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