicide-enzyme interaction.
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Affiliation(s)
- David M Chipman
- Department of Life Sciences, Ben-Gurion University POB 653, Beer-Sheva 84105, Israel
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32
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Abstract
Acetohydroxyacid synthase (Ec 2.2.1.6) catalyses the thiamine diphosphate-dependent reaction between two molecules of pyruvate yielding 2-acetolactacte and CO2. The enzyme will also utilise hydroxypyruvate with a k(cat) value that is 12% of that observed with pyruvate. When hydroxypyruvate is the substrate, the enzyme undergoes progressive inactivation with kinetics that are characteristic of suicide inhibition. It is proposed that the dihydroxyethyl-thiamine diphosphate intermediate can expel a hydroxide ion forming an enol that rearranges to a bound acetyl group.
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Affiliation(s)
- Ronald G Duggleby
- Department of Biochemistry and Molecular Biology, The University of Queensland, Brisbane, Qld 4072, Australia.
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33
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Abstract
Acetohydroxyacid synthase (AHAS) is the first common enzyme in the pathway for the biosynthesis of branched-chain amino acids. Interest in the enzyme has escalated over the past 20 years since it was discovered that AHAS is the target of the sulfonylurea and imidazolinone herbicides. However, several questions regarding the reaction mechanism have remained unanswered, particularly the way in which AHAS "chooses" its second substrate. A new method for the detection of reaction intermediates enables calculation of the microscopic rate constants required to explain this phenomenon.
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Affiliation(s)
- Jennifer A McCourt
- School of Molecular and Microbial Sciences, University of Queensland, St Lucia, Brisbane, QLD 4072, Australia
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34
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Tittmann K, Vyazmensky M, Hübner G, Barak Z, Chipman DM. The carboligation reaction of acetohydroxyacid synthase II: steady-state intermediate distributions in wild type and mutants by NMR. Proc Natl Acad Sci U S A 2005; 102:553-8. [PMID: 15640355 PMCID: PMC545553 DOI: 10.1073/pnas.0408210101] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The thiamin diphosphate (ThDP)-dependent enzyme acetohydroxyacid synthase (AHAS) catalyzes the first common step in branched-chain amino acid biosynthesis. By specific ligation of pyruvate with the alternative acceptor substrates 2-ketobutyrate and pyruvate, AHAS controls the flux through this branch point and determines the relative rates of synthesis of isoleucine, valine, and leucine, respectively. We used detailed NMR analysis to determine microscopic rate constants for elementary steps in the reactions of AHAS II and mutants altered at conserved residues Arg-276, Trp-464, and Met-250. In Arg276Lys, both the condensation of the enzyme-bound hydroxyethyl-ThDP carbanion/enamine (HEThDP) with the acceptor substrates and acetohydroxyacid release are slowed several orders of magnitude relative to the wild-type enzyme. We propose that the interaction of the guanidinium moiety of Arg-264 with the carboxylate of the acceptor ketoacid provides an optimal alignment of substrate and HEThDP orbitals in the reaction trajectory for acceptor ligation, whereas its interaction with the carboxylate of the covalent HEThDP-acceptor adduct plays a similar role in product release. Both Trp-464 and Met-250 affect the acceptor specificity. The high preference for ketobutyrate in the wild-type enzyme is lost in Trp464Leu as a consequence of similar forward rate constants of carboligation and product release for the alternative acceptors. In Met250Ala, the turnover rate is determined by the condensation of HEThDP with pyruvate and release of the acetolactate product, whereas the parallel steps with 2-ketobutyrate are considerably faster. We speculate that the specificity of carboligation and product liberation may be cumulative if the former is not completely committed.
