The determination of the concentration of hydrolytic enzyme solutions: alpha-chymotrypsin, trypsin, papain, elastase, subtilisin, and acetylcholinesterase

Interactions of human acetylcholinesterase with phenyl valerate and acetylthiocholine: Thiocholine as an enhancer of phenyl valerate esterase activity

Phenyl valerate (PV) is a neutral substrate for measuring the PVase activity of neuropathy target esterase (NTE), a key molecular event of organophosphorus-induced delayed neuropathy. This substrate has been used to discriminate and identify other proteins with esterase activity and potential targets of organophosphorus (OP) binding. A protein with PVase activity in chicken (model for delayed neurotoxicity) was identified as butyrylcholinesterase (BChE). Further studies in human BChE suggest that other sites might be involved in PVase activity. From the theoretical docking analysis, other more favorable sites for binding PV related to the Asn289 residue located far from the catalytic site ("PVsite") were deduced.In this paper, we demonstrate that acetylcholinesterase is also able to hydrolyze PV. Robust kinetic studies of interactions between substrates PV and acetylthiocholine (AtCh) were performed. The kinetics did not fit the classic competition models among substrates. While PV interacts as a competitive inhibitor in AChE activity, AtCh at low concentrations enhances PVase activity and inhibits this activity at high concentrations. Kinetic behavior suggests that the potentiation effect is caused by thiocholine released at the active site, where AtCh could act as a Trojan Horse. We conclude that the products released at the active site could play an important role in the hydrolysis reactions of different substrates in biological systems.

Development of an active site titration reagent for α-amylases

α-Amylases are among the most widely used classes of enzymes in industry and considerable effort has gone into optimising their activities. Efforts to find better amylase mutants, such as through high-throughput screening, would be greatly aided by access to precise and robust active site titrating agents for quantitation of active mutants in crude cell lysates. While active site titration reagents designed for retaining β-glycosidases quantify these enzymes down to nanomolar levels, convenient titrants for α-glycosidases are not available. We designed such a reagent by incorporating a highly reactive fluorogenic leaving group onto unsaturated cyclitol ethers, which have been recently shown to act as slow substrates for retaining glycosidases that operate via a covalent 'glycosyl'-enzyme intermediate. By appending this warhead onto the appropriate oligosaccharide, we developed efficient active site titration reagents for α-amylases that effect quantitation down to low nanomolar levels.

Enabling protein-hosted organocatalytic transformations

In this review, the development of organocatalytic artificial enzymes will be discussed. This area of protein engineering research has underlying importance, as it enhances the biocompatibility of organocatalysis for applications in chemical and synthetic biology research whilst expanding the catalytic repertoire of enzymes. The approaches towards the preparation of organocatalytic artificial enzymes, techniques used to improve their performance (selectivity and reactivity) as well as examples of their applications are presented. Challenges and opportunities are also discussed.

Two novel S1 peptidases from Amycolatopsis keratinophila subsp. keratinophila D2T degrading keratinous slaughterhouse by-products

Two proteases, named C- and T-like proteases, respectively, were purified from the culture supernatant of Amycolatopsis keratinophila subsp. keratinophila D2^T grown on a keratinous slaughterhouse by-product of pig bristles and nails as sole nitrogen and carbon source. The two proteases belong to peptidase family S1 as identified by mass spectrometric peptide mapping, have low mutual sequence identity (25.8%) and differ in substrate specificity. T-like protease showed maximum activity at 40 °C and pH 8-9, and C-like protease at 60 °C and pH 8-10. Peptides released from the keratinous by-product were identified by mass spectrometry and indicated P1 specificity for arginine and lysine of T-like and alanine, valine and isoleucine of C-like protease as also supported by the activity of the two proteases towards synthetic peptide and amino acid substrates. The specific activities of the C- and T-like proteases and proteinase K on keratin azure and azokeratin were comparable. However, C- and T-like proteases showed 5-10-fold higher keratin/casein (K/C) activity ratios than that of another S1 and two keratin-degrading S8 peptidases used for comparison. The findings support that the range of peptidase families considered to contain keratinases should be expanded to include S1 peptidases. Furthermore, the results indicated the quite thermostable C-like protease to be a promising candidate for use in industrial degradation of keratinous slaughterhouse by-products.

