L-amino acid oxidase as biocatalyst: a dream too far?

Engineered Biocatalysts for the Asymmetric Synthesis of d-Phenylalanines

The enzymatic synthesis of d-phenylalanines, important chiral building blocks for several pharmaceuticals and fine chemicals, has been widely explored. Their asymmetric synthesis of high atom economy and accessible prochiral starting materials is highly attractive, while the expanding toolbox of protein engineering facilitates access to biocatalysts tailored for these processes. Accordingly, this Review provides an overview of the protein engineering efforts of enzymes involved in the asymmetric synthetic pathways for d-phenylalanines. The engineering efforts on d-amino acid dehydrogenases, d-amino acid transaminases, and phenylalanine ammonia-lyases to produce d-phenylalanines are thoroughly examined, while their application in (chemo)enzymatic cascades is also discussed. For an improved efficiency of the cascades, the protein engineering of l-amino acid deaminases and/or l-amino acid oxidases for an increased transformation of phenylalanines is also addressed.

Engineered Macrophage Membrane-Camouflaged Nanodecoys Reshape the Infectious Microenvironment for Efficient Periodontitis Treatment

A vicious cycle between microbiota dysbiosis and hyperactivated inflammation, hardly disrupted by conventional therapies, remains a significant clinical challenge for periodontitis treatment. Herein, by cloaking a cascade catalysis system in an engineered macrophage membrane, a nanodecoy-based strategy, with targeted bacteria-killing and immunomodulatory abilities, is proposed for reshaping the hostile periodontitis microenvironment. Specifically, recombinant human antimicrobial peptide, LL-37, is anchored to a Toll-like receptor-enriched macrophage membrane via genetic engineering, which facilitates the specific bacteria elimination and efficient tissue retention of the nanodecoys. Moreover, the cascade catalysis system integrates L-amino acid oxidase (LAAO) with hollowed manganese dioxide (hMnO2) by reciprocal elevation of the catalytic efficiency of hMnO2 and LAAO, leading to accelerated O2 generation under a hypoxic microenvironment and disrupted metabolism of periodontopathogenic bacteria. Notably, the nanodecoys trigger the nuclear translocation of NF-E2-related factor-2 (NRF2) to reduce oxidative stress response and rewire the polarization of macrophages, thereby boosting the osteogenic differentiation of osteoblasts. Furthermore, the alveolar bone regeneration therapeutically benefits from the nanodecoys in vivo. Altogether, these results highlight the attractive functions of engineered macrophage membrane-cloaked nanodecoys for effective periodontitis treatment.

Designing the Aplysia punctata Arginine-Depleting Enzyme for Tumor Targeting

l-Amino acid oxidases (LAAO) deaminate amino acids to α-keto acids and generate hydrogen peroxide, a reactive oxygen species (ROS) with potential value for cancer therapy. We recombinantly expressed the LAAO from Aplysia punctata, called APIT (Cuvier 1803). The resulting wild-type APIT (APITwt) was conjugated to polyethylene glycol (APIT-PEG). Furthermore, an APIT mutant with an affibody targeting the human epidermal growth factor receptor 2 (HER2; zHER2-APIT) was genetically engineered resulting in a binding affinity KD of ∼ 2.2 nM to the HER2 receptor ectodomain. Further, we evaluated if the APIT and tumor-targeted APIT can be used as an APIT-drug conjugate by covalently amidating the lysine residues on the protein surface. However, for the HER2-targeted APIT, the affibody contains lysines as well, and amidation of these lysines could have impaired the affibody's affinity to the HER2 receptor. Therefore, we designed a lysine-free variant of the tumor-targeting part of zHER2-APIT using an in silico mutation analysis, suggesting the replacement of the lysines of the affibody by arginine or alanine. This new variant is referred to as zHER2(K-del)-APIT. To simulate a covalent drug loading to APIT and the targeting constructs, we attached biotin by amidation. Biotin-zHER2(K-del)-APIT successfully allowed binding to HER2-positive but not HER2-negative cells in vitro. The biodistribution of these novel constructs was tested in xenografted mice with a HER2-positive and negative tumor in each animal. The zHER2(K-del)-APIT lost its ability to target HER2-positive tumors despite the in vitro data suggesting otherwise. The zHER2-APIT accumulated within the HER2-positive tumors but not in the negative tumors. APIT-PEG had increased uptake in HER2-positive and negative tumors compared to APITwt, which can be attributed to a prolonged serum half-life achieved by PEGylation, due to the absence of any tumor-targeting effect. These biodistribution studies point to HER2-targeting LAAOs for cancer therapy and PEGylation increasing tumor accumulation.

Imine Synthesis by Engineered d-Amino Acid Oxidase from Porcine Kidney

Various symmetric and asymmetric imines were synthesized using the novel amine oxidase, obtained as variants of d-amino acid oxidase (pkDAO) from porcine kidney (Y228L/R283G) and (I230A/R283G). Active primary imines produced as intermediates in the oxidation of methylbenzylamine (MBA) derivatives were trapped by aliphatic, aromatic amines and diamines as nucleophiles forming new imines. (R)-Fluoro-MBA was the best substrate for symmetric imine synthesis, providing almost stoichiometric conversion (100 mM) and achieving nearly 100% yield. Several (R)-MBA derivatives were used as substrates, and the corresponding symmetric and asymmetric imines were synthesized. The turnover number of N-benzylidenebenzylamine synthesis from benzylamine was calculated to be 1.61 × 105 (number of moles of reactant consumed per mole of catalyst/h), which is more than 103 higher than metal-, photo-, and organo-catalysts reported so far. The diastereomers of bis(1-phenylethyl)amine, the reduced products of (R)-MBA, were identified as a mixture of 84.9% (R,R)-bis(1-phenylethyl)amine and 15.1% (R,S)-bis(1-phenylethyl)amine to consider the reaction mechanism.

Open and closed structures of L-arginine oxidase by cryo-electron microscopy and X-ray crystallography

L-arginine oxidase (AROD, EC 1.4.3.25) is an oxidoreductase that catalyses the deamination of L-arginine, with flavin adenine dinucleotide (FAD) as a cofactor. Recently identified AROD from Pseudomonas sp. TPU 7192 (PT-AROD) demonstrates high selectivity for L-arginine. This enzyme is useful for accurate assays of L-arginine in biological samples. The structural characteristics of the FAD-dependent AROD, however, remain unknown. Here, we report the structure of PT-AROD at a resolution of 2.3 Å by cryo-electron microscopy. PT-AROD adopts an octameric structure with D4 symmetry, which is consistent with its molecular weight in solution, estimated by mass photometry. Comparative analysis of this structure with that determined using X-ray crystallography reveals open and closed forms of the lid-like loop at the entrance to the substrate pocket. Furthermore, mutation of Glu493, located at the substrate binding site, diminishes substrate selectivity, suggesting that this residue contributes significantly to the high selectivity of PT-AROD.