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Affiliation(s)
- Kai Tittmann
- Department of Life Sciences, Ben-Gurion University of the Negev, 84105 Beer-Sheva, Israel
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35
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Kim J, Beak DG, Kim YT, Choi JD, Yoon MY. Effects of deletions at the C-terminus of tobacco acetohydroxyacid synthase on the enzyme activity and cofactor binding. Biochem J 2004; 384:59-68. [PMID: 15521822 PMCID: PMC1134088 DOI: 10.1042/bj20040427] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2004] [Revised: 06/17/2004] [Accepted: 07/22/2004] [Indexed: 11/17/2022]
Abstract
AHAS (acetohydroxyacid synthase) catalyses the first committed step in the biosynthesis of branched-chain amino acids, such as valine, leucine and isoleucine. Owing to the unique presence of these biosynthetic pathways in plants and micro-organisms, AHAS has been widely investigated as an attractive target of several classes of herbicides. Recently, the crystal structure of the catalytic subunit of yeast AHAS has been resolved at 2.8 A (1 A=0.1 nm), showing that the active site is located at the dimer interface and is near the herbicide-binding site. In this structure, the existence of two disordered regions, a 'mobile loop' and a C-terminal 'lid', is worth notice. Although these regions contain the residues that are known to be important in substrate specificity and in herbicide resistance, they are poorly folded into any distinct secondary structure and are not within contact distance of the cofactors. In the present study, we have tried to demonstrate the role of these regions of tobacco AHAS by constructing variants with serial deletions, based on the structure of yeast AHAS. In contrast with the wild-type AHAS, the truncated mutant which removes the C-terminal lid, Delta630, and the internal deletion mutant without the mobile loop, Delta567-582, impaired the binding affinity for ThDP (thiamine diphosphate), and showed different elution profiles representing a monomeric form in gel-filtration chromatography. Our results suggest that these regions are involved in the binding/stabilization of the active dimer and ThDP binding.
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Affiliation(s)
- Joungmok Kim
- *Department of Chemistry, College of Natural Science, Hanyang University, Seoul 133-791, South Korea
| | - Dong-Gil Beak
- *Department of Chemistry, College of Natural Science, Hanyang University, Seoul 133-791, South Korea
| | - Young-Tae Kim
- †Department of Microbiology, Pukyung National University, Busan 608-737, South Korea
| | - Jung-Do Choi
- ‡School of Life Science and Research Institute for Genetic Engineering, Chungbuk National University, Cheongju 361-763, South Korea
| | - Moon-Young Yoon
- *Department of Chemistry, College of Natural Science, Hanyang University, Seoul 133-791, South Korea
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36
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Engel S, Vyazmensky M, Vinogradov M, Berkovich D, Bar-Ilan A, Qimron U, Rosiansky Y, Barak Z, Chipman DM. Role of a Conserved Arginine in the Mechanism of Acetohydroxyacid Synthase. J Biol Chem 2004; 279:24803-12. [PMID: 15044456 DOI: 10.1074/jbc.m401667200] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The thiamin diphosphate (ThDP)-dependent bio-synthetic enzyme acetohydroxyacid synthase (AHAS) catalyzes decarboxylation of pyruvate and specific condensation of the resulting ThDP-bound two-carbon intermediate, hydroxyethyl-ThDP anion/enamine (HEThDP(-)), with a second ketoacid, to form acetolactate or acetohydroxybutyrate. Whereas the mechanism of formation of HEThDP(-) from pyruvate is well understood, the role of the enzyme in control of the carboligation reaction of HEThDP(-) is not. Recent crystal structures of yeast AHAS from Duggleby's laboratory suggested that an arginine residue might interact with the second ketoacid substrate. Mutagenesis of this completely conserved residue in Escherichia coli AHAS isozyme II (Arg(276)) confirms that it is required for rapid and specific reaction of the second ketoacid. In the mutant proteins, the normally rapid second phase of the reaction becomes rate-determining. A competing alternative nonnatural but stereospecific reaction of bound HEThDP(-) with benzaldehyde to form phenylacetylcarbinol (Engel, S., Vyazmensky, M., Geresh, S., Barak, Z., and Chipman, D. M. (2003) Biotechnol. Bioeng. 84, 833-840) provides a new tool for studying the fate of HEThDP(-) in AHAS, since the formation of the new product has a very different dependence on active site modifications than does acetohydroxyacid acid formation. The effects of mutagenesis of four different residues in the site on the rates and specificities of the normal and unnatural reactions support a critical role for Arg(276) in the stabilization of the transition states for ligation of the incoming second ketoacid with HEThDP(-) and/or for the breaking of the product-ThDP bond. This information makes it possible to engineer the active site so that it efficiently and preferentially catalyzes a new reaction.
