Which vitamin is essential for gamma-carboxylation of clotting factors?

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

Answer: (A)

Gamma-carboxylation of clotting factors is driven by a vitamin K–dependent enzyme that requires reduced vitamin K to convert specific glutamate residues into gamma-carboxyglutamate. This modification creates calcium-binding sites (Gla residues) on factors II, VII, IX, X and on proteins C and S, allowing them to attach to phospholipid surfaces and participate effectively in the coagulation cascade. Without this gamma-carboxylation, those factors can’t bind calcium properly and are functionally inactive in clot formation. Other vitamins don’t participate in this posttranslational modification, which is why vitamin K is essential here. Clinically, inhibiting the vitamin K cycle (as warfarin does) reduces gamma-carboxylation and produces an anticoagulant effect.