Precipitated-crosslinked multi-enzyme hybrid nanoflowers for efficient synthesis of α-ketoglutaric acid

Cascade catalysis of glutamate oxidase (GLOX) and catalase (CAT) to perform one-pot synthetic route for α-ketoglutarate (α-KG) production offers several advantages including simplicity of operation and the generation of few reaction by-products. Nevertheless, the instability of free GLOX and CAT, the high production cost and the difficulty of recycling severely limits its industrial utilisation. Here, catalase-inorganic hybrid nanoflowers were first prepared, and cross-linked with GLOX precipitates by a macromolecular cross-linking agent dextran polyaldehyde to form a novel dual enzyme precipitation-cross-linking hybrid nanoflower (GLOX@CAT-HNFs). The resultant GLOX@CAT-HNFs exhibited higher catalytic activity than conventional combined cross-linked enzyme aggregates (combi-CLEAs) and hybrid nanoflowers (GLOX&CAT-HNFs). The GLOX@CAT-HNFs exhibited 92 % activity recovery whereas combi-CLEAs and GLOX&CAT-HNFs was 87 % and 72 %, respectively. Meanwhile, the GLOX@CAT-HNFs showed better thermal stability, pH and storage stability than free enzymes. After incubation at 60 °C for 100 min, GLOX@CAT-HNFs maintained 70 % of its initial activity while free enzyme was only 18.52 %. Furthermore, after 7 cycles of use, GLOX@CAT-HNFs maintained 68.79 % of its initial activity, indicating excellent reusability. Benefiting from the excellent stability and reusability of GLOX@CAT-HNFs, a nearly 100 % (99.64 %) conversion of L-glutamate to α-KG was achieved, over 1.79 times higher than that of the free GLOX system (55.53 %). This work provides a feasibility for constructing a high-performance cascade catalyst of multiple enzymes.

Structural and functional analysis of l-methionine oxidase identified through sequence data mining

l-Amino acid oxidase (LAAO), an FAD-dependent enzyme, catalyzes the oxidation of l-amino acids (l-AAs) to their corresponding imino acids. While LAAOs, which can oxidize charged or aromatic l-AAs specifically, have been extensively characterized across various species, LAAOs that have high specificity toward alkyl-chain l-AAs, such as l-Met, are hardly characterized for now. In this study, we screened a highly specific l-Met oxidizing LAAOs from Burkholderiales bacterium (BbMetOx) and Undibacterium sp. KW1 (UndMetOx) using sequence similarity network (SSN) analysis. These enzymes displayed an order of magnitude higher specific activity towards l-Met compared to other l-AAs. Enzyme activity assays showed that these LAAOs operate optimally at moderate condition because the optimal pH and Tm values were pH 7.0 and 58-60°C. We determined the crystal structures of wild-type BbMetOx (BbMetOx(WT)) and an inactivated mutant, BbMetOx (K304A), at 2.7 Å and 2.2 Å resolution, respectively. The overall structure of BbMetOx is closely similar to other known LAAOs of which structures were determined. Comparative analysis of the BbMetOx structures revealed significant conformational changes in the catalytic domain, particularly a movement of approximately 8 Å in the Cα atom of residue Y180. Further analysis highlighted four residues, i.e., Y180, M182, F300, and M302, as critical for l-Met recognition, with alanine substitution at these positions resulting in loss of activity. This study not only underscores the utility of SSN for discovering novel LAAOs but also advances our understanding of substrate specificity in this enzyme family.

Enhancement of tryptophan 2-monooxygenase thermostability by semi-rational enzyme engineering: a strategic design to minimize experimental investigation

Tryptophan 2-monooxygenase (TMO) is an FAD-bound flavoenzyme which catalyzes the oxidative decarboxylation of l-tryptophan to produce indole-3-acetamide (IAM) and carbon dioxide. The reaction of TMO is the first step of indole-3-acetic acid (IAA) biosynthesis. Although TMO is of interest for mechanistic studies and synthetic biology applications, the enzyme has low thermostability and soluble expression yield. Herein, we employed a combined approach of rational design using computational tools with site-saturation mutagenesis to screen for TMO variants with significantly improved thermostability properties and soluble protein expression. The engineered TMO variants, TMO-PWS and TMO-PWSNR, possess melting temperatures (T m) of 65 °C, 17 °C higher than that of the wild-type enzyme (TMO-WT). At 50 °C, the stabilities (t 1/2) of TMO-PWS and TMO-PWSNR were 85-fold and 92.4-fold higher, while their soluble expression yields were 1.4-fold and 2.1-fold greater than TMO-WT, respectively. Remarkably, the kinetic parameters of these variants were similar to those of the wild-type enzymes, illustrating that they are promising candidates for future studies. Molecular dynamic simulations of the wild-type and thermostable TMO variants identified key interactions for enhancing these improvements in the biophysical properties of the TMO variants. The introduced mutations contributed to hydrogen bond formation and an increase in the regional hydrophobicity, thereby, strengthening the TMO structure.

Crystal structure and enzyme engineering of the broad substrate spectrum l-amino acid oxidase 4 from the fungus Hebeloma cylindrosporum

l-Amino acid oxidases (LAAOs) catalyze the oxidative deamination of l-amino acids to α-keto acids. Recombinant production of LAAOs with broad substrate spectrum remains a formidable challenge. We previously achieved this for the highly active and thermostable LAAO4 of Hebeloma cylindrosporum (HcLAAO4). Here, we crystallized a proteolytically truncated surface entropy reduction variant of HcLAAO4 and solved its structure in substrate-free form and in complex with diverse substrates. The ability to support the aliphatic portion of a substrate's side chain by an overall hydrophobic active site is responsible for the broad substrate spectrum of HcLAAO4, including l-amino acids with big aromatic, acidic and basic side chains. Based on the structural findings, we generated an E288H variant with increased activity toward pharmaceutical building blocks of high interest.

Improvement of Heterologous Soluble Expression of L-amino Acid Oxidase Using Logistic Regression

Successful implementation of enzymes in practical application hinges on the development of efficient mass production techniques. However, in a heterologous expression system, the protein is often unable to fold correctly and, thus, forms inclusion bodies, resulting in the loss of its original activity. In this study, we present a new and more accurate model for predicting amino acids associated with an increased L-amino acid oxidase (LAO) solubility. Expressing LAO from Rhizoctonia solani in Escherichia coli and combining random mutagenesis and statistical logistic regression, we modified 108 amino acid residues by substituting hydrophobic amino acids with serine and hydrophilic amino acids with alanine. Our results indicated that specific mutations in Euclidean distance, glycine, methionine, and secondary structure increased LAO expression. Furthermore, repeated mutations were performed for LAO based on logistic regression models. The mutated LAO displayed a significantly increased solubility, with the 6-point and 58-point mutants showing a 2.64- and 4.22-fold increase, respectively, compared with WT-LAO. Ultimately, using recombinant LAO in the biotransformation of α-keto acids indicates its great potential as a biocatalyst in industrial production.

Synthesis of Extracellular L-lysine-α-oxidase along with Degrading Enzymes by Trichoderma cf. aureoviride Rifai VKM F-4268D: Role in Biocontrol and Systemic Plant Resistance

When cultivating on wheat bran or deactivated fungal mycelium as a model of "natural growth", the ability of Trichoderma to synthesize extracellular L-lysine-α-oxidase (LysO) simultaneously with cell-wall-degrading enzymes (proteases, xylanase, glucanases, chitinases, etc.), responsible for mycoparasitism, was shown. LysO, in turn, causes the formation of H2O2 and pipecolic acid. These compounds are known to be signaling molecules and play an important role in the induction and development of systemic acquired resistance in plants. Antagonistic effects of LysO have been demonstrated against phytopathogenic fungi and Gram-positive or Gram-negative bacteria with dose-dependent cell death. The antimicrobial effect of LysO decreased in the presence of catalase. The generating intracellular ROS in the presence of LysO was also shown in both bacteria and fungi, which led to a decrease in viable cells. These results suggest that the antimicrobial activity of LysO is due to two factors: the formation of exogenous hydrogen peroxide as a product of the enzymatic oxidative deamination of L-lysine and the direct interaction of LysO with the cell wall of the micro-organisms. Thus, LysO on its own enhances the potential of the producer in the environment; namely, the enzyme complements the strategy of the fungus in biocontrol and indirectly participates in inducing SAR and regulating the relationship between pathogens and plants.