Quantification of the total neuraminidase content of recent commercially-available influenza vaccines: Introducing a neuraminidase titration reagent

The protective immunological effects of the influenza neuraminidase (NA) surface protein are of renewed interest but NA content in vaccines remains unstandardized and methods to easily and reliably quantify NA content are unsatisfactory. We describe the use of a recently developed fluorometric titration reagent, TR1, to efficiently quantify the total enzymatically active NA content of six commercially-available influenza vaccines, including split/subunit, inactivated/live and standard /high dose products distributed from 2015/16 to 2017/18 in North America. Considerable differences in active NA content were measured between influenza vaccine products for the same season, with relative content differences between brands generally maintained across seasons. These results highlight the simplicity of use of this reagent, and its unique ability to quantitate enzymatically active NA without the need for specific activities of individual enzymes. The reagent could also prove valuable in assessing the importance of using fully active enzyme to generate protective immune responses.

Ultrasensitive Fluorogenic Reagents for Neuraminidase Titration

Influenza viral neuraminidase plays a crucial role during infections. It is a major target for the development of anti-influenza drugs and is also attracting increasing attention as a vaccine target as evidence accumulates that neuraminidase-neutralizing antibodies contribute to protection. However, no method currently exists to accurately and efficiently measure concentrations of active neuraminidase in virus samples or other crude mixtures, which hampers development on both fronts. In this report, we describe the development of a selective and sensitive active-site titration reagent for neuraminidase that can quantify viral neuraminidases down to sub-nanomolar levels in crude samples, with no background from non-viral neuraminidases. By using this reagent, we determined accurate k_cat values for six influenza A and two influenza B neuraminidases for the first time. We also quantified the neuraminidase content in a commercial influenza vaccine, thus demonstrating that this titration reagent opens the possibility for better vaccine analysis.

High performance enzyme kinetics of turnover, activation and inhibition for translational drug discovery

INTRODUCTION

Enzymes are the macromolecular catalysts of many living processes and represent a sizable proportion of all druggable biological targets. Enzymology has been practiced just over a century during which much progress has been made in both the identification of new enzymes and the development of novel methodologies for enzyme kinetics. Areas covered: This review aims to address several key practical aspects in enzyme kinetics in reference to translational drug discovery research. The authors first define what constitutes a high performance enzyme kinetic assay. The authors then review the best practices for turnover, activation and inhibition kinetics to derive critical parameters guiding drug discovery. Notably, the authors recommend global progress curve analysis of dose/time dependence employing an integrated Michaelis-Menten equation and global curve fitting of dose/dose dependence. Expert opinion: The authors believe that in vivo enzyme and substrate abundance and their dynamics, binding modality, drug binding kinetics and enzyme's position in metabolic networks should be assessed to gauge the translational impact on drug efficacy and safety. Integrating these factors in a systems biology and systems pharmacology model should facilitate translational drug discovery.

Oximate metal complexes breaking the limiting esterolytic reactivity of oximate anions

Zinc(II) and cadmium(II) complexes of a tridentate oximate ligand cleave 4-nitrophenyl acetate with rate constants surpassing by two orders of magnitude those reported as the maximum possible level for highly basic free oximate anions as a result of removal of the "solvational imbalance" of the nucleophile by metal coordination.

Identification of the first synthetic inhibitors of the type II transmembrane serine protease TMPRSS2 suitable for inhibition of influenza virus activation

TMPRSS2 (transmembrane serine proteinase 2) is a multidomain type II transmembrane serine protease that cleaves the surface glycoprotein HA (haemagglutinin) of influenza viruses with a monobasic cleavage site, which is a prerequisite for virus fusion and propagation. Furthermore, it activates the fusion protein F of the human metapneumovirus and the spike protein S of the SARS-CoV (severe acute respiratory syndrome coronavirus). Increased TMPRSS2 expression was also described in several tumour entities. Therefore TMPRSS2 emerged as a potential target for drug design. The catalytic domain of TMPRSS2 was expressed in Escherichia coli and used for an inhibitor screen with previously synthesized inhibitors of various trypsin-like serine proteases. Two inhibitor types were identified which inhibit TMPRSS2 in the nanomolar range. The first series comprises substrate analogue inhibitors containing a 4-amidinobenzylamide moiety at the P1 position, whereby some of these analogues possess inhibition constants of approximately 20 nM. An improved potency was found for a second type derived from sulfonylated 3-amindinophenylalanylamide derivatives. The most potent derivative of this series inhibits TMPRSS2 with a K(i) value of 0.9 nM and showed an efficient blockage of influenza virus propagation in human airway epithelial cells. On the basis of the inhibitor studies, a series of new fluorogenic substrates containing a D-arginine residue at the P3 position was synthesized, some of them were efficiently cleaved by TMPRSS2.

