Gamma-carboxylation of glutamate residues in clotting factors requires which vitamin?

• A. Vitamin K
• B. Vitamin C
• C. Vitamin D
• D. Vitamin A

Answer: (A)

Gamma-carboxylation of glutamate residues in clotting factors is a modification that enables calcium binding and proper interaction with phospholipid surfaces during coagulation. This reaction relies on a cofactor provided by reduced vitamin K, which drives the carboxylation step via gamma-glutamyl carboxylase and is reused through the vitamin K cycle. Without enough vitamin K, clotting factors like II, VII, IX, and X (and the anticoagulant proteins C and S) become under-carboxylated and function poorly, leading to impaired coagulation. Other vitamins don’t play this carboxylation role—Vitamin C is involved in collagen synthesis, Vitamin D in calcium homeostasis, and Vitamin A in vision and epithelial health. Thus, vitamin K is required.