Apoenzyme or apoprotein is an enzymatically inactive protein part of an enzyme, which requires a cofactor for its activity. Apart from catalytic RNA, most of the enzymes are proteins. Not all the enzymes require a cofactor. Enzymes that do not require any cofactor are known as simple enzymes, e.g. pepsin, trypsin, etc.
A cofactor required by an apoenzyme can be a metal ion, e.g. Mg2+, Fe3+, etc. or an organic molecule called coenzyme such as NAD+, NADP+, FAD2+, etc.
Coenzymes are non-protein organic complexes, which are loosely attached to the protein part, e.g. NAD+, NADP+, etc.
The cofactor, which is tightly bound to an apoenzyme is known as a prosthetic group, e.g. heme prosthetic group attached to catalase, peroxidase, etc.
An apoenzyme becomes enzymatically active when bound to its cofactors and now known as a holoenzyme.
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