Novel mechanism of hydrolysis of therapeutic beta-lactams by Stenotrophomonas maltophilia L1 metallo-beta-lactamase.

Stopped-flow tryptophan fluorescence under single turnover and pseudo-first-order conditions has been used to investigate the kinetic mechanism of beta-lactam hydrolysis by the Stenotrophomonas maltophilia L1 metallo-beta-lactamase. For the cephalosporin substrates nitrocefin and cefaclor and the carbapenem meropenem, a substantial quench of fluorescence is observed on association of substrate with enzyme. We have assigned this to a rearrangement event subsequent to formation of an initial collision complex. For the colorimetric compound nitrocefin, decay of this dark inter- mediate represents the overall rate-determining step for the reaction and is equivalent to decay of a previously observed state in which the beta-lactam amide bond has already been cleaved. For both cefaclor and meropenem, the rate-determining step for hydrolysis is loss of a second, less quenched state, in which, however, the beta-lactam amide bond remains intact. We suggest, therefore, that the mechanism of hydrolysis of nitrocefin by binuclear metallo-beta-lactamases may be atypical and that cleavage of the beta-lactam amide bond is the rate-determining step for breakdown of the majority of beta-lactam substrates by the L1 enzyme.     

Unusual kinetic behavior of Endothia parasitica protease in hydrolysis of small peptides

Endothia parasitica protease hydrolyzes l-leucyl-l-leucine amide and l-leucyl-l-phenylalanine amide at the peptide bond. l-Phenylalanyl-l-leticine amide, N-carbobenzoxy-l-leucyl-l-phenylalanine amide, N-carbobenzoxy-l-leucyl-l-pheml-alanine, N-carbobenzoxy-l-phenylalanyl-l-valine amide, and l-leucyl-β-naphthyl-amide are not hydrolyzed. In contrast to the kinetics of hydrolysis of casein and oxidized B-chain of insulin and activation of trypsinogen by Endothia parasitica protease which are normal, reaction progress curves for hydrolysis of l-leucyl-l-leucine amide and l-leucyl-l-phenylalanine amide are sigrnoidal. Initially, the reaction rates were of the order of 0.5–2.5% of the maximum rates eventually attained. With increasing time of incubation the reaction rates became faster and faster until maximum rates were achieved. This abnormal behavior was not eliminated by recrystallization of substrate or by incubation of enzyme alone or with products of the reaction prior to addition of substrate. Addition of a new aliquot of substrate, vizl-leucyl-l-leucine amide, to the reaction prior to complete hydrolysis of all of a previous aliquot of the same substrate, or reactions containing a mixture of oxidized B chain of insulin and l-leucyl-l-leucine amide, gave normal reaction progress curves. The duration of abnormal behavior before a maximum rate was attained was a function of enzyme concentration and temperature but not of substrate concentration even though substrate was in less than saturating amounts. The reaction data follow second-order autocatalytic kinetics with respect to enzyme concentration. It is proposed that most of the enzyme is in an inactive form in absence of substrate but is rapidly converted to the active form on combination with a good substrate such as trypsinogen, casein, or oxidized B chain of insulin. However, with a poor substrate such as l-leucyl-l-leucine amide, conversion to active enzyme is mediated through formation of an active enzyme-inactive enzyme complex followed by combination with substrate and hydrolysis.     

Steady-state and pre-steady-state kinetic evaluation of severe acute respiratory syndrome coronavirus (SARS-CoV) 3CLpro cysteine protease: development of an ion-pair model for catalysis

