Influence of solvent and water activity on kinetically controlled peptide synthesis

Effects of subzero temperatures on the kinetics of protease catalyzed dipeptide synthesis in organic media

A depeptide synthesis was drastically influenced by the reaction temperature, in the range from -30 degrees to 25 degrees C. This article shows the kinetic reasons of this effect. alpha-Chymotrypsin was immobilized on celite and used in four different water-miscible solvents containing small amounts of water-miscible solvents containing small amounts of water. The reaction studied was the aminolysis of N-acetyl-L-phenylalanine ethyl ester (Ac-PheOEt) with L-alaninamide (Ala-NH(2)) and water for the acylenzyme complex, the nucleophile was favoured by low reaction temperatures. This effect (quantified as p-values) was observed in all four solvents, and it was greatest in acetonitrile and tetrahydrofuran. The esterase and amidase activities of the enzyme were studies using AcPheOEt and N-acetyl-L-phenylalanyl-L-ananinamide (AcPheAla-NH(2)) as substrates. The Michaelis-Menten parameters, K(m,app) and V(max), were determined for ester hydrolysis and dipeptide hydrolysis. Both K(m,app) and V(max) tended to increase with increasing temperature. Secondary hydrolysis was reduced at subzero temperatures because ester hydrolysis was favoured in relation to depeptide hydrolysis. Depeptide synthesis was thus favored by low temperatures in two ways: first, in the competition between the nucleophile and water for the acyl enzyme; and, second, in the competition between the ester substrate and the peptide substrate for the free enzyme. As a result, in acetonitrile containing 10% water, the maximal yield was 99% at -20%C compared with 84% at 25 degrees C. (c) 1995 John Wiley & Sons, Inc.

Investigating the effects of polymer chemistry on activity of biocatalytic plastic materials

The affects of polymer chemistry on the organic solvent activity of alpha-chymotrypsin-containing biocatalytic plastic materials are investigated in this study. To incorporate alpha-chymotrypsin into the polymer, the enzyme is first acryloylated, then solubilized into organic solvents via hydrophobic ion paring with surfactant molecules. Once in the organic solvent, a vinyl monomer and crosslinker are added and copolymerized with the enzyme. Due to the intimate contact between the enzyme and the resulting polymer network, the polymer chemistry plays an important role in the activity of these biocatalytic materials. The chemical composition of the monomer/polymer has the greatest effect on catalytic activity. The activity spans a range of 100-fold and appears to correlate with the hydrophilicity of the monomer, with the lowest activity exhibited for poly(methyl methacrylate) and the highest for poly(2-hydroxyethyl methacrylate). The effect of the chemical structure of the monomer/polymer appears to be an intrinsic kinetic effect, whereas other polymer chemistry conditions investigated, including crosslinker concentration and length and ratio of solvent:monomer during synthesis, appear to effect the rate of substrate diffusion, thereby affecting observed enzyme activity. Changes in the conditions of polymer synthesis can cause as much as a 20-fold change in activity for a given polymeric material. This is most likely due to an increase in the porosity of the materials, and thus a relaxation of diffusional limitations.

Enhancement of both reaction yield and rate of synthesis of structured triacylglycerol containing eicosapentaenoic acid under vacuum with water activity control

Production of structured triacylglycerols (sTAG) containing eicosapentaenoic acid (EPA) at the sn-1 (or 3) position using Lipozyme in a solvent-free system was studied. Optimal water activity (a(w)) for the synthesis of the sTAG was investigated. Vacuum was applied to shift reaction equilibrium toward the synthesis reaction by removing by-products. During vacuum application, the water level of the reaction system was controlled at the optimal level by addition of a suitable amount of water at a predetermined interval. Intermittent periodic addition of a suitable amount of water into the reaction mixture made the reaction rate faster than that without adding water. A molar yield of 89.7% of the targeted sTAG was obtained after 16 h reaction with a(w) control during the vacuum application as compared with the yield of 87.0% after 24 h of reaction without a(w) control during the vacuum application.

