Treatment of Enzyme Kinetic Data

Abstract

It has become evident in the past few years that there exists a considerable number of enzymes with activities that are markedly affected by compounds (metabolites) which are not substrates for the reaction involved. It has become equally evident that in many cases these effects are or may be of considerable physiological importance. The concept of feedback inhibition which describes the situation in which the product of a series of consecutive enzymatic reactions specifically inhibits the first of these steps is an excellent example of this type of effect (1). In such cases the effect may be considered to be physiological since it prevents the accumulation of excessive amounts of the product and directs the substrate of the first of the enzymes in the series to other metabolic pathways. In other cases, the physiological effect of compounds unrelated to the substrate on the rate of the enzymatic reaction may not be so simple, but these effects are probably of great importance in metabolic control and regulation. The purpose of the present paper is to attempt to combine two aspects of the problems relating to this type of enzyme. On the one hand, experimental kinetic treatments have been made on the particular enzymes which have shown these effects, but in many instances the kinetic properties have not been fully investigated and the full kinetic implications have not been realized. On the other hand, theoretical kinetic treatments, dealing with the effects of “modifiers” on enzymatic reactions have been described, but the full implication of what the physiological effects might be and how to treat such cases was not fully examined. Presented here is an attempt to deal with the question of how to treat the data for enzymes which are influenced by substances binding specifically to sites other than the enzymatically active site. Such binding is assumed, of course, to give rise to either activation or inhibition, and these cases will be dealt with in considerable detail.