Steps in the reactions of proteolytic enzymes with their substrates

Abstract

Kinetic experiments should be designed to answer specific questions about a reaction mechanism. The present paper is intended to show how a number of specific questions have been answered. Chymotrypsin and trypsin are mainly used to illustrate the different approaches, but many of the arguments used are equally applicable to the reactions of other hydrolytic enzymes with serine-OH or cysteine-SH at the active site. T he recognition of serine-OH and cysteine-SH as essential groups at the active sites of different hydrolytic enzymes did not rest on kinetic evidence. This was deduced from the correlation of enzyme activity with the extent of modification of specially reactive groups. The investigation of proton dissociation equilibria and the assignment of dissociation constants to groups with specified functions in substrate binding, catalysis or protein conformation was the first objective of serious kinetic studies of enzyme reactions. Steady state rate measurements over a wide range of pH showed that groups with p K 6.25 and 6.85 respectively are involved in the catalytic activity of trypsin and chymotrypsin with certain specific substrates (Hammond & Gutfreund 1955). In the case of chymotrypsin it was also shown by Hammond & Gutfreund (1955) that a group with a more alkaline pK is involved in substrate binding. This latter group was subsequently identified and its function was elucidated through the elegant experiments of Oppenheimer, Labouresse & Hess (1966). The identification of histidine as the group with p K _A near neutrality, involved in the catalytic mechanism of trypsin and chymotrypsin, was subsequently confirmed by direct chemical methods by Schoelmann & Shaw (1963). Only kinetic analysis can demonstrate the involvement of proton donors or acceptors with specific properties in enzyme-substrate interaction or in catalysis. The clear identification of chemical groups capable of performing such functions is coming from the crystallographic analysis of the three-dimensional structure at the site of enzyme-substrate interaction, as illustrated in other papers presented in this discussion. Very interesting chemical information is obtained when the effect of structure on reactivity is synthesized from the composite of crystallographic and kinetic data.