Development of an Enzyme Cascade System for the Synthesis of Enantiomerically Pure D-Amino Acids Utilizing Ancestral L-Amino Acid Oxidase

Enantiomerically pure D-amino acids hold significant potential as precursors for synthesizing various fine chemicals, including peptide-based drugs and other pharmaceuticals. This study focuses on establishing an enzymatic cascade system capable of converting various L-amino acids into their D-isomers. The system integrates four enzymes: ancestral L-amino acid oxidase (AncLAAO-N4), D-amino acid dehydrogenase (DAADH), D-glucose dehydrogenase (GDH), and catalase. AncLAAO-N4 initiates the process by converting L-amino acids to corresponding keto acids, which are then stereo-selectively aminated to D-amino acids by DAADH using NADPH and NH4Cl. Concurrently, any generated H2O2 is decomposed into O2 and H2O by catalase, while GDH regenerates NADPH from D-glucose. Optimization of reaction conditions and substrate concentrations enabled the successful synthesis of five D-amino acids, including a D-Phe derivative, three D-Trp derivatives, and D-phenylglycine, all with high enantiopurity (>99 % ee) at a preparative scale (>100 mg). This system demonstrates a versatile approach for producing a diverse array of D-amino acids.

The recombinant L-lysine α-oxidase from the fungus Trichoderma harzianum promotes apoptosis and necrosis of leukemia CD34 + hematopoietic cells

BACKGROUND

In hematologic cancers, including leukemia, cells depend on amino acids for rapid growth. Anti-metabolites that prevent their synthesis or promote their degradation are considered potential cancer treatment agents. Amino acid deprivation triggers proliferation inhibition, autophagy, and programmed cell death. L-lysine, an essential amino acid, is required for tumor growth and has been investigated for its potential as a target for cancer treatment. L-lysine α-oxidase, a flavoenzyme that degrades L-lysine, has been studied for its ability to induce apoptosis and prevent cancer cell proliferation. In this study, we describe the use of L-lysine α-oxidase (LO) from the filamentous fungus Trichoderma harzianum for cancer treatment.

RESULTS

The study identified and characterized a novel LO from T. harzianum and demonstrated that the recombinant protein (rLO) has potent and selective cytotoxic effects on leukemic cells by triggering the apoptotic cascade through mitochondrial dysfunction.

CONCLUSIONS

The results support future translational studies using the recombinant LO as a potential drug for the treatment of leukemia.

A chemo-biocatalyst based on glutamate oxidase-integrated biomimetic trimanganese tetraoxide as cascade composite nano-catalyst for synthesis of α‑Ketoglutaric acid

The combination of chemo- and biocatalysts to perform one-pot synthetic route has presented great challenges for decades. Herein, glutamate oxidase (GLOX) and trimanganese tetraoxide (Mn3O4) nanocrystals were combined for the first time by one-step biomineralization to construct a mimic multi-enzyme system (GLOX@Mn3O4) for chemoenzymatic synthesis of α‑ketoglutaric acid (α‑KG). Mn3O4 not only served as a support for the enzyme immobilization, but also contributed its catalytic activity to co-operate with natural enzymes for the cascade reactions. The as-synthesized chemo-enzyme catalysts with directly contacted catalytic sites of the enzyme and inorganic catalyst maximizes the substrate channeling effffects for in situ rapid decomposition of the oxidative intermediate, H2O2, during the enzymatic oxidation of sodium glutamate, thus relieving the inhibition of H2O2 accumulation for GLOX. Benefiting from the excellent stability and reusability of GLOX@Mn3O4, a nearly 100% conversion (99.7%) of l-glutamate to α-KG was achieved, over 4.7 times higher than that of the free GLOX system (21.2%). This work provides a feasibility for constructing a high-performance chemo-enzyme catalyst for cascade catalysis, especially for those reactions with toxic intermediates.

Co-Immobilization of a Multi-Enzyme Cascade: (S)-Selective Amine Transaminases, l-Amino Acid Oxidase and Catalase

An enzyme cascade was established previously consisting of a recycling system with an l-amino acid oxidase (hcLAAO4) and a catalase (hCAT) for different α-keto acid co-substrates of (S)-selective amine transaminases (ATAs) in kinetic resolutions of racemic amines. Only 1 mol % of the co-substrate was required and l-amino acids instead of α-keto acids could be applied. However, soluble enzymes cannot be reused easily. Immobilization of hcLAAO4, hCAT and the (S)-selective ATA from Vibrio fluvialis (ATA-Vfl) was addressed here. Immobilization of the enzymes together rather than on separate beads showed higher reaction rates most likely due to fast co-substrate channeling between ATA-Vfl and hcLAAO4 due to their close proximity. Co-immobilization allowed further reduction of the co-substrate amount to 0.1 mol % most likely due to a more efficient H2 O2 -removal caused by the stabilized hCAT and its proximity to hcLAAO4. Finally, the co-immobilized enzyme cascade was reused in 3 cycles of preparative kinetic resolutions to produce (R)-1-PEA with high enantiomeric purity (97.3 %ee). Further recycling was inefficient due to the instability of ATA-Vfl, while hcLAAO4 and hCAT revealed high stability. An engineered ATA-Vfl-8M was used in the co-immobilized enzyme cascade to produce (R)-1-(3-ethoxy-4-methoxyphenyl)-2-(methylsulfonyl)ethanamine, an apremilast-intermediate, with a 1,000 fold lower input of the co-substrate.

Ammonia Production Using Bacteria and Yeast toward a Sustainable Society

Ammonia is an important chemical that is widely used in fertilizer applications as well as in the steel, chemical, textile, and pharmaceutical industries, which has attracted attention as a potential fuel. Thus, approaches to achieve sustainable ammonia production have attracted considerable attention. In particular, biological approaches are important for achieving a sustainable society because they can produce ammonia under mild conditions with minimal environmental impact compared with chemical methods. For example, nitrogen fixation by nitrogenase in heterogeneous hosts and ammonia production from food waste using microorganisms have been developed. In addition, crop production using nitrogen-fixing bacteria has been considered as a potential approach to achieving a sustainable ammonia economy. This review describes previous research on biological ammonia production and provides insights into achieving a sustainable society.

Reaction Mechanism of Ancestral l-Lys α-Oxidase from Caulobacter Species Studied by Biochemical, Structural, and Computational Analysis

The flavin-dependent amine oxidase superfamily contains various l-amino acid oxidases (LAAOs) bearing different substrate specificities and enzymatic properties. LAAOs catalyze the oxidation of the α-amino group of l-amino acids (L-AAs) to produce imino acids and H2O2. In this study, an ancestral l-Lys α-oxidase (AncLLysO2) was designed utilizing genome-mined sequences from the Caulobacter species. The AncLLysO2 exhibited high specificity toward l-Lys; the k cat/K m values toward l-Lys were one and two orders larger than those of l-Arg and l-ornithine, respectively. Liquid chromatography-high resolution mass spectrometry analysis indicated that AncLLysO2 released imino acid immediately from the active site after completion of oxidation of the α-amino group. Crystal structures of the ligand-free, l-Lys- and l-Arg-bound forms of AncLLysO2 were determined at 1.4-1.6 Å resolution, indicating that the active site of AncLLysO2 kept an open state during the reaction and more likely to release products. The structures also indicated the substrate recognition mechanism of AncLLysO2; ε-amino, α-amino, and carboxyl groups of l-Lys formed interactions with Q357, A551, and R77, respectively. Biochemical and molecular dynamics simulation analysis of AncLLysO2 indicated that active site residues that indirectly interact with the substrate are also important to exhibit high activity; for example, the aromatic group of Y219 is important to ensure that the l-Lys substrate is placed in the correct position to allow the reaction to proceed efficiently. Taken together, we propose the reaction mechanism of AncLLysO2.