DNA-based sensor for real-time measurement of the enzymatic activity of human topoisomerase I

Sensors capable of quantitative real-time measurements may present the easiest and most accurate way to study enzyme activities. Here we present a novel DNA-based sensor for specific and quantitative real-time measurement of the enzymatic activity of the essential human enzyme, topoisomerase I. The basic design of the sensor relies on two DNA strands that hybridize to form a hairpin structure with a fluorophore-quencher pair. The quencher moiety is released from the sensor upon reaction with human topoisomerase I thus enabling real-time optical measurement of enzymatic activity. The sensor is specific for topoisomerase I even in raw cell extracts and presents a simple mean of following enzyme kinetics using standard laboratory equipment such as a qPCR machine or fluorimeter. Human topoisomerase I is a well-known target for the clinically used anti-cancer drugs of the camptothecin family. The cytotoxic effect of camptothecins correlates directly with the intracellular topoisomerase I activity. We therefore envision that the presented sensor may find use for the prediction of cellular drug response. Moreover, inhibition of topoisomerase I by camptothecin is readily detectable using the presented DNA sensor, suggesting a potential application of the sensor for first line screening for potential topoisomerase I targeting anti-cancer drugs.

Highly potent inhibitors of proprotein convertase furin as potential drugs for treatment of infectious diseases

Optimization of our previously described peptidomimetic furin inhibitors was performed and yielded several analogs with a significantly improved activity. The most potent compounds containing an N-terminal 4- or 3-(guanidinomethyl)phenylacetyl residue inhibit furin with K(i) values of 16 and 8 pM, respectively. These analogs inhibit other proprotein convertases, such as PC1/3, PC4, PACE4, and PC5/6, with similar potency, whereas PC2, PC7, and trypsin-like serine proteases are poorly affected. Incubation of selected compounds with Madin-Darby canine kidney cells over a period of 96 h revealed that they exhibit great stability, making them suitable candidates for further studies in cell culture. Two of the most potent derivatives were used to inhibit the hemagglutinin cleavage and viral propagation of a highly pathogenic avian H7N1 influenza virus strain. The treatment with inhibitor 24 (4-(guanidinomethyl)phenylacetyl-Arg-Val-Arg-4-amidinobenzylamide) resulted in significantly delayed virus propagation compared with an inhibitor-free control. The same analog was also effective in inhibiting Shiga toxin activation in HEp-2 cells. This antiviral effect, as well as the protective effect against a bacterial toxin, suggests that inhibitors of furin or furin-like proprotein convertases could represent promising lead structures for future drug development, in particular for the treatment of infectious diseases.

Gly or Ala substitutions for Pro(210)Thr(211)Asn(212) at the β8-β9 turn of subtilisin Carlsberg increase the catalytic rate and decrease thermostability

A comparison of the primary structures among psychrophilic, mesophilic, and thermophilic subtilases revealed that the turn between the β8 and β9 strands (β8-β9 turn, BPN' numbering) of psychrophilic subtilases are more flexible than those of their mesophilic and thermophilic counterparts. To investigate the relationship between structure of this turn and enzyme activity as well as thermostability of mesophilic subtilisin Carlsberg (sC), we analyzed 6 mutants of sC with a single, double, or triple Gly or Ala substitutions for Pro(210)Thr(211)Asn(212) at the β8-β9 turn. Among the single Gly substitutions, the P210G substitution most significantly (1.5-fold) increased the specific activity on N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide (AAPF) substrate and 12-fold decreased the thermostability. All mutants tested showed the increased k(cat) for the AAPF substrate and reduced thermostability compared with the wild-type sC. The k(cat) values of the P210G, P210G/T211G, and P210G/T211G/N212G mutants were 1.5-, 1.7-, and 1.8-fold higher than that of the wild-type sC. There were significant positive correlations between k(cat) and thermal inactivation rates as well as k(cat) and K(m) of the wild-type and mutants. These results demonstrate that the structure of β8-β9 turn, despite its distance from the active site, has significant effects on the catalytic rate and thermostability of sC through a global network of intramolecular interactions and suggest that the lack of flexibility of this turn stabilizes the wild-type sC against thermal inactivation in compensation for some loss of catalytic activity.

