Enzymes are biological catalysts. They are substrate-specific and play an essential role in various biochemical reactions. Most of the enzymes, except catalytic RNAs or ribozymes, are proteins and made up of amino acids. Some enzymes also require a non-protein part for their activity. Based on these, enzymes are categorised into two types:
- Simple enzymes – They are only made up of proteins, e.g. trypsin, pepsin, etc.
- Conjugate enzymes or holoenzymes – They consist of a protein and a non-protein part essential for the activity. The protein part of the holoenzyme is known as the apoenzyme, which is inactive. The non-protein part is called a cofactor and is necessary for the catalytic function of the enzymes. E.g. metal ions (Mg2+, Fe3+, Zn2+), organic molecules or coenzymes (NAD+, NADP+, FAD2+) and prosthetic groups. E.g. catalase, alcohol dehydrogenase, pyruvate kinase, DNA polymerase, etc.
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So holoenzyme is an active enzyme-cofactor complex, i.e. an apoenzyme attached to a cofactor.
Apoenzyme (Inactive) + Cofactor ⇌ Holoenzyme (Active)
Difference between Apoenzyme and Holoenzyme
The table below shows the main differences between Apoenzyme and Holoenzyme:
| The catalytically inactive protein part of an enzyme | The catalytically active apoenzyme-cofactor complex |
| Contains only protein | It contains protein as well as cofactors such as metal ions or other organic complexes known as coenzymes |
| Inactive and becomes active only after attaching to a cofactor | Active and fully functional to catalyse a biochemical reaction |
| It does not contain cofactors | It contains cofactors like metal ions or coenzymes |
| The protein moiety of the holoenzymes, e.g. carbonic anhydrase without Zn2+ ions. | DNA polymerase, RNA polymerase, carbonic anhydrase, etc. |
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