Cytotoxic activity of l-lysine alpha-oxidase against leukemia cells

Cancer cells exhibit higher proliferation rates than normal cells, and as a consequence, a higher nutritional demand for metabolites such as amino acids. Such cells demonstrate high expression of amino acid transporters and are significantly dependent on the external uptake of amino acids. Moreover, some types of cancer cells exhibit oncogenic mutations that render them auxotrophic to certain amino acids. This metabolic difference between tumor and normal cells has been explored for developing anticancer drugs. Enzymes capable of depleting certain amino acids in the bloodstream can be employed to inhibit the proliferation of cancer cells and promote cell death. Certain microbial enzymes, such as l-asparaginase and l-amino acid oxidases, have been studied for this purpose. In this paper, we discuss the role of l-asparaginase, the only enzyme currently used as a chemotherapeutic agent. We also review the studies on a new potential antineoplastic agent, l-lysine α-oxidase, an enzyme of l-amino acid oxidase family.

Biosynthesis of phenylpyruvic acid from l-phenylalanine using chromosomally engineered Escherichia coli

The efficiency of whole-cell biotransformation is often affected by the genetic instability of plasmid-based expression systems, which require selective pressure to maintain the stability of the plasmids. To circumvent this shortcoming, we constructed a chromosome engineering strain for the synthesis of phenylpyruvic acid (PPA) from l-phenylalanine. First, l-amino acid deaminase (pmLAAD) from Proteus myxofaciens was incorporated into Escherichia coli BL21 (DE3) chromosome and the copy numbers of pmLAAD were increased by chemically induced chromosomal evolution (CIChE). Fifty-nine copies of pmLAAD were obtained in E. coli BL8. The PPA titer of E. coli BL8 reached 2.22 g/L at 6 h. Furthermore, the deletion of lacI improved PPA production. In the absence of isopropyl-β-d-thiogalactopyranoside, the resulting strain, E. coli BL8△recA△lacI, produced 2.65 g/L PPA at 6 h and yielded a 19.37% increase in PPA production compared to E. coli BL8△recA. Finally, the engineered E. coli BL8△recA△lacI strain achieved 19.14 g/L PPA at 24 h in a 5-L bioreactor.

Molecular dynamics study on the hydrogen bond formation between α-hydrogen atom of L-Phe and N5 atom of FAD in the enzyme-substrate complex of the L-Phe oxidase reaction

It is difficult to observe the structure of the enzyme-substrate complex (ES complex) experimentally, since the complex changes to the enzyme and its product during observation. The molecular dynamics (MD) approach is ideal to observe the structural change of enzyme and of substrate in the ES complex. Analyses on the complex of L-Phe oxidase with L-Phe by MD showed 1) the distance between the α-hydrogen atom of L-Phe and the N5 atom of isoalloxazine ring of FAD to be 2.64 ± 0.19 Å, and 2) the angle CA-HA-N5 atoms to be 141.5 ± 10.7°. This result clearly showed that the α-hydrogen atom forms the hydrogen bond with the N5 atom of isoalloxazine ring of FAD in the enzyme-substrate complex. Thus, the complex is ready for the hydrogen transfer from substrate to FAD in the key step of the oxidation of substrate by the enzyme.

Structural Determinants of the Specific Activities of an L-Amino Acid Oxidase from Pseudoalteromonas luteoviolacea CPMOR-1 with Broad Substrate Specificity

The Pseudoalteromonas luteoviolacea strain CPMOR-1 expresses a flavin adenine dinucleotide (FAD)-dependent L-amino acid oxidase (LAAO) with broad substrate specificity. Steady-state kinetic analysis of its reactivity towards the 20 proteinogenic amino acids showed some activity to all except proline. The relative specific activity for amino acid substrates was not correlated only with K_m or k_cat values, since the two parameters often varied independently of each other. Variation in K_m was attributed to the differential binding affinity. Variation in k_cat was attributed to differential positioning of the bound substrate relative to FAD that decreased the reaction rate. A structural model of this LAAO was compared with structures of other FAD-dependent LAAOs that have different substrate specificities: an LAAO from snake venom that prefers aromatic amino acid substrates and a fungal LAAO that is specific for lysine. While the amino acid sequences of these LAAOs are not very similar, their overall structures are comparable. The differential activity towards specific amino acids was correlated with specific residues in the active sites of these LAAOs. Residues in the active site that interact with the amino and carboxyl groups attached to the α-carbon of the substrate amino acid are conserved in all of the LAAOs. Residues that interact with the side chains of the amino acid substrates show variation. This provides insight into the structural determinants of the LAAOs that dictate their different substrate preferences. These results are of interest for harnessing these enzymes for possible applications in biotechnology, such as deracemization.

Recombinant L-Amino Acid Oxidase with broad substrate spectrum for Co-Substrate Recycling in (S)-Selective Transaminase-Catalyzed Kinetic Resolutions

Chiral and enantiopure amines can be produced by enantioselective transaminases via kinetic resolution of amine racemates. This transamination reaction requires stoichiometric amounts of co-substrate. A dual-enzyme recycling system overcomes this limitation: L-amino acid oxidases (LAAO) recycle the accumulating co-product of ( S )-selective transaminases in the kinetic resolution of racemic amines to produce pure ( R )-amines. However, availability of suitable LAAOs is limited. Here we use the heterologously produced, highly active fungal hcLAAO4 with broad substrate spectrum. H 2 O 2 as by-product of hcLAAO4 is detoxified by a catalase. The final system allows using sub-stoichiometric amounts of 1 mol% of the transaminase co-substrate as well as the initial application of L-amino acids instead of α-keto acids. With an optimized protocol, synthetic potential of this kinetic resolution cascade was proven at the preparative scale (>90 mg) by the synthesis of highly enantiomerically pure ( R )-methylbenzylamine (>99 %ee) at complete conversion (50 %).

Reactive oxygen species as a double-edged sword: The role of oxidative enzymes in antitumor therapy

A number of approaches have been developed over the years to manage cancer, such as chemotherapy using low-molecular-mass molecules and radiotherapy. Here, enzymes can also find useful applications. Among them, oxidases have attracted attention because of their ability to produce reactive oxygen species (ROS, especially hydrogen peroxide) in tumors and potentially modulate the production of this cytotoxic compound when enzymes active on substrates present in low amounts are used, such as the d-amino acid oxidase and d-amino acid couple system. These treatments have been also developed for additional cancer treatment approaches, such as phototherapy, nutrient starvation, and metal-induced hydroxyl radical production. In addition, to improve tumor specificity and decrease undesired side effects, oxidases have been targeted by means of nanotechnologies and protein engineering (i.e., by designing chimeric proteins able to accumulate in the tumor). The most recent advances obtained by using six different oxidases (i.e., the FAD-containing enzymes glucose oxidase, d- and l-amino acid oxidases, cholesterol oxidase and xanthine oxidase, and the copper-containing amine oxidase) have been reported. Anticancer therapy based on oxidase-based ROS production has now reached maturity and can be applied in the clinic.

Seawater activates l-amino acid oxidase from the serum of the red-spotted grouper Epinephelusakaara

l-amino acid oxidases (LAOs) catalyze the oxidative deamination of l-amino acid and generate α-keto acid, ammonia, and hydrogen peroxide as byproducts. LAOs showed the variety of bioactivity by the resulting hydrogen peroxide. The serum of the red-spotted grouper Epinephelus akaara contains an LAO (Ea-LAO) with the potential to kill bacterial pathogens Aeromonas salmonicida and Vibrio anguillarum via hydrogen peroxide. However, it is unknown how the grouper tolerates the harmful effects of the serum Ea-LAO byproducts. In this study, we analyzed the kinetics of fish LAOs to understand how they escape the toxicity of byproducts. The LAO activity of grouper serum was suppressed in low-salt solutions such as NaCl, CaCl₂, MgCl₂, and diluted seawater. The activity was non-linearly increased and fitted to the four-parameter log-logistic model. The EC₅₀ of the seawater was calculated to have a 0.72-fold concentration. This result suggested that the Ea-LAO could be activated by mixing with seawater. The results of circular dichroism spectroscopy showed that the α helix content was estimated to be 12.1% and 5.3% in a salt-free buffer (inactive condition) and the original concentration of seawater (active condition), respectively, indicating that the secondary structure of the Ea-LAO in the active condition was randomized. In addition, the Ea-LAO showed reversible LAO activity regulation according to the salt concentration in the environment. Taken together, this indicates that the Ea-LAO is normally on standby as an inactive form, and it could activate as a host-defense molecule to avoid pathogen invasion via a wound when mixed with seawater.

