Introduction to Enzymes
Part III
Part III. Enzyme Additives (Cofactors) Assisting in Catalysis
Enzymes are often composed of only protein. In this case only AA side chains are used for catalysis. Some enzymes require additives for assisting with catalysis. Additives like vitamins often provide functional groups not available to the enzyme among the side chains of the amino acids.
In these cases the protein of the enzyme binds:
Organic cofactors (Vitamins = organic cofactors)
Metal ions (e.g. Mg2+)
Nucleotides (even RNA)
The Common Cofactors (Enzyme Additives):
1. Biotin aids in carboxylation reactions (carbon dioxide fixation).
2. Cobaltamine (vitamin B-12) aids in alkylation reactions (methylation for instance).
3. Coenzyme A aids in acyl transfers like in the tricarboxylic acid cycle.
4. Flavin (vitamin B-2) aids in oxidation-reduction reactions (eg. nitrate reductase).
5. Lipoic acid aids in acyl transfers via oxidation-reduction processes.
6. Nicotainamide coenzymes like NAD+ act as independent co-substrates.
7. Pyridoxal (vitamin B-6) aids in amino group transfers (provides aldehyde functional group).
8. Tetrahydrofolate aids in one-carbon transfers.
9. Thiamine (vitamin B-1) aids in aldehyde transfers.
The complex of protein and additive is called Holo-Enzyme. When the additive is removed from the enzyme, the remaining protein part of the enzyme is called the Apo-Enzyme.
Apo-Enzyme (inactive) + Additive = Holo-Enzyme (active)
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©Wilbur H. Campbell, 1995; wcampbel@mtu.edu