Which of the following is the primary cofactor required for transamination reactions catalyzed by aminotransferases?

Explanation

## Cofactor for Transamination **Key Point:** Pyridoxal phosphate (PLP), the active form of vitamin B₆, is the universal cofactor for all transamination reactions. ### Mechanism of PLP in Transamination PLP functions as a coenzyme by: 1. Forming a Schiff base (aldimine) with the amino group of the substrate amino acid 2. Stabilizing the carbanion intermediate formed after decarboxylation 3. Facilitating the transfer of the amino group to the α-ketoacid acceptor 4. Regenerating the enzyme-PLP complex ### Role in Amino Acid Metabolism PLP-dependent aminotransferases include: - **Alanine aminotransferase (ALT)** — transfers amino group from alanine to α-ketoglutarate - **Aspartate aminotransferase (AST)** — transfers amino group from aspartate to α-ketoglutarate - **Other aminotransferases** — involved in branched-chain amino acid and other amino acid metabolism **High-Yield:** Vitamin B₆ deficiency impairs transamination reactions, leading to elevated plasma amino acids and reduced synthesis of non-essential amino acids. ### Why Other Cofactors Are Wrong | Cofactor | Primary Role | Used In | |----------|--------------|----------| | FAD | Oxidation-reduction | Oxidative deamination (monoamine oxidase) | | NAD+ | Oxidation-reduction | Oxidative deamination (glutamate dehydrogenase) | | TPP | Decarboxylation | Pyruvate dehydrogenase, α-ketoglutarate dehydrogenase | [cite:Lehninger Principles of Biochemistry Ch 22]