Isoenzymes in metabolic regulation

Abstract

Before the discovery of isoenzymes, metabolic differences between tissues were explained largely in terms of variation in the relative amounts of individual enzymes present, although permeability, compartmentation, concentration of metabolites and hormonal effects were also recognized to be important. With the realization that enzyme heterogeneity is widespread and that isoenzymes may differ not only in their tissue distributions but also in their kinetic properties, another explanation depending upon the qualitative rather than the quantitative aspects of tissue enzymology became possible. Thus although an individual metabolic pathway must by definition consist of the same sequence of enzymatically catalysed reactions, its regulatory characteristics may vary markedly depending upon which particular isoenzyme is present at each step. This is now recognized to have been a major factor in the evolution of the special control systems which are required for individual tissues or cells to carry out their separate metabolic roles. The purpose of this chapter is, therefore, to examine the ways in which variations in isoenzyme properties can allow an enzyme to serve different functions in different tissues.