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Gless BH, Schmied SH, Bejder BS, Olsen CA. Förster Resonance Energy Transfer Assay for Investigating the Reactivity of Thioesters in Biochemistry and Native Chemical Ligation. JACS AU 2023; 3:1443-1451. [PMID: 37234128 PMCID: PMC10207088 DOI: 10.1021/jacsau.3c00095] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 02/24/2023] [Revised: 04/11/2023] [Accepted: 04/13/2023] [Indexed: 05/27/2023]
Abstract
Thioesters are considered to be "energy-rich" functional groups that are susceptible to attack by thiolate and amine nucleophiles while remaining hydrolytically stable at neutral pH, which enables thioester chemistry to take place in an aqueous medium. Thus, the inherent reactivity of thioesters enables their fundamental roles in biology and unique applications in chemical synthesis. Here, we investigate the reactivity of thioesters that mimic acyl-coenzyme A (CoA) species and S-acylcysteine modifications as well as aryl thioesters applied in chemical protein synthesis by native chemical ligation (NCL). We developed a fluorogenic assay format for the direct and continuous investigation of the rate of reaction between thioesters and nucleophiles (hydroxide, thiolate, and amines) under various conditions and were able to recapitulate previously reported reactivity of thioesters. Further, chromatography-based analyses of acetyl- and succinyl-CoA mimics revealed striking differences in their ability to acylate lysine side chains, providing insight into nonenzymatic protein acylation. Finally, we investigated key aspects of native chemical ligation reaction conditions. Our data revealed a profound effect of the tris-(2-carboxyethyl)phosphine (TCEP) commonly used in systems where thiol-thioester exchange occurs, including a potentially harmful hydrolysis side reaction. These data provide insight into the potential optimization of native chemical ligation chemistry.
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Song J, Wang J, Jozwiak AA, Hu W, Swiderski PM, Chen Y. Stability of thioester intermediates in ubiquitin-like modifications. Protein Sci 2010; 18:2492-9. [PMID: 19785004 DOI: 10.1002/pro.254] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
Ubiquitin-like modifications are important mechanisms in cellular regulation, and are carried out through several steps with reaction intermediates being thioester conjugates of ubiquitin-like proteins with E1, E2, and sometimes E3. Despite their importance, a thorough characterization of the intrinsic stability of these thioester intermediates has been lacking. In this study, we investigated the intrinsic stability by using a model compound and the Ubc9 approximately SUMO-1 thioester conjugate. The Ubc9 approximately SUMO-1 thioester intermediate has a half life of approximately 3.6 h (hydrolysis rate k = 5.33 +/- 2.8 x10(-5) s(-1)), and the stability decreased slightly under denaturing conditions (k = 12.5 +/- 1.8 x 10(-5) s(-1)), indicating a moderate effect of the three-dimensional structural context on the stability of these intermediates. Binding to active and inactive E3, (RanBP2) also has only a moderate effect on the hydrolysis rate (13.8 +/- 0.8 x 10(-5) s(-1) for active E3 versus 7.38 +/- 0.7 x 10(-5) s(-1) for inactive E3). The intrinsically high stability of these intermediates suggests that unwanted thioester intermediates may be eliminated enzymatically, such as by thioesterases, to regulate their intracellular abundance and trafficking in the control of ubiquitin-like modifications.
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Affiliation(s)
- Jing Song
- Department of Immunology, Beckman Research Institute of the City of Hope, Duarte, California 91010, USA
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Richards NG, Schuster SM. Mechanistic issues in asparagine synthetase catalysis. ADVANCES IN ENZYMOLOGY AND RELATED AREAS OF MOLECULAR BIOLOGY 1998; 72:145-98. [PMID: 9559053 DOI: 10.1002/9780470123188.ch5] [Citation(s) in RCA: 45] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
The enzymatic synthesis of asparagine is an ATP-dependent process that utilizes the nitrogen atom derived from either glutamine or ammonia. Despite a long history of kinetic and mechanistic investigation, there is no universally accepted catalytic mechanism for this seemingly straightforward carboxyl group activating enzyme, especially as regards those steps immediately preceding amide bond formation. This chapter considers four issues dealing with the mechanism: (a) the structural organization of the active site(s) partaking in glutamine utilization and aspartate activation; (b) the relationship of asparagine synthetase to other amidotransferases; (c) the way in which ATP is used to activate the beta-carboxyl group; and (d) the detailed mechanism by which nitrogen is transferred.