Enzymatic peptide synthesis in low water content systems: preparative enzymatic synthesis of [Leu]- and [Met]-enkephalin derivatives

A novel total enzymatic synthesis of [Leu]- and [Met]-enkephalin derivatives was accomplished in low-water content systems at a preparative scale. alpha-Chymotrypsin, papain, thermolysin and bromelain adsorbed on Celite were used as catalysts. Organic solvents such as acetonitrile and ethyl acetate with small amounts of buffer added or at specific water activity were used as reaction media. Simple readily available amino acid ester derivatives were used as starting building blocks. This feature allowed the possibility of using the products in one step directly as acyl-donor ester, without any chemical or enzymatic modification, in the next enzymatic coupling. The optimal strategy for the synthesis of the enkephalin derivatives was different depending on the carboxy terminal group. The preparation of the carboxy-terminal amide derivatives (R-Tyr-Gly-Gly-Phe-Leu[Met]-NH2) was achieved via 4 + 1 fragment condensation catalyzed by alpha-chymotrypsin. The carboxy-terminal ethyl ester derivatives (R-Tyr-Gly-Gly-Phe-Leu[Met]-OEt) were obtained via 2 + 3 condensation catalyzed by bromelain, a quite unusual protease for peptide synthesis but more effective than papain in this coupling. Both syntheses were carried out in four enzymatic steps and one or two chemical deprotection steps routinely used in peptide synthesis. The overall yields of pentapeptide derivatives were between 40-54% of pure product.

Acyl transfer reactions catalyzed by native and modified alpha-chymotrypsin in acetonitrile with low water content

The characterization of the S' subsite specificity of native and ethylated alpha-chymotrypsin has been studied via acyl transfer reaction in acetonitrile containing 10 vol% of water. Using Ac-Tyr-OEt as acyl donor, we investigated the partitioning of acyl-chymotrypsins between water and amino acid and peptide-derived nucleophiles. For the investigation of S'2 subsite specificity, a series of 19 dipeptides of the general structure Ala-Xaa (Xaa represents all natural amino acids except cysteine) were used. From the values of the apparent partition constants rho app, the order of preference for the P'2 position is estimated to be: positively charged > hydrophilic > or = hydrophobic > aromatic > Pro > negatively charged side chain. Concerning the S'1 specificity, the same preference is deduced based on the study with the series of amino acid amides and Xaa-Ala dipeptides. In contrast to the nucleophilic specificity of alpha-chymotrypsin in aqueous solutions, free dipeptides and hydrophilic amino acid derivatives as nucleophiles exhibit much higher reactivities for acyl transfer in acetonitrile. We have not observed a significant difference in nucleophilic specificity between native and ethylated chymotrypsin.

Thermodynamic predictions for biocatalysis in nonconventional media: theory, tests, and recommendations for experimental design and analysis

This article discusses the application of thermodynamic and related analysis to reaction systems for enzymic or whole cell catalysis, in which there are high proportions of organic liquid, gas, or supercritical fluid. A variety of predictions may be made, especially based on the partitioning of components between the different phases normally present. In many cases, observed behavior can be explained without invoking any direct molecular effects on the biocatalyst. The predictable changes should always be allowed for before seeking explanations for the residual effects, which are often very different from the crude observations. A summary of the general thermodynamics of multiphase systems is presented, and then the main classes of component that distribute between the phases are discussed in turn. Thermodynamic water activity (aw) determines the mass action effects of water on hydrolytic equilibria. It also describes the distribution of water between the various phases that can compete in binding water. Because catalytic activity is very sensitive to the hydration of the enzyme molecules, aw often predicts an unchanging optimum as other aspects of the system are changed. Hence the aw should be measured and/or controlled in these systems, whether the primary aim is to study the effects of water or of other changes. The methods available for measurement and control of aw are discussed. Adverse effects of organic solvents or similar nonpolar species partly reflect their tendency to partition into the relatively polar phase around the biocatalyst, especially when this is dilute aqueous. The well-established log P parameter is a measure of this. But other mechanisms of inactivation can occur: directly through contact of the biocatalyst with the phase interface, or indirectly via hydration changes. In these cases the molecular property log P is probably not the best solvent parameter. In low-water systems the biocatalyst remains in a separate phase even when water-miscible solvents are used. Hence, the categorization of solvents in terms of miscibility becomes less relevant. This accounts for the "two peak" dependence of catalytic activity on water content in some miscible systems. Differential solvation of reactants and products, as the bulk phase is altered, causes changes in concentration-based equilibrium constants and yields. These changes in solvation may be monitored through partition coefficient or solubility measurements. Reactant solvation can also account for differences in biocatalyst kinetics, whether or not partitioning into a dilute aqueous phase is involved. These predictable effects should be allowed for when studying effects of solvent or similar changes on activity or specificity.(ABSTRACT TRUNCATED AT 400 WORDS)