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
- 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
- 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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