Isoenzymes

Isoenzymes
    •Enzymes which exist in multiple forms within a single species of organism or even in a single cell are called isoenzymes or isozymes.
    •Such multiple forms can be detected and separated by gel electrophoresis of cell extracts.
    •Since they are coded by different genes, they differ in amino acid composition and thus in their isoelectric pH values.
    •Lactate dehydrogenase is an example for the isoenzymes which occur as five different forms in the tissues of the human and other vertebrates.
    •All the five isozymes catalyze the same reaction.
    Lactate + NAD+ →Pyruvate + NADH + H+
    •They have the molecular weight of about 134,000 and contain four polypeptides.
    •The five isozymes consist of five different combinations of two different kinds of polypeptides M and H.
    •Kinetic study of lactate dehydrogenase isozymes has revealed that although they catalyze the same reaction, they differ significantly in their Km values for their substrates as well as Vmax values.
    •The two polypeptide chains in LDH are coded by two different genes.
    •Skeletal muscle contains four identical M chains and designated as M4; whereas heart muscle contains four identical H chains and designated as H4.
    •LDH of other tissues are a mixture of the five possible forms H4, H3M, H2M2, HM3 and M4.
    •A determination of the relative amounts of the five LDH isozymes and the total concentration of LDH in a serum sample can provide valuable diagnostic information about which tissues have been damaged and the extent of the damage.

     

Last modified: Tuesday, 26 June 2012, 4:42 AM