Tailor-Made Amino Acids in Pharmaceutical Industry: Synthetic Approaches to Aza-Tryptophan Derivatives

Over the recent years there has been a noticeable upsurge of interest in aza-analogs of tryptophan which are isosteric to the latter and found numerous applications in medicinal, bioorganic chemistry, and peptide research. In the present review article, five aza-tryptophan derivatives are profiled, including aza-substitution in the positions 2, on the five-membered ring, as well as in positions 4, 5, 6, and 7 on the six-membered ring. A detailed and comprehensive literature overview of the synthetic methods for the preparation of these aza-tryptophans is presented and general facets of the biological properties and most promising applications are discussed.

D-Amino Acids and D-Amino Acid-Containing Peptides: Potential Disease Biomarkers and Therapeutic Targets?

In nature, amino acids are found in two forms, L and D enantiomers, except for glycine which does not have a chiral center. The change of one form to the other will lead to a change in the primary structure of proteins and hence may affect the function and biological activity of proteins. Indeed, several D-amino acid-containing peptides (DAACPs) were isolated from patients with cataracts, Alzheimer's and other diseases. Additionally, significant levels of free D-amino acids were found in several diseases, reflecting the disease conditions. Studying the molecular mechanisms of the DAACPs formation and the alteration in D-amino acids metabolism will certainly assist in understanding these diseases and finding new biomarkers and drug targets. In this review, the presence of DAACPs and free D-amino acids and their links with disease development and progress are summarized. Similarly, we highlight some recent advances in analytical techniques that led to improvement in the discovery and analysis of DAACPs and D-amino acids.

Kinetic and Structural Properties of a Robust Bacterial L-Amino Acid Oxidase

L-Amino acid oxidase (LAAO) is a flavin adenine dinucleotide (FAD)-dependent enzyme active on most proteinogenic L-amino acids, catalysing their conversion to α-keto acids by oxidative deamination of the substrate. For this oxidation reaction, molecular oxygen is used as the electron acceptor, generating hydrogen peroxide. LAAO can be used to detect L-amino acids, for the production of hydrogen peroxide as an oxidative agent or antimicrobial agent, and for the production of enantiopure amino acids from racemates. In this work, we characterised a previously reported LAAO from the bacterium Pseudoalteromonas luteoviolacea. The substrate scope and kinetic properties of the enzyme were determined, and the thermostability was evaluated. Additionally, we elucidated the crystal structure of this bacterial LAAO, enabling us to test the role of active site residues concerning their function in catalysis. The obtained insights and ease of expression of this thermostable LAAO provides a solid basis for the development of engineered LAAO variants tuned for biosensing and/or biocatalysis.

Cobalt Phosphate Nanocrystals: A Catalase-Like Nanozyme and In Situ Enzyme-Encapsulating Carrier for Efficient Chemoenzymatic Synthesis of α-Keto Acid

Chemoenzymatic catalysis combining the traits of chemical and enzymatic catalysis provides tremendous possibilities for the design of biosynthetic pathways utilizing inorganic catalysts and enzymes. However, the efficiency of chemoenzymatic catalysis is usually governed by the synergy and compatibility of the two catalysts. Here, we report for the first time the catalase-like activity of cobalt phosphate nanocrystals (CoPs). By a one-pot biomimetic mineralization with CoPs and l-amino acid oxidase (LAAO) under a mild condition, we have fabricated a hybrid nanobiocatalyst, LAAO@CoPs, for the chemoenzymatic synthesis of α-keto acid. The as-fabricated nanobiocatalyst with directly contacted catalytic sites of the enzyme and nanozyme maximizes the substrate channeling effects for in situ chemical decomposition of the oxidative intermediate, H₂O₂, during the enzymatic oxidation of l-tryptophan (l-Trp), thus minimizing the H₂O₂ accumulation and byproduct generation. Benefiting from the superiority of LAAO@CoPs, complete conversion (100.0%) of l-Trp to indole pyruvic acid is achieved, over two times higher than the yield of the free LAAO system (47.6%). Meanwhile, LAAO@CoPs show high stabilities against heat and proteolytic treatments. This work offers a new design approach for constructing a high-performance nanobiocatalyst for cascade reactions, especially for those systems with toxic or reactive intermediates.

Catalytic mechanism of ancestral L-lysine oxidase assigned by sequence data mining

A large number of protein sequences are registered in public databases such as PubMed. Functionally uncharacterized enzymes are included in these databases, some of which likely have potential for industrial applications. However, assignment of the enzymes remained difficult tasks for now. In this study, we assigned a total of 28 original sequences to uncharacterized enzymes in the FAD-dependent oxidase family expressed in some species of bacteria including Chryseobacterium, Flavobacterium, and Pedobactor. Progenitor sequence of the assigned 28 sequences was generated by ancestral sequence reconstruction, and the generated sequence exhibited L-lysine oxidase activity; thus, we named the enzyme AncLLysO. Crystal structures of ligand-free and ligand-bound forms of AncLLysO were determined, indicating that the enzyme recognizes L-Lys by hydrogen bond formation with R76 and E383. The binding of L-Lys to AncLLysO induced dynamic structural change at a plug loop formed by residues 251 to 254. Biochemical assays of AncLLysO variants revealed the functional importance of these substrate recognition residues and the plug loop. R76A and E383D variants were also observed to lose their activity, and the kcat/Km value of G251P and Y253A mutations were approximately 800- to 1800-fold lower than that of AncLLysO, despite the indirect interaction of the substrates with the mutated residues. Taken together, our data demonstrate that combinational approaches to sequence classification from database and ancestral sequence reconstruction may be effective not only to find new enzymes using databases of unknown sequences but also to elucidate their functions.

Sustainable Biological Ammonia Production towards a Carbon-Free Society

A sustainable society was proposed more than 50 years ago. However, it is yet to be realised. For example, the production of ammonia, an important chemical widely used in the agriculture, steel, chemical, textile, and pharmaceutical industries, still depends on fossil fuels. Recently, biological approaches to achieve sustainable ammonia production have been gaining attention. Moreover, unlike chemical methods, biological approaches have a lesser environmental impact because ammonia can be produced under mild conditions of normal temperature and pressure. Therefore, in previous studies, nitrogen fixation by nitrogenase, including enzymatic ammonia production using food waste, has been attempted. Additionally, the production of crops using nitrogen-fixing bacteria has been implemented in the industry as one of the most promising approaches to achieving a sustainable ammonia economy. Thus, in this review, we described previous studies on biological ammonia production and showed the prospects for realising a sustainable society.