Influence of Leaving-Group Electronic Effect on alpha-Chymotrypsin: Catalytic Constants of Specific Substrates

Rate constants and binding constants for the alpha-chymotrypsin-catalyzed hydrolysis of N-acetyltyrosine, tryptophan, and phenylalanine anilides are presented. Both k(cat) and K(m) are independent of electronic effects in the substrate over a range of 9.8 orders of magnitude (as measured by pK of the leaving group). Similarly, K(m) is independent of charge and orientation about the alpha-carbon for various substrates and pseudo-substrates. These results are not consistent with the pretransition state protonation hypothesis; instead, they are discussed in terms of a tetrahedral intermediate that is thermodynamically less stable than the Michaelis complex.

A study of the unfolding of the inhibited subtilisin in guanidine hydrochloride

Inhibited subtilisin (Subtilism Carlsberg; Subtilopeptidase A) is unfolded in the presence of 7 M guanidine hydrochloride. The unfolding reaches a maximum in approximately 6 min at 20C at pH 8.0. This is demonstrated by an increase of the mean residue ellipticity at 222 nm from -8.02 x 10-3 to -1.72 x 10-3 deg. cm-2/decimole. The unfolding is partially reversible and this reversibility is favoured by lower concentrations of enzyme. The fact that the refolding process is not complete may be attributed to either the demonstrated self association of the denatured enzyme or to interference of non-covalently bound autolysis peptides.

Direct measurement of metal ion chelation in the active site of human ferrochelatase

The final step in heme biosynthesis, insertion of ferrous iron into protoporphyrin IX, is catalyzed by protoporphyrin IX ferrochelatase (EC 4.99.1.1). We demonstrate that pre-steady state human ferrochelatase (R115L) exhibits a stoichiometric burst of product formation and substrate consumption, consistent with a rate-determining step following metal ion chelation. Detailed analysis shows that chelation requires at least two steps, rapid binding followed by a slower (k approximately 1 s-1) irreversible step, provisionally assigned to metal ion chelation. Comparison with steady state data reveals that the rate-determining step in the overall reaction, conversion of free porphyrin to free metalloporphyrin, occurs after chelation and is most probably product release. We have measured rate constants for significant steps on the enzyme and demonstrate that metal ion chelation, with a rate constant of 0.96 s-1, is approximately 10 times faster than the rate-determining step in the steady state (kcat = 0.1 s-1). The effect of an additional E343D mutation is apparent at multiple stages in the reaction cycle with a 7-fold decrease in kcat and a 3-fold decrease in kchel. This conservative mutation primarily affects events occurring after metal ion chelation. Further evaluation of structure-function data on site-directed mutants will therefore require both steady state and pre-steady state approaches.

Biochemical and molecular modeling analysis of the ability of two p-aminobenzamidine-based sorbents to selectively purify serine proteases (fibrinogenases) from snake venoms

Snake venoms contain several trypsin-like enzymes with equivalent physicochemical characteristics and similar inhibition profiles. These are rather difficult to separate by classical purification procedures and therefore constitute a good model for affinity chromatography analysis. Some of these trypsin homologues present fibrinogenase activity, mimicking one or more features of the central mammalian coagulation enzyme, thrombin. It was previously demonstrated that a number of amidine derivatives are able to interact specifically with some of these serine proteases. To understand the enzyme-sorbent interactions we have investigated the ability of two commercially available benzamidine affinity matrices to purify thrombin-like serine proteases (TLSP) with similar biological properties from two snake venoms (Bothrops jararacussu and Lachesis muta rhombeata). Curiously, each sorbent retained a single but distinct TLSP from each venom with high yield. Molecular modeling analysis suggested that hydrophobic interactions within a specific region on the surface of these enzymes could be generated to explain this exquisite specificity. In addition, it was demonstrated that a specific tandem alignment of the two benzamidine sorbents enables the purification of three other enzymes from B. jararacussu venom.

Characterization of p-aminobenzamidine-based sorbent and its use for high-performance affinity chromatography of trypsin-like proteases

An affinity sorbent, hydrophilic polymer-based carrier of different pore size (Toyopearl) with immobilized p-aminobenzamidine (ABA), has been prepared. Its basic properties and some applications for protein purification were studied. ABA, which is a synthetic inhibitor for trypsin-like proteases, was covalently immobilized to Toyopearl by reductive amination. The ligand density and binding capacity for porcine trypsin varied depending on the pore size of Toyopearl. The maximum binding capacity of the immobilized p-aminobenzamidine Toyopearl (ABA-Toyopearl) for trypsin was more than 40 mg/ml gel. ABA-Toyopearl thus obtained was very stable below pH 8 and was successfully used for high-performance affinity chromatography of trypsin-like proteases such as trypsin, thrombin, tissue-type plasminogen activator or urokinase in a single step at 25 degrees C.

