Certain enzymes occur in multimolecular forms in the same organism, which are structurally related, but catalyze the same reactions using different kinetic parameters to suit the requirements of the organism. Such enzymes are known as isoenzymes.
Lactate dehydrogenase (LDH) of mammals is one such example. It is a tetramer occurring in five different forms. These forms differ in their behavior, amino acid composition and relative rates of reactions. They have been reported from different organs of vertebrates, chiefly from the heart and skeletal muscles. Both types have the same molecular weight (35,000) and both are produced in the same cell by two different genes. LDH is composed of M and H subunits in five combinations, viz. H₄, H₃M, H₂M₂,HM₃ and M₄. M₄ and H₄ are pure tetramers, the former is predominant in striated muscle fibres undergoing quick muscle twitch, and the latter predominant in tissues of high oxidative capacity. Beside LDH, many other enzymes, such as alkaline phosphatase, glutamate – oxaloacetate transaminase and creatine phosphokinase, also occur as isoenzymes.
     
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