Holoenzyme is a complete, functional enzyme, which is catalytically active. Holoenzyme consists of an apoenzyme together with its cofactors. Holoenzyme contains all the subunits required for the functioning of an enzyme, e.g. DNA polymerase III, RNA polymerase.
Holoenzyme = Apoenzyme + Cofactor
Holoenzyme is also known as a conjugate enzyme. The apoenzyme is the protein part of the enzyme, which is enzymatically inactive without cofactors.
A cofactor is the non-protein part of the holoenzyme, which is essential for its activity. Cofactors can be metal ions (Mg2+, Fe3+, Zn2+) or organic molecules called coenzymes.
Most of the coenzymes are derived from vitamins. They act as a transient carrier of groups and transfer these groups in a biochemical reaction. Examples include:
- Coenzyme A (CoA) – It is derived from compounds such as pantothenic acid, etc. and carries acyl groups.
- Flavin adenine dinucleotide (FAD) – It is derived from vitamin B2 (Riboflavin) and carries electrons.
- Nicotinamide adenine dinucleotide (NAD) – Derived from nicotinic acid (Niacin) and carries hydride ions.
Explore: Facts about enzymes
A prosthetic group is a cofactor bound to the apoenzyme very tightly, often bound covalently. Examples of prosthetic groups include a heme prosthetic group attached to cytochromes. Examples of enzymes with metallic cofactors are:
- Fe2+ or Fe3+ – Catalase, peroxidase, cytochrome oxidase
- K+ – Pyruvate kinase
- Zn2+ – Carbonic anhydrase
So the catalytically active apoenzyme-cofactor complex is known as the holoenzyme or conjugate enzyme.
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