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Affiliation(s)
- N G Richards
- Department of Chemistry, University of Florida, Gainesville 32611, USA
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LOWE G, WILLIAMS A. PAPAIN-CATALYSED HYDROLYSIS OF SOME HIPPURIC ESTERS. A NEW MECHANISM FOR PAPAIN-CATALYSED HYDROLYSIS. Biochem J 1996; 96:199-204. [PMID: 14346990 PMCID: PMC1206922 DOI: 10.1042/bj0960199] [Citation(s) in RCA: 65] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
1. The Michaelis-Menten parameters for the papain-catalysed hydrolysis of a number of alkyl, aryl and alkyl-thiol esters of hippuric acid have been determined. 2. For all the aryl esters and most of the alkyl esters studied, the catalytic constant, k(0), is 2-3sec.(-1) and most probably represents deacylation of the common intermediate, hippuryl-papain. 3. Two alkyl esters and hippurylamide, however, have catalytic rate constants, k(0), less than 2-3sec.(-1). It is possible to interpret all the available kinetic data in terms of a three-step mechanism in which an enzyme-substrate complex is first formed, followed by acylation of the enzyme through an essential thiol group, followed by deacylation of the acyl-enzyme. 4. The logarithm of the ratio of the Michaelis-Menten parameters, which reflect the acylation rate constant, for four aryl esters of hippuric acid studied give a linear Hammett plot against the substituent constant, sigma. Arguments are presented that indicate acid as well as nucleophilic catalysis in the acylation process and that the most likely proton donor is an imidazolium ion. 5. It is suggested that this imidazolium ion is part of the same histidine residue that has been tentatively implicated in the deacylation process (Lowe & Williams, 1965b). 6. A new mechanism is proposed for the papain-catalysed hydrolysis of N-acyl-alpha-amino acid derivatives.
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Vernet T, Tessier DC, Chatellier J, Plouffe C, Lee TS, Thomas DY, Storer AC, Ménard R. Structural and functional roles of asparagine 175 in the cysteine protease papain. J Biol Chem 1995; 270:16645-52. [PMID: 7622473 DOI: 10.1074/jbc.270.28.16645] [Citation(s) in RCA: 112] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
The role of the asparagine residue in the Cys-His-Asn "catalytic triad" of cysteine proteases has been investigated by replacing Asn175 in papain by alanine and glutamine using site-directed mutagenesis. The mutants were expressed in yeast and kinetic parameters determined against the substrate carbobenzoxy-L-phenylalanyl-(7-amino-4-methylcoumarinyl)- L-arginine. At the optimal pH of 6.5, the specificity constant (k(cat)/KM)obs was reduced by factors of 3.4 and 150 for the Asn175-->Gln and Asn175-->Ala mutants, respectively. Most of this effect was the result of a decrease in k(cat), as neither mutation significantly affected KM. Substrate hydrolysis by these mutants is still much faster than the non-catalytic rate, and therefore Asn175 cannot be considered as an essential catalytic residue in the cysteine protease papain. Detailed analyses of the pH activity profiles for both mutants allow the evaluation of the role of the Asn175 side chain on the stability of the active site ion pair and on the intrinsic activity of the enzyme. Alteration of the side chain at position 175 was also found to increase aggregation and proteolytic susceptibility of the proenzyme and to affect the thermal stability of the mature enzyme, reflecting a contribution of the asparagine residue to the structural integrity of papain. The strict conservation of Asn175 in cysteine proteases might therefore result from a combination of functional and structural constraints.
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Affiliation(s)
- T Vernet
- Biotechnology Research Institute, National Research Council of Canada, Montréal, Québec
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Affiliation(s)
- A C Storer
- Biotechnology Research Institute, National Research Council of Canada, Montréal, Quebec
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Brocklehurst K, Willenbrock F, Salih E. Chapter 2 Cysteine proteinases. ACTA ACUST UNITED AC 1987. [DOI: 10.1016/s0167-7306(09)60016-8] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 03/16/2023]
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Moore SA, Jencks WP. Model reactions for CoA transferase involving thiol transfer. Anhydride formation from thiol esters and carboxylic acids. J Biol Chem 1982. [DOI: 10.1016/s0021-9258(18)33907-3] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
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Baines BS, Brocklehurst K. Characterization of papaya peptidase A as a cysteine proteinase of Carica papaya L. with active-centre properties that differ from those of papain by using 2,2'-dipyridyl disulphide and 4-chloro-7-nitrobenzofurazan as reactivity probes. Use of two-protonic-state electrophiles in the identification of catalytic-site thiol groups. Biochem J 1982; 205:205-11. [PMID: 6751321 PMCID: PMC1158464 DOI: 10.1042/bj2050205] [Citation(s) in RCA: 29] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
1. The proteinase papaya peptidase A, one of the major components of the latex of Carica papaya L., was shown to contain 1 thiol group per molecule; this thiol group is essential for catalytic activity and is part of the catalytic site. 2. The usefulness of two-protonic-state reactivity probes coupled with modification/activity-loss data in assigning a thiol group as an integral part of the catalytic site as against merely 'essential' for activity is discussed. 3. The active centre of papaya peptidase A was investigated by using 2,2'-dipyridyl disulphide and 4-chloro-7-nitrobenzofurazan as reactivity probes. The presence in the enzyme in weakly acidic media of an interactive system containing a nucleophile S atom (pKI3.9,pKII7.9) was demonstrated. 5. Papaya peptidase A resembles ficin (EC 3.4.22.3) and actinidin (the cysteine proteinase from Actinidin chinenis) in that it does not appear to possess a carboxy group able to influence the reactivity of the thiol group by change of ionization state at pH values of about 4, a situation that contrasts markedly with that which obtains in papain. 6. Implications of the results for possible variations in cysteine proteinase mechanism are discussed.