3-Bromophenyl 6-acetoxymethyl-2-oxo-2H-1-benzopyran-3-carboxylate inhibits cancer cell invasion in vitro and tumour growth in vivo

In search for new anticancer agents, we have evaluated the antiinvasive and antimigrative properties of recently developed synthetic coumarin derivatives among which two compounds revealed important activity: 3-chlorophenyl 6-acetoxymethyl-2-oxo-2H-1-benzopyran-3-carboxylate and 3-bromophenyl 6-acetoxymethyl-2-oxo-2H-1-benzopyran-3-carboxylate. Both drugs were able to inhibit cell invasion markedly in a Boyden chamber assay, the bromo derivative being more potent than the reference matrix metalloprotease (MMP) inhibitor GI 129471. In vivo, tumour growth was reduced when nude mice grafted with HT1080 or MDA-MB231 cells were treated i.p. 3 days week(-1) with the bromo coumarin derivative. These effects were not associated with the inhibition of urokinase, plasmin, MMP-2 or MMP-9. The mechanism of action of the drugs remains to be elucidated. However, these two coumarin derivatives may serve as new lead compounds of an original class of antitumour agents.

Introduction of the new dipeptide isostere 7-endo-BtA as reverse turn inducer in a Bowman-Birk proteinase inhibitor: synthesis and conformational analysis

Two dipeptide isosteres 7-exo-BTG (1) and 7-endo-BtA (2), belonging to the new class of gamma/delta-bicyclic amino acid BTAa, were inserted into an 11-residue peptide deriving from the Bowman Birk Inhibitor (BBI) class of serine protease inhibitors, and the conformational properties of these modified peptides have been studied by NMR and molecular modelling. The dipeptide isostere 7-endo-BtA [(1R,4S,5R,7R)-4-endo-methyl-6,8-dioxa-3-azabicyclo[3.2.1]octane-7-endo-carboxylic acid] (2), derived from L-alanine and meso tartaric acid, gave rise to the modified BBI peptide 5 whose structure was very similar to that of the original peptide 3, suggesting a possible reverse turn inducing property for this dipeptide isostere.

Kinetic and titration methods for determination of active site contents of enzyme and catalytic antibody preparations

Kinetic characterization of enzymes and analogous catalysts such as catalytic antibodies requires knowledge of the molarity of functional sites. Various stoichiometric titration methods are available for the determination of active-site concentrations of some enzymes and these are exemplified in the second part of this article. Most of these are not general in that they require the existence of certain types of either intermediate or active-site residues that are susceptible to specific covalent modification. Thus they are not readily applicable to many enzymes and they are rarely available currently for titration of catalytic antibody active sites. In the first part of the article we discuss a general kinetic method for the investigation of active-site availability in preparations of macromolecular catalysts. The method involves steady-state kinetics to provide Vmax and Km and single-turnover first-order kinetics using excess of catalyst over substrate to provide the analogous parameters k(obs)lim and K(m)app. The active-site contents of preparations that contain only active catalyst (Ea) and inert material (Ei) may be calculated as [Ea](T) = Vmax)/k(obs)lim. This is true even if nonproductive binding to E(a) occurs. For polyclonal catalytic antibody preparations, which may contain binding but noncatalytic material (Eb) in addition to Ea and Ei, the significance of Vmax/k(obs)lim is more complex but provides an upper limit to E(a). This can be refined by consideration of the relative values of Km and the equilibrium dissociation constant of EbS. Analysis of the Ea, Eb, Ei system requires the separate determination of Ei. For catalytic antibodies this may be achieved by analytical affinity chromatography using an immobilized hapten or hapten analog and an ELISA procedure to ensure the clean separation of Ei from the Ea + Eb mixture.

The rate-determining step in P450 C21-catalyzing reactions in a membrane-reconstituted system

Adrenal cytochrome P450 C21 in a membrane-reconstituted system catalyzed 21-hydroxylation of 17alpha-hydroxyprogesterone at a rate higher than that for progesterone in the steady state at 37 degrees C. The rate of product formation in the steady state increased with the concentration of the complex between P450 C21 and the reductase in the membranes. The complex formation was independent of the volume of the reaction, showing that the effective concentrations of the membrane proteins should be defined with the volume of the lipid phase. The rates of conversion of progesterone and 17alpha-hydroxyprogesterone to the product in a single cycle of the P450 C21 reaction were measured with a reaction rapid quenching device. The first-order rate constant for the conversion of progesterone by P450 C21 was 4.3 +/- 0.7 s(-)1, and that for 17alpha-hydroxyprogesterone was 1.8 +/- 0.5 s(-)1 at 37 degrees C. It was found from the analysis of kinetic data that the rate-determining step in 21-hydroxylation of progesterone in the steady state was the dissociation of product from P450 C21, whereas the conversion to deoxycortisol was the rate-determining step in the reaction of 17alpha-hydroxyprogesterone. The difference in the rate-determining steps in the reactions for the two substrates was clearly demonstrated in the pre-steady-state kinetics.

