UC Irvine

Enzyme action is the result of a large number of discrete steps involving a great variety of processes such as cooperative conformational changes, acid-base catalysis, nucleophylic and/or electrophylic attack from properly positioned groups, etc.; it is widely recognized that in order to be useful for catalysis, the various elementary processes must be space- and time-controlled during enzyme function. In the past decade great progresses have been made in understanding the chemistry and the stereochemistry of enzyme action, with particular emphasis on the role of the spatial effects. Obviously, an analysis of the temporal aspects of enzyme action is equally important. The ultimate goal is the description of the concomitance and/or sequence of individual elementary steps in the catalytic act. This ambitious but difficult goal can be approached by focusing the attention on the time constants of the various elementary processes and assessing their microscopic mechanism by comparative studies on representative model systems. This approach was introduced in enzymology with the development of fast relaxation methods and will be followed in this paper, with the understanding that it suffers from the same intrinsic limitations as an analysis of a musical piece restricted to a list of the sound frequencies occurring in it but devoid of any information about their temporal sequence and relative intensity. Our aims are: l. To review time events detected in enzymes using a proper physical framework, i.e., the theory of the random processes. 2. To identify these events at a molecular level by comparison with processes occurring in appropriate model systems. 3. To discuss the statistical significance of the detected events. We shall start with the simpler model systems and shall then proceed to analyze situations of increasing complexity and eventually consider enzyme-substrate complexes. For each class of events some data will be critically reviewed and their relevance to enzyme catalysis stressed. All data will then be comparatively discussed according to their time scale and some mechanistic conclusions will be derived. The representative enzymes considered in this review were chosen among those which can work as separate entities in an aqueous medium because they are simpler and better known.