Severe acute respiratory syndrome (SARS) was a worldwide epidemic caused by a coronavirus that has a cysteine protease (3CLpro) essential to its life cycle. Steady-state and pre-steady-state kinetic methods were used with highly active 3CLpro to characterize the reaction mechanism. We show that 3CLpro has mechanistic features common and disparate to the archetypical proteases papain and chymotrypsin. The kinetic mechanism for 3CLpro-mediated ester hydrolysis, including the individual rate constants, is consistent with a simple double displacement mechanism. The pre-steady-state burst rate was independent of ester substrate concentration indicating a high commitment to catalysis. When homologous peptidic amide and ester substrates were compared, a series of interesting observations emerged. Despite a 2000-fold difference in nonenzymatic reactivity, highly related amide and ester substrates were found to have similar kinetic parameters in both the steady-state and pre-steady-state. Steady-state solvent isotope effect (SIE) studies showed an inverse SIE for the amide but not ester substrates. Evaluation of the SIE in the pre-steady-state revealed normal SIEs for both amide and ester burst rates. Proton inventory (PI) studies on amide peptide hydrolysis were consistent with two proton-transfer reactions in the transition state while the ester data was consistent with a single proton-transfer reaction. Finally, the pH-inactivation profile of 3CLpro with iodoacetamide is indicative of an ion-pair mechanism. Taken together, the data are consistent with a 3CLpro mechanism that utilizes an "electrostatic" trigger to initiate the acylation reaction, a cysteine-histidine catalytic dyad ion pair, an enzyme-facilitated release of P1, and a general base-catalyzed deacylation reaction.     

N-(phenylacetyl)glycyl-D-aziridine-2-carboxylate, an acyclic amide substrate of beta-lactamases: importance of the shape of the substrate in beta-lactamase evolution

Certain acyclic depsipeptides, but not peptides, are substrates of typical beta-lactamases [Pratt, R.F., & Govardhan, C.P. (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 1302]. This may reflect either the greater chemical reactivity of depsipeptides (and of beta-lactams, the natural substrates) than peptides or the greater ease of distortion of the depsipeptide (ester) than the peptide (amide) group into a penicillin-like conformation. The latter explanation has been shown to be more likely by employment of a novel beta-lactamase substrate. N-(phenylacetyl)glycyl-D-aziridine-2-carboxylate, which combines a high chemical reactivity with a close to tetrahedral amide nitrogen atom. Although this substrate was better (higher kcat/KM) than a comparable depsipeptide for beta-lactamases, it was poorer than the depsipeptide for the Streptomyces R61 D-alanyl-D-alanine peptidase (which catalyzes specific peptide hydrolysis). It therefore seems likely that one vital feature of the putative evolution of a DD-peptidase into a beta-lactamase would have been modification of the active site to, on one hand, accommodate bicyclic beta-lactams and, on the other, exclude productive binding of planar acyclic amides. Certain serine beta-lactamases and the R61 DD-peptidase also catalyze methanolysis and aminolysis by D-phenylalanine of the N-acylaziridine. The latter reaction, the first amide aminolysis shown to be catalyzed by a beta-lactamase, is a very close analogue of the transpeptidase reaction of DD-peptidases. The methanolysis reaction appeared to proceed by way of the same acyl-enzyme intermediate as formed from depsipeptides possessing the same acyl moiety as the aziridine. The kinetics of methanolysis were employed to determine whether acylation or deacylation was rate limiting to the hydrolysis reaction under saturating substrate concentrations. The kinetics of the aminolysis reaction, catalyzed by the Enterobacter cloacae P99 beta-lactamase, showed the characteristics of, and were interpreted in terms of, a sequential mechanism previously deduced for depsipeptides and this enzyme [Pazhanisamy, S., & Pratt, R. F. (1989) Biochemistry 28, 6875-6882]. This mechanism features two separate binding sites, only one of which is productive. Strikingly, the binding of the N-acylaziridine to the nonproductive site was very tight, such that essentially all hydrolysis at substrate concentrations above 0.1Km proceeded via the ternary complex; this could also be true of penicillins.     

Mechanism of Carboxypeptidase Y Catalyzed Hydrolysis and Aminolysis Reactions

The reaction mechanism of carboxypeptidase Y catalyzed reactions is investigated. Presteady state and steady state kinetic measurements are performed on the hydrolysis and aminolysis of an ester and an amide substrate. It is found that deacylation is the rate determining step in hydrolysis of the ester, pivalic acid 4-nitrophenol and acylation in that of the amide, succinyl-L-alanyl-L-alalyl-L-propyl-L-phenylalanine 4-nitroanilide.

The kinetic effects observed in the presence of a nucleophile, L-valine amide, where aminolysis occurs in parallel to the hydrolysis reaction are analysed in details. The results are described satisfactorily by a reaction scheme which involves the binding of the added nucleophile, (i) to the free enzyme, resulting in a simple competitive effect, and (ii) to the acyl-enzyme with the formation of a complex between the enzyme and the aminolysis product, the dissociation of which is rate determining. That scheme can account for both increases and decreases of kinetic parameter values as a function of the nucleophile concentration. There is no indication of binding of the nucleophile to the enzyme-substrate complex before acylation takes place.     

