Which vitamin is a cofactor for gamma-carboxylation of blood clotting enzymes?

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

Answer: (B)

Vitamin K is required to activate blood-clotting proteins by enabling gamma-carboxylation of glutamate residues. This modification creates gamma-carboxyglutamate (Gla) residues that bind calcium, allowing clotting factors such as II, VII, IX, and X (and proteins C and S) to attach to phospholipid surfaces and function properly in the coagulation cascade. The enzyme gamma-glutamyl carboxylase uses reduced vitamin K to add the carboxyl groups, and vitamin K is recycled through the cycle involving vitamin K epoxide reductase. This is why interference with vitamin K—such as with warfarin, which blocks the recycling step—leads to reduced gamma-carboxylation and anticoagulation. Other vitamins don’t play this specific role in activating clotting factors.