Analysis of N-glycosylation in fungal l-amino acid oxidases expressed in the methylotrophic yeast Pichia pastoris

l-amino acid oxidases (LAAOs) catalyze the oxidative deamination of l-amino acids to corresponding α-keto acids. Here, we describe the heterologous expression of four fungal LAAOs in Pichia pastoris. cgLAAO1 from Colletotrichum gloeosporioides and ncLAAO1 from Neurospora crassa were able to convert substrates not recognized by recombinant 9His-hcLAAO4 from the fungus Hebeloma cylindrosporum described earlier thereby broadening the substrate spectrum for potential applications. 9His-frLAAO1 from Fibroporia radiculosa and 9His-laLAAO2 from Laccaria amethystine were obtained only in low amounts. All four enzymes were N-glycosylated. We generated mutants of 9His-hcLAAO4 lacking N-glycosylation sites to further understand the effects of N-glycosylation. All four predicted N-glycosylation sites were glycosylated in 9His-hcLAAO4 expressed in P. pastoris. Enzymatic activity was similar for fully glycosylated 9His-hcLAAO4 and variants without one or all N-glycosylation sites after acid activation of all samples. However, activity without acid treatment was low in a variant without N-glycans. This was caused by the absence of a hypermannosylated N-glycan on asparagine residue N54. The lack of one or all of the other N-glycans was without effect. Our results demonstrate that adoption of a more active conformation requires a specific N-glycosylation during biosynthesis.

Amperometric Biosensors for L-Arginine Determination Based on L-Arginine Oxidase and Peroxidase-Like Nanozymes

There are limited data on amperometric biosensors (ABSs) for L-arginine (Arg) determination based on oxidases that produce hydrogen peroxide (H2O2) as a byproduct of enzymatic reaction, and artificial peroxidases (POs) for decomposition of H2O2. The most frequently proposed Arg-sensitive oxidase-based ABSs contain at least two enzymes in the bioselective layer; this complicates the procedure and increases the cost of analysis. Therefore, the construction of a one-enzyme ABS for Arg analysis is a practical problem. In the current work, fabrication, and characterization of three ABS types for the direct measurement of Arg were proposed. L-arginine oxidase (ArgO) isolated from the mushroom Amanita phalloides was co-immobilized with PO-like nanozymes (NZs) on the surface of graphite electrodes. As PO mimetics, chemically synthesized NZs of CeCu (nCeCU) and NiPtPd (nNiPtPd), as well as green-synthesized hexacyanoferrate of copper (gCuHCF), were used. The novel ABSs exhibited high sensitivity and selectivity to Arg, broad linear ranges and good storage stabilities. Two ABSs were tested on real samples of products containing Arg, including the pharmaceutical preparation “Tivortine”, juices, and wine. A high correlation (R = 0.995) was demonstrated between the results of testing “Tivortine” and juice using nCeCU/GE and nNiPtPd/GE. It is worth mentioning that only a slight difference (less than 1%) was observed for “Tivortin” between the experimentally determined content of Arg and its value declared by the producer. The proposed ArgO-NZ-based ABSs may be promising for Arg analysis in different branches of science, medicine, and industry.

Identification and characterization of l-amino acid oxidase 2 gene in orange-spotted grouper (Epinephelus coioides)

Recently, l-amino acid oxidases (LAAOs) have been identified in several fish species as first-line defense molecules against bacterial infection. Here, we report the cloning and characterization of a fish LAAO gene, EcLAAO2, from orange-spotted grouper (Epinephelus coioides). The full-length cDNA is 3030 bp, with an ORF encoding a protein of 511 amino acids. EcLAAO2 is mainly expressed in the fin, gill, and intestine. Its expression is upregulated in several immune organs after challenge with lipopolysaccharide (LPS) and poly (I:C). The recombinant EcLAAO2 protein (rEcLAAO2), expressed and purified from a baculovirus expression system, was determined to be a glycosylated dimer. According to a hydrogen peroxide-production assay, the recombinant protein was identified as having LAAO enzyme activity with substrate preference for L-Phe and L-Trp, but not L-Lys as other known fish LAAOs. rEcLAAO2 could effectively inhibit the growth of Vibrio parahaemolyticus, Staphylococcus aureus, and Bacillus subtilis while exhibiting less effective inhibition of the growth of Escherichia coli. Finally, protein models based on sequence homology were constructed to predict the three-dimensional structure of EcLAAO2 as well as to explain the difference in substrate specificity between EcLAAO2 and other reported fish LAAOs. In conclusion, this study identifies EcLAAO2 as a novel fish LAAO with a substrate preference distinct from other known fish LAAOs and reveals that it may function against invading pathogens.

Human D-aspartate Oxidase: A Key Player in D-aspartate Metabolism

In recent years, the D-enantiomers of amino acids have been recognized as natural molecules present in all kingdoms, playing a variety of biological roles. In humans, d-serine and d-aspartate attracted attention for their presence in the central nervous system. Here, we focus on d-aspartate, which is involved in glutamatergic neurotransmission and the synthesis of various hormones. The biosynthesis of d-aspartate is still obscure, while its degradation is due to the peroxisomal flavin adenine dinucleotide (FAD)-containing enzyme d-aspartate oxidase. d-Aspartate emergence is strictly controlled: levels decrease in brain within the first days of life while increasing in endocrine glands postnatally and through adulthood. The human d-aspartate oxidase (hDASPO) belongs to the d-amino acid oxidase-like family: its tertiary structure closely resembles that of human d-amino acid oxidase (hDAAO), the enzyme that degrades neutral and basic d-amino acids. The structure-function relationships of the physiological isoform of hDASPO (named hDASPO_341) and the regulation of gene expression and distribution and properties of the longer isoform hDASPO_369 have all been recently elucidated. Beyond the substrate preference, hDASPO and hDAAO also differ in kinetic efficiency, FAD-binding affinity, pH profile, and oligomeric state. Such differences suggest that evolution diverged to create two different ways to modulate d-aspartate and d-serine levels in the human brain. Current knowledge about hDASPO is shedding light on the molecular mechanisms underlying the modulation of d-aspartate levels in human tissues and is pushing novel, targeted therapeutic strategies. Now, it has been proposed that dysfunction in NMDA receptor-mediated neurotransmission is caused by disrupted d-aspartate metabolism in the nervous system during the onset of various disorders (such as schizophrenia): the design of suitable hDASPO inhibitors aimed at increasing d-aspartate levels thus represents a novel and useful form of therapy.

Chemoenzymatic Production of Enantiocomplementary 2-Substituted 3-Hydroxycarboxylic Acids from L-α-Amino Acids

A two-enzyme cascade reaction plus in situ oxidative decarboxylation for the transformation of readily available canonical and non-canonical L-α-amino acids into 2-substituted 3-hydroxy-carboxylic acid derivatives is described. The biocatalytic cascade consisted of an oxidative deamination of L-α-amino acids by an L-α-amino acid deaminase from Cosenzaea myxofaciens, rendering 2-oxoacid intermediates, with an ensuing aldol addition reaction to formaldehyde, catalyzed by metal-dependent (R)- or (S)-selective carboligases namely 2-oxo-3-deoxy-l-rhamnonate aldolase (YfaU) and ketopantoate hydroxymethyltransferase (KPHMT), respectively, furnishing 3-substituted 4-hydroxy-2-oxoacids. The overall substrate conversion was optimized by balancing biocatalyst loading and amino acid and formaldehyde concentrations, yielding 36-98% aldol adduct formation and 91- 98% ee for each enantiomer. Subsequent in situ follow-up chemistry via hydrogen peroxide-driven oxidative decarboxylation afforded the corresponding 2-substituted 3-hydroxycarboxylic acid derivatives.

