13.10 Vitamin H

Biotin is known as vitamin H, which is synthesized by bacteria the live in intestines. Thus, biotin does not have to be incorporated in diet and the deficiencies are generally rare. Biotin is the coenzyme required by enzymes that catalyze carboxylation of a carbon adjacent to a carbonyl group. For example, acetyl-CoA carboxylate converts acetyl-CoA into malonyl-CoA. HCO_3^- is used as the source of carboxyl group which is attached to the substrate (Scheme 3). The reaction is also required ATP and Mg²⁺.

Scheme 3

13.11 Vitamin B₆

The coenzyme pyridoxal phosphate (PLP) is derived vitamin B₆. The deficiency of vitamin B₆ causes anemia. Enzymes that catalyze the reactions of amino acids require PLP. One of the common examples is the transamination using enzyme aminotransferases (Scheme 4).

Scheme 4

13.12 Vitamin B₁₂

Enzymes that catalyze certain rearrangement reactions require coenzyme B₁₂ which is derived from vitamin B₁₂. In vitamin B₁₂, cyano group is coordinated to cobalt. In coenzyme B₁₂, this group is replaced by a 5’-deoxyadenosyl group.

Coenzyme B₁₂ is used by enzymes which catalyze reactions in that a group (Y) bonded to one carbon changes to hydrogen bonded an adjacent carbon (Scheme 5). For example, b-methylaspartate is converted into glutamate where CO_2^- has migrated to the adjacent carbon.

Scheme 5