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Enzymes having similar biological action may be isolated from different sources. Thus many different molecular species of an enzyme have the same function. Such enzymes which exist in more than one form are called isozymes. More than a hundred enzymes are known to exist as isozymes. When the variants of an enzyme are within the same species of an organism they are called intraspecific or ontogenetic variants. When they are from different species they are called interspecific or phylogenetic variants.

Multiple forms of the same enzyme may also be found in the same organisms. In humans there are three different types of haemoglobin, two in the adult and one in the foetus. These are formed by the union of four polypeptide chains. The enzyme lactic dehydrogenase (LHD) is present is most animal tissues as five isozymes. It has two subunits or monomers, the H (heart) polypeptide and M (muscle) polypeptide. Four units, in different combinations, make up the five different molecular forms of LDH. The five isozymes are.

1. HHHH or H4
2. HHHM or H3M
3. HHMM or H2M2
4. HMMM or HM3
5. MMMM or M4

molecular form of LDH

The M subunit is formed under conditions of low oxygen tension, and the H subunit under conditions of low oxygen tension, and the H subunit under conditions of plentiful oxygen supply (aerobic metabolism). It has been found that embryos metabolize through low oxygen supply mechanisms, and in the course of development proceeds the M4 and HM3 forms of LDH diminish, and are replaced by the H4 and H3M forms.

Isozymes may be homologous or analogous. Homologous isozymes have essentially similar molecular structure and catalytic properties. Cytochrome C from different organisms shows homology. Half the amino acids of cytochrome c or such widely separated organisms like yeasts and mammals are identical. Generally isozymes of more closely related organisms show greater similarity than isozymes of widely separted organisms. Analogous isozymes have similar reaction but different molecular structure and catalytic properties. They are arisen from different ancestral genes.

Isozymes provide a clue to the genetic relationships of organisms. The sequence of amino acids in an ezymes is related to the structure of DNA. Therefore similarity in isozymes is correlated with similarity in DNA structure.

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