Kinetics of the hydrolysis of N-benzoyl-l-serine methyl ester catalysed by bromelain and by papain. Analysis of modifier mechanisms by lattice nomography, computational methods of parameter evaluation for substrate-activated catalyses and consequences of postulated non-productive binding in bromelain- and papain-catalysed hydrolyses

1. N-Benzoyl-l-serine methyl ester was synthesized and evaluated as a substrate for bromelain (EC 3.4.22.4) and for papain (EC 3.4.22.2). 2. For the bromelain-catalysed hydrolysis at pH7.0, plots of [S(0)]/v(i) (initial substrate concn./initial velocity) versus [S(0)] are markedly curved, concave downwards. 3. Analysis by lattice nomography of a modifier kinetic mechanism in which the modifier is substrate reveals that concave-down [S(0)]/v(i) versus [S(0)] plots can arise when the ratio of the rate constants that characterize the breakdown of the binary (ES) and ternary (SES) complexes is either less than or greater than 1. In the latter case, there are severe restrictions on the values that may be taken by the ratio of the dissociation constants of the productive and non-productive binary complexes. 4. Concave-down [S(0)]/v(i) versus [S(0)] plots cannot arise from compulsory substrate activation. 5. Computational methods, based on function minimization, for determination of the apparent parameters that characterize a non-compulsory substrate-activated catalysis are described. 6. In an attempt to interpret the catalysis by bromelain of the hydrolysis of N-benzoyl-l-serine methyl ester in terms of substrate activation, the general substrate-activation model was simplified to one in which only one binary ES complex (that which gives rise directly to products) can form. 7. In terms of this model, the bromelain-catalysed hydrolysis of N-benzoyl-l-serine methyl ester at pH7.0, I=0.1 and 25 degrees C is characterized by K(m) (1) (the dissociation constant of ES)=1.22+/-0.73mm, k (the rate constant for the breakdown of ES to E+products, P)=1.57x10(-2)+/-0.32x10(-2)s(-1), K(a) (2) (the dissociation constant that characterizes the breakdown of SES to ES and S)=0.38+/-0.06m, and k' (the rate constant for the breakdown of SES to E+P+S)=0.45+/-0.04s(-1). 8. These parameters are compared with those in the literature that characterize the bromelain-catalysed hydrolysis of alpha-N-benzoyl-l-arginine ethyl ester and of alpha-N-benzoyl-l-arginine amide; K(m) (1) and k for the serine ester hydrolysis are somewhat similar to K(m) and k(cat.) for the arginine amide hydrolysis and K(as) and k' for the serine ester hydrolysis are somewhat similar to K(m) and k(cat.) for the arginine ester hydrolysis. 9. A previous interpretation of the inter-relationships of the values of k(cat.) and K(m) for the bromelain-catalysed hydrolysis of the arginine ester and amide substrates is discussed critically and an alternative interpretation involving substantial non-productive binding of the arginine amide substrate to bromelain is suggested. 10. The parameters for the bromelain-catalysed hydrolysis of the serine ester substrate are tentatively interpreted in terms of non-productive binding in the binary complex and a decrease of this type of binding by ternary complex-formation. 11. The Michaelis parameters for the papain-catalysed hydrolysis of the serine ester substrate (K(m)=52+/-4mm, k(cat.)=2.80+/-0.1s(-1) at pH7.0, I=0.1, 25.0 degrees C) are similar to those for the papain-catalysed hydrolysis of methyl hippurate. 12. Urea and guanidine hydrochloride at concentrations of 1m have only small effects on the kinetic parameters for the hydrolysis of the serine ester substrate catalysed by bromelain and by papain.     