Functional expression on bacteriophage of the mustard trypsin inhibitor MTI-2

The mustard trypsin inhibitor MTI-2 is a potential tool in the study of interactions between pest insects and plants. It can be applied to study the adaptations of digestive proteases in pest insects. Phage display allows a rapid and exhaustive system for the selection of heterologous protein variants with novel specificities. Here we describe a bacteriophage expression system which permits functional expression of MTI-2 variants. Active and inactive mutants of MTI-2 are constructed and displayed on phage. These are used to demonstrate that an active variant can be selected from a background of 10,000 inactive mutants in four rounds of selection and amplification.

Identification of chymotrypsin inhibitors from a second-generation template assisted combinatorial peptide library

In an earlier study (McBride JD, Freeman N, Domingo GJ, Leatherbarrow RJ. Selection of chymotrypsin inhibitors from a conformationally-constrained combinatorial peptide library. J. Mol. Biol. 1996; 259: 819-827) we described a resin-bound cyclic peptide library, constructed based on the sequence of the anti-tryptic reactive site loop of Bowman Birk Inhibitor (BBI), a proteinase inhibitor protein. This library was used to identify re-directed chymotrypsin inhibitors with Ki values as low as 17 nM. We have now extended this work by constructing an enhanced library in which a further position, at the P4 site of the inhibitor, has been randomized. This new library has variation at three target locations (P4, P1 and P2) within the inhibitory loop region, producing 8,000 variants. Screening this library allowed selection of new inhibitor sequences with Ki values as low as 3.4 nM. The success of this approach is reflected by the fact that the inhibition constant given by the selected peptide sequence is slightly lower than that reported against chymotrypsin for the most studied full length BBI protein, Soybean BBI 2-IV.

The kinetic basis of a general method for the investigation of active site content of enzymes and catalytic antibodies: first-order behaviour under single-turnover and cycling conditions

The theoretical foundation has been laid for the investigation of catalytic systems using first-order kinetics and for a general kinetic method of investigation of the active site content, E(a), of enzymes, catalytic antibodies, and other enzyme-like catalysts. The method involves a combination of steady-state and single-turnover kinetics to provide Vmax and Km and k(lim)(obs) and K(app)(m), respectively. The validity of the method is shown to remain valid for two extensions of the simple two-step enzyme catalysis model (a) when the catalyst preparation contains molecules (Eb) that bind substrate but fail to catalyse product formation and (b) when the catalyst itself binds substrate non-productively as well as productively. The former is a particularly serious complication for polyclonal catalytic antibodies and the latter a potential complication for all catalysts. For the simple model and for (b) Vmax/k(lim)(obs) provides the value of [Ea]T and for (a) its upper limit. This can be refined by consideration of the relative values of Km and the equilibrium dissociation constant of EbS. For the polyclonal catalytic antibody preparation investigated, the fact that K(app/m) > Km demonstrates for the first time the presence of a substrate-binding but non-catalytic component in a polyclonal preparation. First-order behaviour in catalytic systems occurs not only with a large excess of catalyst over substrate but also with lower catalyst/substrate ratios, including the equimolar condition, when K(app)(m) >> [S]0, a phenomenon that is not widely appreciated.

Characterization of potato proteinase inhibitor II reactive site mutants

Potato proteinase inhibitor II (PI-2) is composed of two sequence repeats. It contains two reactive site domains. We developed an improved protocol for the production of PI-2 using the yeast Pichia pastoris as the expression host. We then assessed the role of its two reactive sites in the inhibition of trypsin and chymotrypsin by mutating each of the two reactive sites in various ways. From these studies it appears that the second reactive site strongly inhibits both trypsin (Ki = 0.4 nM) and chymotrypsin (Ki = 0.9 nM), and is quite robust towards mutations at positions P2 or P1'. In contrast, the first reactive site inhibits only chymotrypsin (Ki = 2 nM), and this activity is very sensitive to mutations. Remarkably, replacing the reactive site amino acids of domain I with those of domain II did not result in inhibitory activities similar to domain II. The fitness for protein engineering of each domain is discussed.

