Two theoretical problems concerning the irreversible modification kinetics of enzyme activity

Influence of substrates on in vitro dephosphorylation of glycogen phosphorylase a by protein phosphatase-1

The kinetic theory of the substrate reaction during modification of enzyme activity has been applied to a study of the dephosphorylation of phosphorylase a by protein phosphatase-1 (ppase-1). On the basis of the kinetic equation of the substrate reaction in the presence of ppase-1, all the inactivation rate constants for the free enzyme and the enzyme-substrate(s) complexes have been determined. Binding of the allosteric substrate, glucose 1-phosphate, to one subunit of phosphorylase a protects completely against ppase-1 action on either the same subunit or the adjacent subunit, whereas binding of the non-allosteric substrate, glycogen, to one subunit protects this subunit partially, but has no effect on the modification on the neighbouring subunit. Analysis of the data suggests that the allosteric behaviour of phosphorylase a can be interpreted in terms of a modified concerted model. The present method also provides a novel approach for studying dephosphorylation reactions. Since the experimental conditions used resemble more closely the in vivo situation where the substrate is constantly being turned over while the enzyme is being modified, this new method would be particularly useful when the regulatory mechanism of the reversible phosphorylation reaction toward certain enzymes is being assessed.

The inactivation kinetics of papain by guanidine hydrochloride: a re-analysis

The kinetic theory of the substrate reaction during modification of enzyme activity has been applied to study the inactivation kinetics of enzymes by denaturant. However, an important problem related to the determination of the inactivation rate constants has not been considered in a previous publication (Xiao, et al., Biochim. Biophys. Acta, 1164 (1993) 54-60). In most denaturation experiments, the high concentrations of denaturants may greatly affect the kinetic behavior of the system to preclude the use of the kinetic parameters determined in the absence of denaturant. In the present study, the kinetic equation of substrate reaction in presence of denaturant has been derived. A re-examination of the effect of high concentrations of guanidine hydrochloride on the inactivation of papain, taking into consideration the effect of high concentrations of guanidine hydrochloride on the Michaelis constant, showed that, for papain, the substrate gives no protection on its inactivation. It is the purpose of the present communication to stress the importance of observing the effect of the denaturant on the kinetic parameters for kinetic analysis of enzyme inactivation by denaturants generally.

Activation mechanism and modification kinetics of Chinese hamster dihydrofolate reductase by p-chloromercuribenzoate

Substrate effects on the activation kinetics of Chinese hamster dihydrofolate reductase by p-chloromercuribenzoate (pCMB) have been studied. On the basis of the kinetic equation of substrate reaction in the presence of pCMB, all modification kinetic constants for the free enzyme and enzyme-substrate binary and ternary complexes have been determined. The results of the present study indicate that the modification of Chinese hamster dihydrofolate reductase by pCMB shows single-phase kinetics, and that changes in the enzyme activity and tertiary structure proceed simultaneously during the modification process. Both substrates, NADPH and 7,8-dihydrofolate, protect dihydrofolate reductase against modification by pCMB. In the presence of a saturating concentration of NADPH, the value of kcat for 7,8-dihydrofolate in the enzyme-catalysed reaction increased four-fold on modification of Cys-6, accompanied by a two-fold increase in Km for the modified enzyme. The utilization of the binding energy of a group to increase kcat rather than reduce Km implies that the full binding energy of the group is not realized in the formation of the enzyme-substrate complex, but is used to stabilize the enzyme-transition-state complex.

Inactivation kinetics of the reduced spinach chloroplast fructose-1,6-bisphosphatase by subtilisin

The course of inactivation of the reduced spinach chloroplast fructose-1,6-bisphosphatase by digestion with subtilisin has been followed by the progress curve method [Tsou, C. L. (1988) Adv. Enzymol. 61, 381-436] and found to follow first-order kinetics. On the basis of the hydrolysis of the substrate, fructose 1,6-bisphosphate, at different concentrations during proteolysis by subtilisin, the first-order inactivation rate constants for the free enzyme and the enzyme-substrate complex can both be determined. The ratio between the inactivation rate constants for the free enzyme and the enzyme-substrate complex indicates strong protection against subtilisin proteolysis by the substrate. It is proposed that the above ratio can be used as a quantitative measure of substrate protection for enzyme inactivation generally. As it has been found that the site of proteolysis is located in a loop region near the N-terminus and well away from the active site, the substrate protection indicates a conformation change of the enzyme away from the substrate binding site.

Inactivation kinetics of dihydrofolate reductase from Chinese hamster during urea denaturation

The kinetic theory of substrate reaction during modification of enzyme activity has been applied to the study of inactivation kinetics of Chinese hamster dihydrofolate reductase by urea [Tsou (1988) Adv. Enzymol. Relat. Areas Mol. Biol. 61, 381-436]. On the basis of the kinetic equation of substrate reaction in the presence of urea, all microscopic kinetic constants for the free enzyme and enzyme-substrate binary and ternary complexes have been determined. The results of the present study indicate that the denaturation of dihydrofolate reductase by urea follows single-phase kinetics, and changes in enzyme activity and tertiary structure proceed simultaneously in the unfolding process. Both substrates, NADPH and 7,8-dihydrofolate, protect dihydrofolate reductase against inactivation, and enzyme-substrate complexes lose their activity less rapidly than the free enzyme.

Kinetics of inactivation of aminoacylase by 2-chloromercuri-4-nitrophenol: a new type of complexing inhibitor

The kinetic theory of the substrate reaction during modification of enzyme activity previously described [Tsou (1988) Adv. Enzymol. 61, 381-436] has been applied to a study of the inactivation kinetics of aminoacylase by 2-chloromercuri-4-nitrophenol (MNP). The results indicate that the mechanism of reaction between MNP and aminoacylase is a special type of irreversible inhibition. The main features of this type of inhibitor are as follows: (i) the reaction kinetics of inhibitor with enzyme is a single exponential process; (ii) inhibition shows a noncompetitive, complexing behavior; (iii) the first inhibitor-enzyme complex, EI, still has some enzyme activity, and hence the plot of [P]infinity versus the reciprocal of inhibitor concentration gives a straight line with a positive intercept at the ordinate. On the basis of the kinetic equation of substrate reaction in the presence of the inhibitor, a plotting method has been developed for determining the inhibition kinetic constants. As an example, all reaction kinetic constants of aminoacylase with 2-chloromercuri-4-nitrophenol have been determined. The results of the present study suggest that the essential thiol group at the active site of aminoacylase may have a significant effect on the catalytic step but is not involved in substrate binding.