Chicken avidin exhibits pseudo-catalytic properties. Biochemical, structural, and electrostatic consequences

Avidin and its bacterial analogue streptavidin exhibit similarly high affinities toward the vitamin biotin. The extremely high affinity of these two proteins has been utilized as a powerful tool in many biotechnological applications. Although avidin and streptavidin have similar tertiary and quaternary structures, they differ in many of their properties. Here we show that avidin enhances the alkaline hydrolysis of biotinyl p-nitrophenyl ester, whereas streptavidin protects this reaction even under extreme alkaline conditions (pH > 12). Unlike normal enzymatic catalysis, the hydrolysis reaction proceeds as a single cycle with no turnover because of the extremely high affinity of the protein for one of the reaction products (i.e. free biotin). The three-dimensional crystal structures of avidin (2 A) and streptavidin (2.4 A) complexed with the amide analogue, biotinyl p-nitroanilide, as a model for the p-nitrophenyl ester, revealed structural insights into the factors that enhance or protect the hydrolysis reaction. The data demonstrate that several molecular features of avidin are responsible for the enhanced hydrolysis of biotinyl p-nitrophenyl ester. These include the nature of a decisive flexible loop, the presence of an obtrusive arginine 114, and a newly formed critical interaction between lysine 111 and the nitro group of the substrate. The open conformation of the loop serves to expose the substrate to the solvent, and the arginine shifts the p-nitroanilide moiety toward the interacting lysine, which increases the electron withdrawing characteristics and consequent electrophilicity of the carbonyl group of the substrate. Streptavidin lacked such molecular properties, and analogous interactions with the substrate were consequently absent. The information derived from these structures may provide insight into the action of artificial protein catalysts and the evolution of catalytic sites in general.     

Probing the Ser-Ser-Lys catalytic triad mechanism of peptide amidase: computational studies of the ground state, transition state, and intermediate

Peptide amidase (Pam), a hydrolytic enzyme that belongs to the amidase signature (AS) family, selectively catalyzes the hydrolysis of the C-terminal amide bond (CO-NH(2)) of peptides. The recent availability of the X-ray structures of Pam, fatty acid amide hydrolase, and malonamidase E2 has led to the proposal of a novel Ser-Ser-Lys catalytic triad mechanism for the amide hydrolysis by the AS enzymes. The molecular dynamics (MD) simulations using the CHARMM force field were performed to explore the catalytic mechanism of Pam. The 1.8 A X-ray crystal structure of Pam in complex with the amide analogue of chymostatin was chosen for the initial coordinates for the MD simulations. The five systems that were investigated are as follows: (i) enzyme.substrate with Lys123-NH(2), (ii) enzyme.substrate with Lys123-NH(3)(+), (iii) enzyme.substrate with Lys123-NH(3)(+) and Ser226-O(-), (iv) enzyme.transition state, and (v) enzyme.tetrahedral intermediate. Our data support the presence of the hydrogen bonding network among the catalytic triad residues, Ser226, Ser202, and Lys123, where Ser226 acts as the nucleophile and Ser202 bridges Ser226 and Lys123. The MD simulation supports the catalytic role of the crystallographic waters, Wat1 and Wat2. In all the systems that have been studied, the backbone amide nitrogens of Asp224 and Thr223 create an oxyanion hole by hydrogen bonding to the terminal amide oxygen of the substrate, and stabilize the oxyanion tetrahedral intermediate. The results from both our computational investigation and previously published experimental pH profile support two mechanisms. In a mechanism that is relevant at lower pH, the Lys123-NH(3)(+)-Ser202 dyad provides structural support to the catalytic residue Ser226, which in turn carries out a nucleophilic attack at the substrate amide carbonyl in concert with Wat1-mediated deprotonation and stabilization of the tetrahedral transition state by the oxyanion hole. In the mechanism operating at higher pH, the Lys123-NH(2)-Ser202 catalytic dyad acts as a general base to assist addition of Ser226 to the substrate amide carbonyl. The results from the MD simulation of the tetrahedral intermediate state show that both Ser202 and Lys123 are possible candidates for protonation of the leaving group, NH(2), to form the acyl-enzyme intermediate.     

Ornithine carbamoyltransferase from Escherichia coli W. Purification, structure and steady-state kinetic analysis.

