Elementary plant Biochemistry and Biotechnology (2+1)

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