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Affiliation(s)
- Stanislav Engel
- Department of Life Sciences, Ben-Gurion University of the Negev, POB 653, 84105 Beer-Sheva, Israel
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37
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The Cofactors Role on Chemical Mechanism of Recombinant Acetohydroxy Acid Synthase from Tobacco. B KOREAN CHEM SOC 2004. [DOI: 10.5012/bkcs.2004.25.5.721] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
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38
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Le DT, Yoon MY, Kim YT, Choi JD. Homology modeling of the structure of tobacco acetohydroxy acid synthase and examination of the active site by site-directed mutagenesis. Biochem Biophys Res Commun 2004; 317:930-8. [PMID: 15081429 DOI: 10.1016/j.bbrc.2004.03.133] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2004] [Indexed: 11/24/2022]
Abstract
A reliable model of tobacco acetohydroxy acid synthase (AHAS) was obtained by homology modeling based on a yeast AHAS X-ray structure using the Swiss-Model server. Conserved residues at the dimer interface were identified, of which the functional roles of four residues, namely H142, E143, M489, and M542, were determined by site-directed mutagenesis. Eight mutants were successfully generated and purified, five of which (H142T, M489V, M542C, M542I, and M542V) were found to be inactive under various assay conditions. The H142K mutant was moderately altered in all kinetic parameters to a similar extent. In addition, the mutant was more thermo-labile than wild type enzyme. The E143A mutant increased the Km value more than 20-fold while other parameters were not significantly changed. All mutations carried out on residue M542 inactivated the enzyme. Though showing a single band on SDS-PAGE, the M542C mutant lost its native tertiary structure and was aggregated. Except M542C, each of the other mutants showed a secondary structure similar to that of wild type enzyme. Although all the inactive mutants were able to bind FAD, the mutants M489V and M542C showed a very low affinity for FAD. None of the active mutants constructed was strongly resistant to three tested herbicides. Taken together, the results suggest that the residues of H142, E143, M489, and M542 are essential for catalytic activity. Furthermore, it seems that H142 residue is involved in stabilizing the dimer interaction, while E143 residue may be involved in binding with substrate pyruvate. The data from the site-directed mutagenesis imply that the constructed homology model of tobacco AHAS is realistic.
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Affiliation(s)
- Dung Tien Le
- School of Life Sciences, Chungbuk National University, Cheongju 361-763, Republic of Korea
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39
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Engel S, Vyazmensky M, Berkovich D, Barak Z, Chipman DM. Substrate range of acetohydroxy acid synthase I fromEscherichia coli in the stereoselective synthesis of ?-hydroxy ketones. Biotechnol Bioeng 2004; 88:825-31. [PMID: 15558598 DOI: 10.1002/bit.20275] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Acetohydroxy acid synthase I appears to be the most effective of the AHAS isozymes found in Escherichia coli in the chiral synthesis of phenylacetyl carbinol from pyruvate and benzaldehyde. We report here the exploration of a range of aldehydes as substrates for AHAS I and demonstrate that the enzyme can accept a wide variety of substituted benzaldehydes, as well as heterocyclic and heteroatomic aromatic aldehydes, to produce chiral carbinols. The active site of AHAS I does not appear to impose serious steric constraints on the acceptor substrate. The influence of electronic effects on the reaction has been probed using substituted benzaldehydes as substrates. The electrophilicity of the aldehyde acceptor substrates is most important to their reactivity, but the lipophilicity of substituents also affects their reactivity. AHAS I is an effective biosynthetic platform for production of a variety of alpha-hydroxy ketones, compounds with considerable potential as pharmacological precursors.
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Affiliation(s)
- Stanislav Engel
- Department of Biotechnology Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
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Yoon MY, Hwang JH, Choi MK, Baek DK, Kim J, Kim YT, Choi JD. The active site and mechanism of action of recombinant acetohydroxy acid synthase from tobacco. FEBS Lett 2003; 555:185-91. [PMID: 14644413 DOI: 10.1016/s0014-5793(03)01177-3] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Acetohydroxy acid synthase (AHAS) is one of several enzymes that require thiamine diphosphate and a divalent cation as essential cofactors. Recently, the three-dimensional structure of the enzyme from yeast has been determined [Pang et al., J. Mol. Biol. 317 (2002) 249-262]. While this structure sheds light on the binding of the cofactors and the reaction mechanism, the interactions between the substrates and the enzyme remain unclear. We have studied the pH dependence of kinetic parameters in order to obtain information about the chemical mechanism in the active site. Data are consistent with a mechanism in which substrate selectively catalyzed to the enzyme with an unprotonated base having a pK of 6.48, and a protonated group having a pK of 8.25 for catalysis. The temperature dependence of kinetic parameters was pH-dependent, and the enthalpies of ionization, DeltaH(ion), calculated from the slope of pK(1) and pK(2) are both pH-independent. The solvent perturbation of kinetic parameters was pH-dependent, and the pK(1) from the acidic side and the pK(2) from the basic side were shifted down 0.4 pH units and shifted up 0.6 units as water was replaced by 15% ethanol, respectively. The data are discussed in terms of the acid-base chemical mechanism.