A new l-arginine oxidase engineered from l-glutamate oxidase

The alternation of substrate specificity expands the application range of enzymes in industrial, medical, and pharmaceutical fields. l-Glutamate oxidase (LGOX) from Streptomyces sp. X-119-6 catalyzes the oxidative deamination of l-glutamate to produce 2-ketoglutarate with ammonia and hydrogen peroxide. LGOX shows strict substrate specificity for l-glutamate. Previous studies on LGOX revealed that Arg305 in its active site recognizes the side chain of l-glutamate, and replacement of Arg305 by other amino acids drastically changes the substrate specificity of LGOX. Here we demonstrate that the R305E mutant variant of LGOX exhibits strict specificity for l-arginine. The oxidative deamination activity of LGOX to l-arginine is higher than that of l-arginine oxidase form from Pseudomonas sp. TPU 7192. X-ray crystal structure analysis revealed that the guanidino group of l-arginine is recognized not only by Glu305 but also Asp433, Trp564, and Glu617, which interact with Arg305 in wild-type LGOX. Multiple interactions by these residues provide strict specificity and high activity of LGOX R305E toward l-arginine. LGOX R305E is a thermostable and pH stable enzyme. The amount of hydrogen peroxide, which is a byproduct of oxidative deamination of l-arginine by LGOX R305E, is proportional to the concentration of l-arginine in a range from 0 to 100 μM. The linear relationship is maintained around 1 μM of l-arginine. Thus, LGOX R305E is suitable for the determination of l-arginine.

Improved ammonia production from soybean residues by cell surface-displayed l-amino acid oxidase on yeast

Ammonia is critical for agricultural and chemical industries. The extracellular production of ammonia by yeast (Saccharomyces cerevisiae) using cell surface engineering can be efficient approach because yeast can avoid growth deficiencies caused by knockout of genes for ammonia assimilation. In this study, we produced ammonia outside the yeast cells by displaying an l-amino acid oxidase with a wide substrate specificity derived from Hebeloma cylindrosporum (HcLAAO) on yeast cell surfaces. The HcLAAO-displaying yeast successfully produced 12.6 m m ammonia from a mixture of 20 proteinogenic amino acids (the theoretical conversion efficiency was 63%). We also succeeded in producing ammonia from a food processing waste, soybean residues (okara) derived from tofu production. The conversion efficiency was 88.1%, a higher yield than reported in previous studies. Our study demonstrates that ammonia production outside of yeast cells is a promising strategy to utilize food processing wastes.

Structural basis of enzyme activity regulation by the propeptide of l-lysine α-oxidase precursor from Trichoderma viride

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The suppression mechanism of activity by propeptide remains unclear for most LAAOs.

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The crystal structures of the LysOX precursor (prLysOX) have been determined.

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The propeptide indirectly changes the active site structure to suppress the activity.

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prLysOX can adopt another conformation similar to mature LysOX.

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prLysOX is able to be activated without proteolytic processing.

Harmuful proteins are usually synthesized as inactive precursors and are activated by proteolytic processing. l-Amino acid oxidase (LAAO) is a flavoenzyme that catalyzes the oxidative deamination of l-amino acid to produce a 2-oxo acid with ammonia and highly toxic hydrogen peroxide and, therefore, is expressed as a precursor. The LAAO precursor shows significant variation in size and the cleavage pattern for activation. However, the molecular mechanism of how the propeptide suppresses the enzyme activity remains unclear except for deaminating/decarboxylating Pseudomonasl-phenylalanine oxidase (PAO), which has a short N-terminal propeptide composed of 14 residues. Here we show the inactivation mechanism of the l-lysine oxidase (LysOX) precursor (prLysOX), which has a long N-terminal propeptide composed of 77 residues, based on the crystal structure at 1.97 Å resolution. The propeptide of prLysOX indirectly changes the active site structure to inhibit the enzyme activity. prLysOX retains weak enzymatic activity with strict specificity for l-lysine and shows raised activity in acidic conditions. The structures of prLysOX crystals that soaked in a solution with various concentrations of l-lysine have revealed that prLysOX can adopt two conformations; one is the inhibitory form, and the other is very similar to mature LysOX. The propeptide region of the latter form is disordered, and l-lysine is bound to the latter form. These results indicate that prLysOX uses a different strategy from PAO to suppress the enzyme activity and suggest that prLysOX can be activated quickly in response to the environmental change without proteolytic processing.

Comparative analysis of the chemical and biochemical synthesis of keto acids

Keto acids are essential organic acids that are widely applied in pharmaceuticals, cosmetics, food, beverages, and feed additives as well as chemical synthesis. Currently, most keto acids on the market are prepared via chemical synthesis. The biochemical synthesis of keto acids has been discovered with the development of metabolic engineering and applied toward the production of specific keto acids from renewable carbohydrates using different metabolic engineering strategies in microbes. In this review, we provide a systematic summary of the types and applications of keto acids, and then summarize and compare the chemical and biochemical synthesis routes used for the production of typical keto acids, including pyruvic acid, oxaloacetic acid, α-oxobutanoic acid, acetoacetic acid, ketoglutaric acid, levulinic acid, 5-aminolevulinic acid, α-ketoisovaleric acid, α-keto-γ-methylthiobutyric acid, α-ketoisocaproic acid, 2-keto-L-gulonic acid, 2-keto-D-gluconic acid, 5-keto-D-gluconic acid, and phenylpyruvic acid. We also describe the current challenges for the industrial-scale production of keto acids and further strategies used to accelerate the green production of keto acids via biochemical routes.

Biocatalysed synthesis of chiral amines: continuous colorimetric assays for mining amine-transaminases

Versatile and sensitive continuous colorimetric assays were developed for the high throughput screening of a large collection of amine-TAs from biodiversity, and allowed the discovery of a set of diverse biocatalysts with high synthetic potential.

Ancestral L-amino acid oxidases for deracemization and stereoinversion of amino acids

L-amino acid oxidases (LAAOs) can be applied to convert racemic amino acids to D-isomers, which are potential precursors of pharmaceuticals. However, this application is hampered by the lack of available stable and structure-determined LAAOs. In this study, we attempt to address this limitation by utilizing two ancestral LAAOs: AncLAAO-N4 and AncLAAO-N5. AncLAAO-N4 has the highest thermal and temporal stabilities among the designed LAAOs that can be used for deracemization and stereoinversion. AncLAAO-N5 can provide X-ray crystal structures, which are helpful to reveal substrate recognition and reaction mechanisms of LAAOs at the molecular level. Next, we attempted to improve activity of AncLAAO-N4 toward L-Val through a semi-rational protein engineering method. Three variants with enhanced activity toward L-Val were obtained. Taken together, we believe that the activity and substrate selectivity of AncLAAOs give them the potential to be key enzymes in various chemoenzymatic reactions.

Process properties of an l-amino acid oxidase from Hebeloma cylindrosporum for the synthesis of phenylpyruvic acid from l-phenylalanine

The biocatalytic oxidation of amino acids represents an attractive approach towards the synthesis of α-keto acids, which are interest for various industrial applications. As l-amino acids are readily available from fermentation processes, these natural amino acids can serve as substrates in combination with an l-amino acid oxidase. Besides an aqueous phase as reaction medium, a further advantage of such a process is the utilization of air as oxidation agent. In this study, we studied the organic-synthetic properties of a literature-known recombinant l-amino acid oxidase from the fungus Hebeloma cylindrosporum with respect to its suitability to catalyze the formation of α-keto acids exemplified for the synthesis of phenylpyruvic acid starting from l-phenylalanine as a substrate. In our study the enzyme displayed a reasonable operational stability in the reaction system and as well as promising applicability data with respect to substrate and product inhibition. In a biotransformation, 20 mM of substrate were converted after 4 h reaction. The formation of undesired by-products was suppressed using a commercially available catalase enzyme.