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Allen KG, Stewart JA, Johnson PE, Wettlaufer DG. Identification of the functional ionic groups of papain by pH/rate profile analysis. EUROPEAN JOURNAL OF BIOCHEMISTRY 1978; 87:575-82. [PMID: 28228 DOI: 10.1111/j.1432-1033.1978.tb12409.x] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
The pH dependence of papain catalysis was analyzed by a scheme which evaluates the kinetic contribution of both protonated and unprotonated species of functional groups involved in catalysis. Kinetic measurements were made at constant pH, without buffers, by automatic titration. The rate-determining step for papain-catalyzed hydrolysis of alpha-N-benzoyl-L-arginine ethyl ester, determined by nucleophile competition, changed from acylation below pH 6.5 to mixed acylation-deacylation above pH 6.5. Kinetic analysis indicated that three prototropic groups governed the pH-specificity of alpha-N-benzoyl-L-arginine ethyl ester hydrolysis. These prototropic groups had pKa values of 4.8, 6.5 to 6.7, and 8.7. Theoretical treatment of the kinetics provided an excellent fit with the experimentally found profile when the contribution of all three prototropic groups was considered. Analysis showed that, in acid, the pathways of papain catalysis were functional with either two or three active-site protons. In base, a single functional ionic pathway is associated with an active site with only one proton. Pathways involving an unprotonated active site are catalytically inoperative in both acid and base. These results indicate that papain exhibits several catalytically functional ionic pathways. The results are discussed in terms of pKa assignments, and the mechanism of papain catalysis.
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Ordman AB, Kirkwood S. UDPglucose dehydrogenase. Kinetics and their mechanistic implications. BIOCHIMICA ET BIOPHYSICA ACTA 1977; 481:25-32. [PMID: 191082 DOI: 10.1016/0005-2744(77)90133-4] [Citation(s) in RCA: 26] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
Initial velocity and product inhibition studies were carried out on UDP-glucose dehydrogenase (UDPglucose: NAD+ 6-oxidoreductase, EC 1.1.1.22) from beef liver to determine if the kinetics of the reaction are compatible with the established mechanism. An intersecting initial velocity pattern was observed with NAD+ as the variable substrate and UDPG as the changing fixed substrate. UDPglucuronic acid gave competitive inhibition of UDPG and non-competitive inhibition of NAD+. Inhibition by NADH gave complex patterns.Lineweaver-Burk plots of 1/upsilon versus 1/NAD+ at varied levels of NADH gave highly non-linear curves. At levels of NAD+ below 0.05 mM, non-competitive inhibition patterns were observed giving parabolic curves. Extrapolation to saturation with NAD+ showed NADH gave linear uncompetitive inhibition of UDPG if NAD+ was saturating. However, at levels of NAD+ above 0.10 mM, NADH became a competitive inhibitor of NAD+ (parabolic curves) and when NAD+ was saturating NADH gave no inhibition of UDPG. NADH was non-competitive versus UDPG when NAD+ was not saturating. These results are compatible with a mechanism in which UDPG binds first, followed by NAD+, which is reduced and released. A second mol of NAD+ is then bound, reduced, and released. The irreversible step in the reaction must occur after the release of the second mol of NADH but before the release of UDPglucuronic acid. This is apparently caused by the hydrolysis of a thiol ester between UDPglucoronic acid and the essential thiol group of the enzyme. Examination of rate equations indicated that this hydrolysis is the rate-limiting step in the overall reaction. The discontinuity in the velocities observed at high NAD+ concentrations is apparently caused by the binding of NAD+ in the active site after the release of the second mol of NADH, eliminating the NADH inhibition when NAD+ becomes saturating.