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Affiliation(s)
- Moon-Young Yoon
- Department of Chemistry, Hanyang University, Seoul 133-791, South Korea.
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Pang SS, Duggleby RG, Schowen RL, Guddat LW. The crystal structures of Klebsiella pneumoniae acetolactate synthase with enzyme-bound cofactor and with an unusual intermediate. J Biol Chem 2003; 279:2242-53. [PMID: 14557277 DOI: 10.1074/jbc.m304038200] [Citation(s) in RCA: 61] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Acetohydroxyacid synthase (AHAS) and acetolactate synthase (ALS) are thiamine diphosphate (ThDP)-dependent enzymes that catalyze the decarboxylation of pyruvate to give a cofactor-bound hydroxyethyl group, which is transferred to a second molecule of pyruvate to give 2-acetolactate. AHAS is found in plants, fungi, and bacteria, is involved in the biosynthesis of the branched-chain amino acids, and contains non-catalytic FAD. ALS is found only in some bacteria, is a catabolic enzyme required for the butanediol fermentation, and does not contain FAD. Here we report the 2.3-A crystal structure of Klebsiella pneumoniae ALS. The overall structure is similar to AHAS except for a groove that accommodates FAD in AHAS, which is filled with amino acid side chains in ALS. The ThDP cofactor has an unusual conformation that is unprecedented among the 26 known three-dimensional structures of nine ThDP-dependent enzymes, including AHAS. This conformation suggests a novel mechanism for ALS. A second structure, at 2.0 A, is described in which the enzyme is trapped halfway through the catalytic cycle so that it contains the hydroxyethyl intermediate bound to ThDP. The cofactor has a tricyclic structure that has not been observed previously in any ThDP-dependent enzyme, although similar structures are well known for free thiamine. This structure is consistent with our proposed mechanism and probably results from an intramolecular proton transfer within a tricyclic carbanion that is the true reaction intermediate. Modeling of the second molecule of pyruvate into the active site of the enzyme with the bound intermediate is consistent with the stereochemistry and specificity of ALS.
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Affiliation(s)
- Siew Siew Pang
- Department of Biochemistry and Molecular Biology, the University of Queensland, Brisbane, Queensland 4072, Australia
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Engel S, Vyazmensky M, Geresh S, Barak Z, Chipman DM. Acetohydroxyacid synthase: a new enzyme for chiral synthesis of R-phenylacetylcarbinol. Biotechnol Bioeng 2003; 83:833-40. [PMID: 12889023 DOI: 10.1002/bit.10728] [Citation(s) in RCA: 64] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
We have found that acetohydroxyacid synthase (AHAS) is an efficient catalyst for the enantiospecific (> or =98% enantiomeric excess) synthesis of (R)-phenylacetylcarbinol (R-PAC) from pyruvate and benzaldehyde, despite the fact that its normal physiological role is synthesis of (S)-acetohydroxyacids from pyruvate and a second ketoacid. (R)-phenylacetylcarbinol is the precursor of important drugs having alpha and beta adrenergic properties, such as L-ephedrine, pseudoephedrine, and norephedrin. It is currently produced by whole-cell fermentations, but the use of the isolated enzyme pyruvate decarboxylase (PDC) for this purpose is the subject of active research and development efforts. Some of the AHAS isozymes of Escherichia coli have important advantages compared to PDC, including negligible acetaldehyde formation and high conversion of substrates (both pyruvate and benzaldehyde) to PAC. Acetohydroxyacid synthase isozyme I is particularly efficient. The reaction is not limited to condensation of pyruvate with benzaldehyde and other aromatic aldehydes may be used.