Estimation of inhibitory organophosphates with purified pig liver carboxylesterase

Organophosphates that inhibit acetylcholinesterase normally also inhibit pig liver carboxylesterase irreversibly. Since this liver esterase is well characterized and easily accessible in large amounts, we propose the use of this enzyme for the quantitation of low concentrations of such organophosphates. The principle of two estimation methods is described. Both methods involve the addition of an unknown amount of organophosphate to an assay mixture of purified esterase, buffer and a low affinity esterase substrate. In the first of these methods, the inhibitor concentration is calculated from the esterase activities before and after the addition of the inhibitor. In the second method, the amounts of inhibitor or of enzyme are changed in several assays, until equimolar conditions can be detected from the observed reaction kinetics. The theoretical background of these methods is discussed and practical examples for the estimation of paraoxon (order of 0.1 nmoles) are given.

From detergent additive to semisynthetic peroxidase-simplified and up-scaled synthesis of seleno-subtilisin

A simplified and up-scaled synthesis of the semisynthetic peroxidase seleno-subtilisin was developed. Highly purified to technical grade subtilisin preparations from Bacillus licheniformis and Bacillus amyloliquefaciens were applied as starting materials. Activation of Ser 221 with phenylmethanesulfonyl fluoride, nucleophilic substitution by sodium hydrogen selenide, and oxidation to the seleninic acid with hydrogen peroxide completed the chemical active-site modification. The reactions were accomplished with a minimum of simple work-up procedures in 10 g scale. Kinetics and enantioselectivity of the preparations were tested using 1-phenylethyl hydroperoxide. For the first time, an up-scaled synthesis of a semisynthetic enzyme is available. Copyright 1998 John Wiley & Sons, Inc.

The role of threonine in the P2 position of Bowman-Birk proteinase inhibitors: studies on P2 variation in cyclic peptides encompassing the reactive site loop

Previously, we have described a template-assisted combinatorial peptide library based on the anti-tryptic reactive site loop of a Bowman-Birk inhibitor (BBI). Sequences that displayed inhibitory activity re-directed towards chymotrypsin were found to have a consensus binding motif, with their most striking feature being that exclusively threonine was found at the P2 position. The present study investigates the reason for this surprising specificity by maintaining the binding motif but systematically varying the P2 residue. From analysis of 26 variants, it is found that the requirements for inhibitory activity at P2 are finely tuned, and in agreement with the library work, threonine at P2 provides optimal inhibition. In addition, peptides with threonine at P2 are significantly less susceptible to hydrolysis. Examination of all available BBI sequences shows that threonine is very highly conserved at P2, which implies that the functional requirement extends to the full-length BBI protein. Our results are consistent with a dual requirement for hydrophobic recognition within the S2 pocket and maintenance of an inhibitory conformation via hydrogen bonding within the reactive-site loop. As the isolated peptide loop reproduces the active region of full-length BBI, these results explain why threonine is well conserved at P2 in this class of inhibitor. Furthermore, they illustrate that proteinase inhibitor specificity can have characteristics that are not easily predicted from information on the substrate preferences of a proteinase.

Adaptation of Spodoptera exigua larvae to plant proteinase inhibitors by induction of gut proteinase activity insensitive to inhibition

Tobacco plants were transformed with a cDNA clone of chymotrypsin/trypsin-specific potato proteinase inhibitor II (PI2) under the control of a constitutive promoter. Although considerable levels of transgene expression could be demonstrated, the growth of Spodoptera exigua larvae fed with detached leaves of PI2-expressing plants was not affected. Analysis of the composition of tryptic gut activity demonstrated that only 18% of the proteinase activity of insects reared on these transgenic plants was sensitive to inhibition by PI2, whereas 78% was sensitive in insects reared on control plants. Larvae had compensated for this loss of tryptic activity by a 2.5-fold induction of new activity that was insensitive to inhibition by PI2. PI2-insensitive proteolytic activity was also induced in response to endogenous proteinase inhibitors of tobacco; therefore, induction of such proteinase activity may represent the mechanism by which insects that feed on plants overcome plant proteinase inhibitor defense.