Ornithine carbamoyltransferase from Escherichia coli W was purified to homogeneity. The enzyme has a molecular weight of 105000. It is composed of three apparently identical subunits with molecular weights of 35000. The mechanism of the ornithine carbamoyltransferase enzyme system from E. coli W was investigated kinetically by using the approach of product inhibition and dead-end inhibition of both forward and reverse reactions. On the basis of the kinetic data and binding studies it appears that the mechanism of the reaction involves a compulsory sequence of substrate binding to the enzyme, in which carbamoylphosphate is the first substrate to bind to the enzyme and phosphate the last product to be released. The same studies also indicate that the mechanism involves dead-end complexes. The reaction mechanism appears consistent with that proposed by Theorell and Chance. Values have been determined for the Michaelis and dissociation constants involved in the combination of each reactant with the enzyme. Comparison of the values for the kinetic constants which are common to both forward and reverse reaction have shown that they are always of a comparable magnitude.     

Glycosylation of macrolide antibiotics. Purification and kinetic studies of a macrolide glycosyltransferase from Streptomyces antibioticus

The oleD gene has been identified in the oleandomycin producer Streptomyces antibioticus and it codes a macrolide glycosyltransferase that is able to transfer a glucose moiety from UDP-glucose (UDP-Glc) to many macrolides. The glycosyltransferase coded by the oleD gene has been purified 371-fold from a Streptomyces lividans clone expressing this protein. The reaction product was isolated, and its structure determined by NMR spectroscopy. The kinetic mechanism of the reaction was analyzed using the macrolide antibiotic lankamycin (LK) as substrate. The reaction operates via a compulsory order mechanism. This has been shown by steady-state kinetic studies and by isotopic exchange reactions at equilibrium. LK binds first to the enzyme, followed by UDP-glucose. A ternary complex is thus formed prior to transfer of glucose. UDP is then released, followed by the glycosylated lankamycin (GS-LK). A pH study of the reaction was performed to determine values for the molecular pK values, suggesting possible amino acid residues involved in the catalytic process.     

A rate-limiting stage of enteropeptidase hydrolysis

It has been shown for the first time that deacylation is the rate-limiting stage in the enteropeptidase-catalyzed hydrolysis of highly efficient oligopeptide substrates containing four Asp residues in positions P2-P5. On the other hand, the rate-limiting stage in the hydrolysis of low-efficiency peptide substrates containing less than four Asp or Glu residues in positions P2-P5 is acylation, as has previously been suggested for all amide and peptide substrates of serine proteases on the basis of the classic studies by Bender et al. The method of introduction of an additional nucleophile or another effector that selectively affects the deacylation stage was used to determine the rate-limiting stage in the enteropeptidase hydrolysis of Nalpha-benzyloxycarbonyl-L-lysine thiobenzyl ester, the highly efficient amide substrate GlyAsp4-Lys beta-naphthyl amide, and the low-efficiency peptide substrate VLSAADK-GNVKAAWG (where a hyphen denotes the hydrolysis site). The English version of the paper: Russian Journal of Bioorganic Chemistry, 2008, vol. 34, no. 2; see also http://www.maik.ru.     

A rate-limiting step of enteropeptidase hydrolysis

It has been shown for the first time that deacylation is the rate-limiting step in the enteropeptidase-catalyzed hydrolysis of highly effective oligopeptide substrates containing four Asp residues in positions P2–P5. On the other hand, the rate-limiting step in the hydrolysis of low-efficiency peptide substrates containing less than four Asp or Glu residues in positions P2–P5 is acylation, as it has previously been suggested for all amide and peptide substrates of serine proteases on the basis of classical works of Bender et al. The method of introduction of an additional nucleophile or another effector that selectively affects the deacylation step was used to determine the rate-limiting step in the enteropeptidase hydrolysis of N ^α-benzyloxycarbonyl-L-lysine thiobenzyl ester, the highly efficient amide substrate GlyAsp₄-Lys β-naphthyl amide, and the low-efficiency peptide substrate VLSAADK-GNVKAAWG (where a hyphen denotes the hydrolysis site).     

Energetics of thrombin-fibrinogen interaction.

The kinetic mechanism of thrombin-fibrinogen interaction has been elucidated by steady-state measurements of synthetic substrate hydrolysis by human alpha-thrombin in the presence of human fibrinogen used as a competitive inhibitor and sucrose used as a viscogenic agent. Sucrose greatly affects the FKm for thrombin-fibrinogen interaction, without altering the intrinsic properties of the system. Under conditions of pH 7.5 and 0.1 M NaCl, fibrinogen behaves like a sticky substrate for thrombin, with acylation being comparable to dissociation in the temperature range 20-37 degrees C. In the same temperature range, deacylation is much faster than acylation. The van't Hoff enthalpy of binding for thrombin-fibrinogen interaction is -24 +/- 3 kcal/mol and the entropy is -55 +/- 11 cal mol-1 deg-1. A chemical compensation effect is present in the binding of fibrinogen and synthetic amide substrates to thrombin, with the delta H and delta G values being linked through a linear relationship.     

