Which vitamin is required for gamma-carboxylation of glutamate residues in clotting factors?

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

Answer: (D)

Gamma-carboxylation of glutamate residues in clotting factors requires vitamin K as a cofactor for gamma-glutamyl carboxylase. This modification adds carboxyl groups to specific glutamate residues, creating gamma-carboxyglutamate that can bind calcium. The calcium-bound clotting factors then interact properly with phospholipid surfaces, which is essential for the coagulation cascade to proceed effectively. Vitamin K is recycled in this process through the vitamin K epoxide reductase system, which is why anticoagulants like warfarin disrupt it by blocking that recycling step. Other vitamins don’t participate in this post-translational modification: vitamin C aids collagen synthesis, vitamin D regulates calcium metabolism, and vitamin A supports vision and differentiation.