The specificity in the elementary steps of alpha-chymotrypsin catalysis. A temperature study with a series of N-acetyl-L-amino acid methyl esters

The method of 'added nucleophile' (methanol) was employed to study the steady-state kinetics of the alpha-chymotryptic hydrolysis of methyl esters of N-acetyl-L-amino acids of the RCH(NHCOCH3)-C(O)OCH3 type, which are derivatives of alpha-aminobutyric acid, norvaline, norleucine, phenylalanine, alpha-aminoheptanoic acid and alpha-aminooctanoic acid. In the 5 to 30 degrees C temperature range at pH 8.0 the elementary rate and binding constants for both hydrolysis (Ks, k2, k3) and methanolysis (k-2) of the intermediate acylenzyme have been determined. The following results are discussed: 1. The enthalpy-entropy compensation phenomenon, which has previously been established to take place in alpha-chymotryptic catalysis, is characterized by two values of isokinetic temperatures (Tc), i.e., about 200 K at the step of binding the substrate and 430--460 K at the chemical steps of the enzymatic reaction. 2. It is demonstrated how the free energy and enthalpy, as well as entropy, change along all the reaction coordinate. To explain the results obtained, a mechanism for enzyme-substrate interaction is suggested according to which the sorption of substrate group R on the enzyme is different in the metastable intermediates and in the transition states of the reaction: (i) in the enzyme-substrate complex and in the acylenzyme, substrate group R merely emerges from water to be buried in the protein. Two facts point to the hydrophobic nature of the E-R interaction. First, the change in the free energy is exactly delta GRextr, that is the free energy of the transfer (extraction) of substrate group R from water to the organic solvent. Second, the motive force of the E-R interaction is the change in the entropy only. (ii) In the transition state the situation is different, i.e., the E-R interaction becomes thermodynamically still more favourable (the free energy change is 2 delta GRtrans) and involves thermodynamically favourable changes not only in the entropy (as is the case in the intermediates), but also in the enthalpy. Hence the E-R interaction in the transition states is accompanied by conformational and solvational changes of the enzyme-substrate system and, first and foremost, of the entire protein.

A kinetic study of hog kidney aminoacylase

The kinetic and thermodynamic parameters of the hog kidney acylase-catalyzed reactions of N-acetyl-L-methionine hydrolysis and synthesis have been investigated. The equilibrium constants were determined at high concentrations of the products (acetate and L-amino acid) for a number of amino acids. A kinetic scheme of the enzymatic reaction was proposed that describes the dependence of the rate of hydrolytic and synthetic reactions on the composition of the reaction system. The kinetic parameters determined from the progress curves proved very close to those obtained by the initial rate analysis. The kinetic and thermodynamic constants fitted the Haldane equation.

Enzymatic synthesis in biphasic aqueous-organic systems. II. Shift of ionic equilibria

Ionic equilibria in "water-water-immiscible organic solvent" systems have been studied. It has been shown that in such systems shift of the apparent pK value of acids and bases takes place (compared to aqueous solutions), the value of the shift being rather high, up to 5 and more pH units (with 2,4-dinitrophenyltryptophan as an acid and neutral red as a base). The pK shift of ionogenic reagents observed in biphasic systems can be used in preparative organic synthesis for increasing the yield of end products in enzyme-catalyzed reactions. In connection with this, the physico-chemical reasons for the equilibrium shift in a chemical reaction that involves one or two ionogenic reagents are theoretically analyzed. The above approach has been tested with two alpha-chymotrypsin-catalyzed reactions, i.e., synthesis of N-benzoyl-L-phenylalanine ethyl ester (from NBz-LPhe-OH and ethanol) and synthesis of N-acetyl-L-tryptophanyl-L-leucine amide (from NAc-LTrp-OH and LLeu-NH2). In water the equilibria in these reactions are shifted almost entirely towards the starting reagents with the yield of end product being negligibly low. In biphasic systems consisting of chloroform +5% (v/v) water or ethyl acetate +2% (v/v) water, the yield of both the ester and the dipeptide reaches 100%.