ISOZYMES
Sometimes an enzyme present in the same organism is found to have different molecular forms but catalyzing the same reaction. These are called isozymes. The 1964 Committee recommended that "multiple enzyme forms" in a single species should be known as isozymes. Among many enzymes known to have isozymes, the most studied is lactate dehydrogenase (LDffl, which is an important enzyme of carbohydrate metabolism. This enzyme exists in five possible forms in most vertebrates. In fact two basically different types of LDH occur predominantly in the heart and muscle. The former consists of four identical monomers (H). The latter (muscle LDH) also forms due to the combination of four identical monomers (M) but having different amino acid composition, in comparison to the former. Different combinations of H and M monomers yield three additional hybrid enzymes possessing four monomers each. The possible combinations of H and M monomers therefore, produce five isozymes of LDH. These are:
H4. H3M, H2M2, HM3 and M4