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

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

Answer: (C)

Gamma-carboxylation of glutamate residues in clotting factors is the modification that enables these proteins to bind calcium and interact with phospholipid surfaces, which is essential for their role in the coagulation cascade. This process requires vitamin K as a cofactor for the enzyme gamma-glutamyl carboxylase, which converts specific glutamate residues into gamma-carboxyglutamate. Without enough vitamin K, the clotting factors cannot bind calcium effectively, undermining their activity and leading to impaired clot formation. Vitamins A, D, and C have other roles in the body and do not participate in this gamma-carboxylation step. Clinically, antagonists like warfarin disrupt vitamin K recycling, reducing gamma-carboxylation and producing anticoagulation by lowering the activity of these vitamin K–dependent factors.