A study of the stabilization of the oxyanion of tetrahedral adducts by trypsin, chymotrypsin and subtilisin

Subtilisin and delta-chymotrypsin have been alkylated using 2-13C-enriched benzyloxycarbonylglycylglycylphenylalanylchloromethane. A single signal due to the 13C-enriched carbon was detected in both the intact subtilisin and delta-chymotrypsin derivatives. The signal titrated from 98.9 p.p.m. to 103.6 p.p.m. with a pKa value of 6.9 in the subtilisin derivative and it is assigned to a tetrahedral adduct formed between the hydroxy group of serine-221 and the inhibitor. The signal in the delta-chymotrypsin derivative titrated from 98.5 p.p.m. to 103.2 p.p.m. with a pKa value of 8.92 and it is assigned to a tetrahedral adduct formed between the hydroxy group of serine-195 and the inhibitor. In both derivatives the titration shift is assigned to the formation of the oxyanion of the tetrahedral adduct. delta-Chymotrypsin has been inhibited by benzyloxycarbonylphenylalanylchloromethane and two signals due to 13C-enriched carbons were detected. One of these signals titrated from 98.8 p.p.m. to 103.6 p.p.m. with a pKa value of 9.4 and it was assigned in the same way as in the previous delta-chymotrypsin derivative. The second signal had a chemical shift of 204.5 +/- 0.5 p.p.m. and it did not titrate from pH 3.5 to 9.0. This signal was assigned to alkylated methionine-192. We discuss how subtilisin and chymotrypsin could stabilize the oxyanion of tetrahedral adducts.

Interleukin-8 processing by neutrophil elastase, cathepsin G and proteinase-3

Activated neutrophils secrete two forms of IL-8 with 77 and 72 amino acids, IL-8(77) and IL-8(72), along with proteinases that could process these cytokines. Significant conversion of IL-8(77) to more potent, N-terminally truncated forms was observed upon incubation with neutrophil granule lysates and purified proteinase-3. IL-8(72) was considerably more resistant to proteolytic processing than IL-8(77). The present observations indicate that neutrophil proteinases released in inflamed tissues convert IL-8 to more active forms and therefore tend to conserve or enhance, rather than decrease IL-8 activity.

Catalytic properties and potential of an extracellular protease from an extreme halophile

An extracellular protease has been isolated and partially purified from the extreme halophile Halobacterium halobium (ATCC 43214). The major enzyme component has a M(r) of 66,000 and is highly dependent upon salt concentrations near saturation for catalytic activity and stability. In aqueous solutions, a decrease in the NaCl concentration from 4 to 1 M results in an increase of nearly three orders of magnitude in the first-order rate constant of inactivation at 30 degrees C. Salt effects the stability of the enzyme in a cooperative manner, with a Hill coefficient of 4.1, which is similar to that of other enzymes from extreme halophiles. The enzyme activity is dramatically affected by the salt concentration, with a loss of 2.5 orders of magnitude in kcat/Km in going from 4 to 0 M NaCl. This loss in catalytic efficiency is primarily due to a dramatic increase in the Km for the substrate in low-salt media. Thermodynamic analysis revealed that this Km increase was mainly the result of increased solubility of the synthetic peptide substrate in low-salt media, which dramatically increases the ground-state stability of the substrate. This results in an effectively reduced substrate partitioning from the bulk solution into the enzyme's active site and an increased value of Km. The halophilic protease is also active in DMF/water mixtures, albeit with novel catalytic properties. In 33% (v/v) DMF in aqueous buffer, the esterase activity of the enzyme is ca. 80-fold higher than the corresponding amidase activity. This contrasts to the situation in pure aqueous buffer, in which the esterase activity is only fourfold higher than the amidase activity. The increased esterase activity relative to amidase activity prompted us to investigate the use of the protease in kinetically controlled peptide synthesis. The enzyme has a broad acyl donor substrate specificity and can effectively use amino acid esters of Phe, Tyr, Trp, Ser, Gly, and Ala. The enzyme is significantly more selective for the amino acid amide, preferring Gly in the P'1 site. A series of glycine-containing oligopeptides have been prepared in yields up to 76% without degradation due to secondary hydrolysis.

Acylation of subtilisin with long fatty acyl residues affects its activity and thermostability in aqueous medium

Subtilisin Carlsberg has been artificially hydrophobized by acylation with octanoyl or palmitoyl chlorides. Samples with several degrees of substitution were obtained. Hydrophobization facilitates in some cases the binding of synthetic or natural substrates. Furthermore, derivatized subtilisins show improved thermal stability (15-fold at 45 degrees C) in aqueous solution. As a result, octanoyl-subtilisin exhibits enhanced thermostability without losing biological activity.