Methylated derivatives of l-tyrosine in reaction catalyzed by l-amino acid oxidase: isotope and inhibitory effects

l-Amino acid oxidase (LAAO) is widely distributed in nature and shows important biological activity. It induces cell apoptosis and has antibacterial properties. This study was designed to investigate the effect of methyl substituent on its activity as methylated derivatives of l-tyrosine, labelled with short-lived B+ emitters, have been used in oncological diagnostics. To study isotope effects in the oxidative deamination of O-methyl-l-tyrosine, the deuterated isotopomer, i.e. O-methyl-[2-2H]-l-tyrosine, was synthesized by isotope exchange, catalyzed enzymatically by tryptophanase. Isotope effects were determined using the spectrophotometric non-competitive method. The values of isotope effects indicate that the α-C-H bond cleavage occurs in the rate determining step of the investigated reaction and α-hydrogen plays a role in the substrate binding process at the enzyme active site. The inhibitory effect on LAAO activity was studied with α-methyl-l-tyrosine and N-methyl-l-tyrosine. The mode of inhibition was determined based on Lineweavear-Burk plots intersections. α-Methyl-l-tyrosine has been found a mixed type inhibitor of the investigated enzyme, whereas N-methyl-l-tyrosine is a non-competitive inhibitor of LAAO.

Delivery of amino acid oxidase via catalytic nanocapsules to enable effective tumor inhibition

Amino acids are the fundamental building blocks of proteins in tumor cells. The consumption of amino acid can be an effective approach for destroying the tumor cytoskeleton and malfunctioning of the intracellular metabolic balance. Following this concept, herein, amino acid oxidase (AAO) is delivered by hollow Fe3+/tannic acid nanocapsules (HFe-TA) and incorporated within the cancer cell membrane (M) for the first time for synergistic tumor therapy. In this system (M@AAO@HFe-TA), the intracellularly delivered AAO molecules catalyze the oxidative deamination effectively and consume amino acids significantly. The upregulation of intracellular acid and H2O2 concentration facilitates the HFe-TA mediated Fenton reaction and enhances the induction of cytotoxic ˙OH. With the combined effects, considerable in vitro and in vivo tumor inhibition was achieved by M@AAO@Fe-TA due to the activated Bcl-2/Bax/Cyt C/caspase 3 mitochondrial apoptotic pathway. This study offers an alternative therapeutic platform, functioning as a biomimetic cascade nanozyme, to enable synergistic starvation and chemodynamic tumor therapy with high efficacy.

Structural basis of strict substrate recognition of l-lysine α-oxidase from Trichoderma viride

l-Lysine oxidase (LysOX) is a FAD-dependent homodimeric enzyme that catalyzes the oxidative deamination of l-lysine to produce α-keto-ε-aminocaproate with ammonia and hydrogen peroxide. LysOX shows strict substrate specificity for l-lysine, whereas most l-amino acid oxidases (LAAOs) exhibit broad substrate specificity for l-amino acids. Previous studies of LysOX showed that overall structural similarity to the well-studied snake venom LAAOs. However, the molecular mechanism of strict specificity for l-lysine was still unclear. We here determined the structure of LysOX in complex with l-lysine at 1.7 Å resolution. The structure revealed that the hydrogen bonding network formed by D212, D315, and A440 with two water molecules is responsible for the recognition of the side chain amino group. In addition, a narrow hole formed by five hydrophobic residues in the active site contributes to strict substrate specificity. Mutation studies demonstrated that D212 and D315 are essential for l-lysine recognition, and the D212A/D315A double mutant LysOX showed different substrate specificity from LysOX. Moreover, the structural basis of the substrate specificity change has also been revealed by the structural analysis of the mutant variant and its substrate complexes. These results clearly explain the molecular mechanism of the strict specificity of LysOX and suggest that LysOX is a potential candidate for a template to design LAAOs specific to other l-amino acids.

Efficient Synthesis of Phenylacetate and 2-Phenylethanol by Modular Cascade Biocatalysis

The green and sustainable synthesis of chemicals from renewable feedstocks by a biotransformation approach has gained increasing attention in recent years. In this work, we developed enzymatic cascades to efficiently convert l-phenylalanine into 2-phenylethanol (2-PE) and phenylacetic acid (PAA), l-tyrosine into tyrosol (p-hydroxyphenylethanol, p-HPE) and p-hydroxyphenylacetic acid (p-HPAA). The enzymatic cascade was cast into an aromatic aldehyde formation module, followed by an aldehyde reduction module, or aldehyde oxidation module, to achieve one-pot biotransformation by using recombinant Escherichia coli. Biotransformation of 50 mM l-Phe produced 6.76 g/L PAA with more than 99 % conversion and 5.95 g/L of 2-PE with 97 % conversion. The bioconversion efficiencies of p-HPAA and p-HPE from l-Tyr reached to 88 and 94 %, respectively. In addition, m-fluoro-phenylalanine was further employed as an unnatural aromatic amino acid substrate to obtain m-fluoro-phenylacetic acid; >96 % conversion was achieved. Our results thus demonstrated high-yielding and potential industrial synthesis of above aromatic compounds by one-pot cascade biocatalysis.

Spontaneous conversions of glutamine, histidine and arginine into α-hydroxycarboxylates with NH4VO3 or V2O5

Glutamine gets transformed to 2-hydroxy-5-oxoproline with NH₄VO₃ in a neutral solution as a product of 2,2'-bipyridine oxidovanadium(v) 2-hydroxy-5-oxoproline [V^V₂O₃(hop)₂(bpy)₂]·7H₂O [1, H₂hop = 2-hydroxy-5-oxoproline] with yields of 65.6%. Similarly, histidine and arginine are converted into the corresponding α-hydroxycarboxylates as 2,2'-bipyridine oxidovanadium(iv) 3-(1H-imidazolyl-5-yl)-2-hydroxyacrylate [V^IV₂O₂(imha)₂(bpy)₂]·bpy [2, H₂imha = 3-(1H-imidazolyl-5-yl)-2-hydroxyacrylic acid] and guanidinium oxidovanadium(v) 1-(aminoiminomethyl)-2-hydroxyproline (CN₃H₆)[V^VO₂(Haimhp)₂]·2H₂O [3, H₂aimhp = 1-(aminoiminomethyl)-2-hydroxyproline] with V₂O₅ in low yields respectively, where an aggregate of oxidovanadium(v) arginine (H₂arg)_n(V^VO₃)_n·½nH₂O (4, Harg = arginine) has been isolated preferentially in an initial experiment for 3. α-Hydroxycarboxylates chelate bidentately with vanadium viaα-alkoxy and α-carboxy groups in 1-3, as observed from structural analyses. Their racemizations have been observed after the conversions. There is no coordination in 4 based on solid-state ¹³C NMR spectra, and only strong hydrogen bonds exist in the anion chains (V^VO₃)^- and protonated arginines. 1 and 4 were fully characterized by elemental analysis, UV-vis, IR and solid-state ¹³C NMR spectroscopies, TG and X-ray structural analyses, and theoretical bond valence calculations (BVS). The conversions of glutamine, histidine and arginine occur spontaneously in a solution under ambient conditions.

Recombinant expression of an l-amino acid oxidase from the fungus Hebeloma cylindrosporum in Pichia pastoris including fermentation

l‐amino acid oxidases (LAAOs) are flavoenzymes that catalyze the oxidative deamination of l‐amino acids to the corresponding α‐keto acids, ammonia, and hydrogen peroxide. Here, we show the overexpression, purification, and the characterization of LAAO4 from the fungus Hebeloma cylindrosporum in the yeast Pichia pastoris with a 9His‐tag and compare this with the recently characterized 6His‐hcLAAO4 expressed in E. coli. The expression of the enzyme with an ER‐signal sequence in P. pastoris resulted in a glycosylated, secreted protein. The enzymatic activity without activation was higher after expression in P. pastoris compared to E. coli. Due to treatment with acidic pH, a striking increase of activity could be detected for both expression systems resulting in similar specific activities after acid activation. Regarding the substrate spectrum, temperature stability, K_m, and v_max values, hcLAAO4 showed very few differences when produced in these two expression systems. A higher yield of hcLAAO4 could be obtained by fermentation.