Isoenzymes

isozymes, different structural forms of enzymes with a single type of catalytic activity in organisms of the same species (or tissue).

Isoenzymes catalyze the same reaction but differ in amino acid composition and in some of their physical, immunological, and catalytic properties. Isoenzymes consist of several polypeptide chains (subunits), combined in various ways to form a quaternary enzyme structure. Thus, two forms of the lactate dehydrogenase enzyme are found in the chick—one characteristic of skeletal muscles, the other of heart muscle. In all, five isoforms of this enzyme have been found in chicks and other organisms, each form (tetramer) made up of four protein subunits of two types. Electrophoresis can be used to separate isoenzymes. In organisms of the same species (or in the same tissue), isoenzymes form a characteristic set, or spectrum, which may be altered with pathological tissue changes (useful in diagnosis) and in the process of ontogeny.