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Affiliation(s)
- Stanislav Engel
- Department of Biotechnology Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
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Inoue H, Nishito A, Eriguchi SI, Tamura T, Inagaki K, Tanaka H. Purification and substrate characterization of α-ketobutyrate decarboxylase from Pseudomonas putida. ACTA ACUST UNITED AC 2003. [DOI: 10.1016/s1381-1177(03)00089-4] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
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Duggleby RG, Pang SS, Yu H, Guddat LW. Systematic characterization of mutations in yeast acetohydroxyacid synthase. Interpretation of herbicide-resistance data. EUROPEAN JOURNAL OF BIOCHEMISTRY 2003; 270:2895-904. [PMID: 12823560 DOI: 10.1046/j.1432-1033.2003.03671.x] [Citation(s) in RCA: 73] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Acetohydroxyacid synthase (AHAS, EC 4.1.3.18) catalyses the first step in branched-chain amino acid biosynthesis and is the target for sulfonylurea and imidazolinone herbicides, which act as potent and specific inhibitors. Mutants of the enzyme have been identified that are resistant to particular herbicides. However, the selectivity of these mutants towards various sulfonylureas and imidazolinones has not been determined systematically. Now that the structure of the yeast enzyme is known, both in the absence and presence of a bound herbicide, a detailed understanding of the molecular interactions between the enzyme and its inhibitors becomes possible. Here we construct 10 active mutants of yeast AHAS, purify the enzymes and determine their sensitivity to six sulfonylureas and three imidazolinones. An additional three active mutants were constructed with a view to increasing imidazolinone sensitivity. These three variants were purified and tested for their sensitivity to the imidazolinones only. Substantial differences are observed in the sensitivity of the 13 mutants to the various inhibitors and these differences are interpreted in terms of the structure of the herbicide-binding site on the enzyme.
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Affiliation(s)
- Ronald G Duggleby
- Department of Biochemistry and Molecular Biology, The University of Queensland, Brisbane, QLD 4072, Australia.
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Pang SS, Guddat LW, Duggleby RG. Molecular basis of sulfonylurea herbicide inhibition of acetohydroxyacid synthase. J Biol Chem 2003; 278:7639-44. [PMID: 12496246 DOI: 10.1074/jbc.m211648200] [Citation(s) in RCA: 127] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Acetohydroxyacid synthase (AHAS) (acetolactate synthase, EC ) catalyzes the first step in branched-chain amino acid biosynthesis and is the target for sulfonylurea and imidazolinone herbicides. These compounds are potent and selective inhibitors, but their binding site on AHAS has not been elucidated. Here we report the 2.8 A resolution crystal structure of yeast AHAS in complex with a sulfonylurea herbicide, chlorimuron ethyl. The inhibitor, which has a K(i) of 3.3 nm, blocks access to the active site and contacts multiple residues where mutation results in herbicide resistance. The structure provides a starting point for the rational design of further herbicidal compounds.
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Affiliation(s)
- Siew Siew Pang
- Department of Biochemistry and Molecular Biology, The University of Queensland, Brisbane QLD 4072, Australia
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Abstract
Carbohydrates are highly abundant biomolecules found extensively in nature. Besides playing important roles in energy storage and supply, they often serve as essential biosynthetic precursors or structural elements needed to sustain all forms of life. A number of unusual sugars that have certain hydroxyl groups replaced by a hydrogen, an amino group, or an alkyl side chain play crucial roles in determining the biological activity of the parent natural products in bacterial lipopolysaccharides or secondary metabolite antibiotics. Recent investigation of the biosynthesis of these monosaccharides has led to the identification of the gene clusters whose protein products facilitate the unusual sugar formation from the ubiquitous NDP-glucose precursors. This review summarizes the mechanistic studies of a few enzymes crucial to the biosynthesis of C-2, C-3, C-4, and C-6 deoxysugars, the characterization and mutagenesis of nucleotidyl transferases that can recognize and couple structural analogs of their natural substrates and the identification of glycosyltransferases with promiscuous substrate specificity. Information gleaned from these studies has allowed pathway engineering, resulting in the creation of new macrolides with unnatural deoxysugar moieties for biological activity screening. This represents a significant progress toward our goal of searching for more potent agents against infectious diseases and malignant tumors.
