BIC 101 :: Lecture 18 :: APOENZYMES, COENZYMES AND COFACTORS, ISOZYMES

A complete, catalytically active enzyme together with its coenzyme and/or metal ions is called holoenzyme.

  • The protein part of an enzyme is called apoenzyme or apoprotein.
  • Enzymes require an additional non-protein component to carry out its catalytic functions.
  • Generally these non-protein components are called as  cofactors.
  • The cofactors may be either one or more inorganic ions such as Fe2+, Mg2+, Mn2+ and Zn2+ or a complex organic molecules called coenzymes.
  • A coenzyme or metal ion that is covalently bound to the enzyme protein is called prosthetic group.
  • Some enzymes require both coenzyme and one or more metal ions for their activity
  • Coenzymes function as transient carriers of specific functional groups

Cofactors

  • Metals are required as cofactors in approximately two thirds of all enzymes.
  • Metalloenzymes contain a definite quantity of functional metal ion that is retained

throughout whereas metal-activated enzymes bind metals less tightly but require added metals.

  • The distinction between metalloenzymes and metal activated enzymes thus rests on the affinity of a particular enzyme for its metal ion.
  • The mechanisms whereby metal ions perform their function appear to be similar both in metalloenzymes and metal activated enzymes.
  • Metals participate through their ability to act as Lewis acids and through chelate formation. Eg. For metal functioning as a Lewis acid is the zinc in carbonic anhydrase.
  • The metal can also promote catalysis by binding substrate at the site of bond cleavage. In carboxypeptidase, the carbonyl oxygen is chelated to the zinc.

The iron-sulfur enzymes are unique class of metalloenzymes in which the active centre consists of one or more clusters of sulfur-bridged iron chelates. These are of greater importance in plant systems
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.
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