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Bendall MR, Lowe G. Co-operative ionisation of aspartic-acid-158 and histidine-159 in papain. Evidence from 19F nuclear-magnetic-resonance and fluorescence spectroscopy. EUROPEAN JOURNAL OF BIOCHEMISTRY 1976; 65:481-91. [PMID: 949979 DOI: 10.1111/j.1432-1033.1976.tb10364.x] [Citation(s) in RCA: 26] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
The chemical shift of the single resonance in the 19F nuclear magnetic resonance spectrum of papain which has been irreversibly inhibited by 3-bromo-1,1,1-trifluoropropanone, exhibits pH-dependence. The fluorescence intensity of this papain derivative shows pH-dependence on two groups which exhibit co-operative ionisation. This co-operative behaviour is probably a function of the probe since the fluorescence intensity of S-ethane-thio-papain is dependent on a single ionisation constant, whereas that of S-(2-hydroxyethane)-thio-papain is dependent on two ionisable groups again acting co-operatively. The 1,1,1-trifluoroketone probe will be hydrated in aqueous solution and would be capable of hydrogen bonding with the protein. The two groups detected are considered to be aspartic-acid-158 and histidine-159. The co-operative ionisation of these groups in substrate hydrolysis is discussed.
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Gleisner JM, Liener IE. Chemical modification of the histidine residue located at the active site of ficin. ACTA ACUST UNITED AC 1973; 317:482-91. [DOI: 10.1016/0005-2795(73)90240-7] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
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Abstract
7-Chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD chloride) and 7-(2'-hydroxyethylthio)-NBD (obtained from NBD chloride and mercaptoethanol) undergo a reversible spectral change in alkaline solution that depends respectively on a single apparent pK(a) 9.76 (at 25 degrees C) and 8.81 (at 32 degrees C). In acid solution however no spectral change was observed. NBD chloride reacts slowly with papain at pH7, but the rate of inhibition increases at lower pH and depends on an apparent pK(a) of 3.7 (at 35 degrees C), which has been tentatively assigned to the carboxyl group of aspartic acid-158. The spectral properties of NBD-papain indicate that the thiol group of cysteine-25 is the site of reaction. The intensity of the fluorescence-emission spectrum of NBD-papain depends on a single pK(a) of 4.2 (at 26.7 degrees C). The intensity of the fluorescence-emission spectrum of the mixed disulphide formed from papain and 7-(2'-mercaptoethylamino)-NBD (obtained from NBD chloride and cysteamine) depended on a single pK(a) of 3.94 in water and 3.89 in aq. 19.2% (v/v) dioxan (at 27 degrees C). This small change to lower pK(a) value in a medium of lower dielectric constant is characteristic of a cationic acid. The only acid of this type in the active-site region is the conjugate acid of histidine-159.
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An Essential Histidine in the Catalytic Activities of 3-Phosphoglyceraldehyde Dehydrogenase. J Biol Chem 1970. [DOI: 10.1016/s0021-9258(18)63287-9] [Citation(s) in RCA: 49] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
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Kramer DE, Whitaker JR. Ficin-Catalyzed Reactions. Hydrolysis of alpha-N-Benzoyl-l-Arginine Ethyl Ester and alpha-N-Benzoyl-l-Argininamide. PLANT PHYSIOLOGY 1969; 44:609-14. [PMID: 16657108 PMCID: PMC396133 DOI: 10.1104/pp.44.4.609] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/08/2023]
Abstract
The effect of pH on the hydrolysis of alpha-N-benzoyl-l-arginine ethyl ester (BAEE) and alpha-N-benzoyl-l-argininamide (BAA) by a proteolytic enzyme component purified from Ficus carica var. Kadota latex has been studied in detail over the pH range of 3 to 9.5. k(cat) (lim) values for the hydrolysis of BAEE and BAA were essentially identical (5.20 and 5.01 sec(-1), respectively at 30 degrees ). k(cat) values for hydrolysis of BAEE and BAA were dependent on prototropic groups with apparent pK values of 4.24 and 8.53 and 4.10 and 8.59, respectively. k(cat) (lim) values for tht hydrolysis of BAEE and BAA were essentially identical (5.20 and groups of pK 4.33 and 8.60 and 4.55 and 8.51, respectively. Thus the pH optimum is 6.5 for both substrates. K(m) (app) values for BAEE and BAA were 3.32 x 10(-2)m and 6.03 x 10(-2)m respectively over the pH range of 3.9 to 8.0. These data are interpreted in terms of the involvement of a carboxyl and a sulfhydryl group in the active center of the enzyme. The data do not support the concept that deacylation of the acyl-enzyme is completely the rate controlling step in the hydrolyses. Rather, it appears that the magnitude of k(2) and k(3) are not greatly different.