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Affiliation(s)
- Xuemei M He
- Division of Medicinal Chemistry, College of Pharmacy, and Department of Chemistry and Biochemistry, University of Texas, Austin, Texas 78712, USA.
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Lapidot M, Raveh D, Sivan A, Arad SM, Shapira M. Stable chloroplast transformation of the unicellular red alga Porphyridium species. PLANT PHYSIOLOGY 2002; 129:7-12. [PMID: 12011332 PMCID: PMC1540221 DOI: 10.1104/pp.011023] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/18/2023]
Abstract
Red algae are extremely attractive for biotechnology because they synthesize accessory photosynthetic pigments (phycobilins and carotenoids), unsaturated fatty acids, and unique cell wall sulfated polysaccharides. We report a high-efficiency chloroplast transformation system for the unicellular red microalga Porphyridium sp. This is the first genetic transformation system for Rhodophytes and is based on use of a mutant form of the gene encoding acetohydroxyacid synthase [AHAS(W492S)] as a dominant selectable marker. AHAS is the target enzyme of the herbicide sulfometuron methyl, which effectively inhibits growth of bacteria, fungi, plants, and algae. Biolistic transformation of synchronized Porphyridium sp. cells with the mutant AHAS(W492S) gene that confers herbicide resistance gave a high frequency of sulfomethuron methyl-resistant colonies. The mutant AHAS gene integrated into the chloroplast genome by homologous recombination. This system paves the way for expression of foreign genes in red algae and has important biotechnological implications.
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Affiliation(s)
- Miri Lapidot
- Department of Life Sciences, Ben Gurion University of the Negev, Beer-Sheva 84105, Israel
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Pang SS, Duggleby RG, Guddat LW. Crystal structure of yeast acetohydroxyacid synthase: a target for herbicidal inhibitors. J Mol Biol 2002; 317:249-62. [PMID: 11902841 DOI: 10.1006/jmbi.2001.5419] [Citation(s) in RCA: 162] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Acetohydroxyacid synthase (AHAS; EC 4.1.3.18) catalyzes the first step in branched-chain amino acid biosynthesis. The enzyme requires thiamin diphosphate and FAD for activity, but the latter is unexpected, because the reaction involves no oxidation or reduction. Due to its presence in plants, AHAS is a target for sulfonylurea and imidazolinone herbicides. Here, the crystal structure to 2.6 A resolution of the catalytic subunit of yeast AHAS is reported. The active site is located at the dimer interface and is near the proposed herbicide-binding site. The conformation of FAD and its position in the active site are defined. The structure of AHAS provides a starting point for the rational design of new herbicides.
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Affiliation(s)
- Siew Siew Pang
- Centre for Protein Structure Function and Engineering, Department of Biochemistry and Molecular Biology, School of Molecular and Microbial Sciences, The University of Queensland, Brisbane, QLD 4072, Australia
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Byrne KL, Meacock PA. Thiamin auxotrophy in yeast through altered cofactor dependence of the enzyme acetohydroxyacid synthase. MICROBIOLOGY (READING, ENGLAND) 2001; 147:2389-2398. [PMID: 11535779 DOI: 10.1099/00221287-147-9-2389] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
The THI1 gene of Saccharomyces cerevisiae has been identified and found to be allelic with the previously characterized gene ILV2 that encodes acetohydroxyacid synthase (AHAS). This enzyme catalyses the first step in the parallel biosyntheses of the branched-chain amino acids isoleucine and valine, using thiamin pyrophosphate (TPP) as a cofactor. The ilv2-thi1 allele encodes a functional AHAS enzyme with an altered dependence for the cofactor TPP resulting in the thiamin auxotrophic phenotype. Nucleotide sequence analysis and site-directed mutagenesis revealed that the thi1 mutation is a single base substitution which causes the conserved amino acid substitution D176E in the AHAS protein. This study therefore implicates aspartate 176 as another amino acid residue important either for the efficient binding of TPP by AHAS or for the functional stability of the holoenzyme.
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Affiliation(s)
- Kerry L Byrne
- Department of Genetics, University of Leicester, University Road, Leicester LE1 7RH, UK1
| | - Peter A Meacock
- Department of Genetics, University of Leicester, University Road, Leicester LE1 7RH, UK1
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