Which vitamin acts as a coenzyme in transamination?

Pyridoxine, also known as vitamin B6, acts as a coenzyme in transamination reactions, which are essential for amino acid metabolism. Transamination involves the transfer of an amino group from one amino acid to a keto acid, allowing for the synthesis of new amino acids. Pyridoxine's role as a coenzyme is critical because it is involved in the formation of the enzyme's active site during this process. This vitamin assists in the conversion of amino acids, thereby contributing significantly to protein metabolism.

Other vitamins listed have different functions: thiamin is primarily involved in carbohydrate metabolism and does not play a direct role in transamination, riboflavin functions mainly as a precursor to flavin coenzymes that are essential in other metabolic pathways, and vitamin B12 is critical for the metabolism of certain fatty acids and amino acids but does not act as a coenzyme specifically in transamination. Thus, the association of pyridoxine with transamination establishes it as the correct answer.