Examination of the mechanism of human brain aspartoacylase through the binding of an intermediate analogue

Canavan disease is a fatal neurological disorder caused by the malfunctioning of a single metabolic enzyme, aspartoacylase, that catalyzes the deacetylation of N-acetyl-L-aspartate to produce L-aspartate and acetate. The structure of human brain aspartoacylase has been determined in complex with a stable tetrahedral intermediate analogue, N-phosphonomethyl-L-aspartate. This potent inhibitor forms multiple interactions between each of its heteroatoms and the substrate binding groups arrayed within the active site. The binding of the catalytic intermediate analogue induces the conformational ordering of several substrate binding groups, thereby setting up the active site for catalysis. The highly ordered binding of this inhibitor has allowed assignments to be made for substrate binding groups and provides strong support for a carboxypeptidase-type mechanism for the hydrolysis of the amide bond of the substrate, N-acetyl- l-aspartate.     

Streptomyces K15 DD-peptidase-catalysed reactions with ester and amide carbonyl donors.

In water, the purified 26 000-Mr membrane-bound DD-peptidase of Streptomyces K15 hydrolyses the ester carbonyl donor Ac2-L-Lys-D-Ala-D-lactate (release of D-lactate) and the amide carbonyl donor Ac2-L-Lys-D-Ala-D-Ala (release of D-alanine) with accumulation of acyl- (Ac2-L-Lys-D-alanyl-)enzyme. Whereas hydrolysis of the ester substrate proceeds to completion, hydrolysis of the amide substrate is negligible because of the capacity of the K15 DD-peptidase for utilizing the released D-alanine in a transfer reaction (Ac2-L-Lys-D-Ala-D-Ala + D-Ala----Ac2-L-Lys-D-Ala-D-Ala + D-Ala) that maintains the concentration of the amide substrate at a constant level. In the presence of an amino acceptor X-NH2 (Gly-Gly or Gly-L-Ala) related to the Streptomyces peptidoglycan, both amide and ester carbonyl donors are processed without detectable accumulation of acyl-enzyme. Under proper conditions, the acceptor activity of water and, in the case of the amide substrate, the acceptor activity of the released D-alanine can be totally overcome so that the two substrates are quantitatively converted into transpeptidated product Ac2-L-Lys-D-Ala-NH-X (and hydrolysis is prevented). Experimental evidence suggests that the amino acceptor modifies both the binding of the carbonyl donor to the enzyme and the ensuing rate of enzyme acylation.     

Prenyltransferase: the mechanism of the reaction.

The enzyme, prenyltransferase, which normally catalyzes the addition of an allylic pyrophosphate to isopentenyl pyrophosphate, has been found to catalyze the hydrolysis of its allylic substrate. The rate of this hydrolysis is markedly stimulated by inorganic pyrophosphate. Competition experiments with 2-fluoroisopentenyl pyrophosphate and inorganic pyrophosphate demonstrated that inorganic pyrophosphate stimulated hydrolysis by binding at the isopentenyl pyrophosphate site. Hydrolysis carried out in H218O or with (1S)-[1-3H]geranyl pyrophosphate show the C-O bond is broken and the C1 carbon of geranyl pyrophosphate is inverted in the process. These results are interpreted to favor a carbonium ion mechanism for the prenyltransferase reaction.     

17 beta-Hydroxysteroid dehydrogenase of the sheep ovary : purification, properties and substrate binding site.