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Affiliation(s)
- D E Kramer
- Department of Food Science and Technology, University of California, Davis, California 95616
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Abstract
Papain was irreversibly inhibited by 1,3-dibromoacetone, a reagent designed to react first with the active-site cysteine residue and subsequently with a second nucleophile. The molecular weight of the inhibited enzyme was indistinguishable from that of papain itself, and no evidence of dimeric or oligomeric species was found. The optical-rotatory-dispersion curves of chloroacetone-inhibited papain and 1,3-dibromoacetone-inhibited papain were essentially similar. Amino acid analysis of the 1,3-dibromo[2-(14)C]acetone-inhibited enzyme and the performic acid-oxidized material clearly showed that a cysteine and histidine residue had been alkylated through the thiol and N-1 of the imidazole group respectively. These groups must therefore be within 5å of each other in the tertiary structure of papain. Possible mechanistic implications are briefly discussed.
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Hollaway MR. The proflavin-induced increase in the catalytic activity of ficin. EUROPEAN JOURNAL OF BIOCHEMISTRY 1968; 5:366-75. [PMID: 5680355 DOI: 10.1111/j.1432-1033.1968.tb00379.x] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
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Husain SS, Lowe G. The location of the active-site histidine residue in the primary sequence of papain. Biochem J 1968; 108:861-6. [PMID: 5673531 PMCID: PMC1198893 DOI: 10.1042/bj1080861] [Citation(s) in RCA: 27] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
Abstract
Papain that had been irreversibly inhibited with 1,3-dibromo[2-(14)C]acetone was reduced with sodium borohydride and carboxymethylated with iodoacetic acid. After digestion with trypsin and alpha-chymotrypsin the radioactive peptides were purified chromatographically. Their amino acid composition indicated that cysteine-25 and histidine-106 were cross-linked. Since cysteine-25 is known to be the active-site cysteine residue, histidine-106 must be the active-site histidine residue.
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Whitaker JR, Perez-Villase ñor J. Chemical modification of papain. I. Reaction with the chloromethyl ketones of phenylalanine and lysine and with phenylmethyl-sulfonyl fluoride. Arch Biochem Biophys 1968; 124:70-8. [PMID: 5661631 DOI: 10.1016/0003-9861(68)90304-4] [Citation(s) in RCA: 149] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
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Stein MJ, Liener IE. Inhibition of ficin by the chloromethyl ketone derivatives of n-tosyl-l-lysine and n-tosyl-l-phenylalanine. Biochem Biophys Res Commun 1967; 26:376-82. [PMID: 6034362 DOI: 10.1016/0006-291x(67)90135-0] [Citation(s) in RCA: 25] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
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25
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Lake AW, Lowe G. The kinetics of papain- and ficin-catalysed hydrolyses in the presence of alcohols. Biochem J 1966; 101:402-10. [PMID: 5966277 PMCID: PMC1270121 DOI: 10.1042/bj1010402] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
Abstract
1. The maximum rate of production of p-nitrophenol (V(max.)) for both papain- and ficin-catalysed hydrolyses of p-nitrophenyl hippurate is independent of methanol concentration up to 2m for papain and 1.5m for ficin. 2. The observed catalytic constant (k(0)) for the production of hippuric acid for both papain- and ficin-catalysed hydrolyses of methyl hippurate decreases with increasing methanol concentration, 1/k(0) being linearly dependent on the methanol concentration. The k(MeOH)/k(H2O) ratio is determined. 3. These results provide strong evidence against general base catalysis for the rate-determining step in the deacylation of hippuryl-papain and hippuryl-ficin and probably for other specific acyl-papains and acyl-ficins. 4. The rate-determining step for the deacylation of the non-specific trans-cinnamoyl-papain appears to be different from that for the specific hippuryl-papain, and is probably subject to general base catalysis. It is possible, however, to accommodate all these observations in a single four-step reaction pathway. 5. Propan-2-ol did not influence the rate of production of hippuric acid for the papain-catalysed hydrolysis of methyl hippurate. A similar result has previously been reported for the ficin-catalysed hydrolysis of methyl hippurate. Ethanol and of course methanol (see 2) decrease the rate of production of hippuric acid for both papain- and ficin-catalysed hydrolyses of methyl hippurate. It is suggested that the secondary alcohol is incapable for structural reasons of approaching the bond to be hydrolysed.
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