Sheep ovarian 17 beta HSDH has been purified about 1000 fold to a specific activity of 0.5 IU/mg protein, using DEAE cellulose chromatography, affinity chromatography on estrone-amino caproate-Sepharose and a second DEAE cellulose chromatography. The molecular weight is 70,000 ; the pH optimum for activity is 9.2 and the energy of activation is 16.5 Kcal/mole. The kinetics of the oxidation of estradiol and many analogues have been studied at various concentrations and in the presence of different amounts of coenzyme. The data are in agreement with a compulsory order mechanism with the binding of NAD+ as the first substrate. Sheep ovarian 17 beta HSDH accepts subtituents in position C3, C11, C13 ; the substrate binding site is open in this region. On the contrary, the binding requirements are strict for the region of C10 since the presence of a C19 methyl group impairs binding and (or) oxidation of the steroid. Sheep ovarian and human placental 17 beta HSDH have close analogies : molecular weight, pH optimum, substrate binding site requirements. Their reaction mechanisms are different : random for the placental 17 beta HSDH, compulsory order for the ovarian 17 beta HSDH : this can be explained by the effect of the coenzyme upon the binding of the substrate : without effect on placental enzyme, the coenzyme fixation enhances the affinity of the ovarian 17 beta HSDH for any substrate.     

Influence of solvent and water activity on kinetically controlled peptide synthesis.

alpha-Chymotrypsin deposited on Celite was used to catalyse peptide synthesis reactions between N-protected amino acid esters and leucine amide in organic media with low water content. The influence of the solvent and the thermodynamic water activity on the reaction kinetics was studied. The substrate specificity in the reactions was shown to be a combination of the substrate specificity of the enzyme in aqueous media and the influence of the solvents. The magnitude of the solvent effects differed greatly depending on the substrates used. In hydrophobic solvents high reaction rates were observed and the competing hydrolysis of the ester substrate occurred to only a minor extent. Reactions occurred at water activities as low as 0.11, but the rate constants increased with increasing water activity and were about two orders of magnitude higher at the highest water activity tested (0.97).     

Modeling of the enzymatic kinetic synthesis of cephalexin--influence of substrate concentration and temperature

During enzymatic kinetic synthesis of cephalexin, an activated phenylglycine derivative (phenylglycine amide or phenylglycine methyl ester) is coupled to the nucleus 7-aminodeacetoxycephalosporanic acid (7-ADCA). Simultaneously, hydrolysis of phenylglycine amide and hydrolysis of cephalexin take place. This results in a temporary high-product concentration that is subsequently consumed by the enzyme. To optimize productivity, it is necessary to develop models that predict the course of the reaction. Such models are known from literature but these are only applicable for a limited range of experimental conditions. In this article a model is presented that is valid for a wide range of substrate concentrations (0-490 mM for phenylglycine amide and 0-300 mM for 7-ADCA) and temperatures (273-298 K). The model was built in a systematic way with parameters that were, for an important part, calculated from independent experiments. With the constants used in the model not only the synthesis reaction but also phenylglycine amide hydrolysis and cephalexin hydrolysis could be described accurately. In contrast to the models described in literature, only a limited number (five) of constants was required to describe the reaction at a certain temperature. For the temperature dependency of the constants, the Arrhenius equation was applied, with the constants at 293 K as references. Again, independent experiments were used, which resulted in a model with high statistic reliability for the entire temperature range. Low temperatures were found beneficial for the process because more cephalexin and less phenylglycine is formed. The model was used to optimize the reaction conditions using criteria such as the yield on 7-ADCA or on activated phenylglycine. Depending on the weight of the criteria, either a high initial phenylglycine amide concentration (yield on 7-ADCA) or a high initial 7-ADCA concentration (yield on phenylglycine amide) is beneficial.     

Activity and regulation of an amidase (acylamide amidohydrolase,EC 3.5.1.4) with a wide substrate spectrum from a Brevibacterium sp.

The Brevibacterium R 312 strain has an amidase with a wide substrate spectrum previously named acetamidase. The study of its activity showed that this enzyme was able to hydrolyze a large number of amides into their corresponding organic acids. The affinity of this enzyme for the substrates varied according to the length of the carbon chain and the spatial crowding of the molecule. The comparison of the specific rates of hydrolysis showed that propionamide was the amide substrate most quickly hydrolyzed.We confirmed the inducible feature of this enzyme and noted that only acetamide and N-methylacetamide were inducers of this enzyme among the compounds tested. Thioacetamide and N-methylpropionamide, both as amide analogues, were shown to inhibit the biosynthesis of acetamidase. Similarly, the organic acids, products of the hydrolysis reaction, showed a strong repression action on the biosynthesis of the enzyme.