Vitamin K is essential for post-translational gamma-carboxylation of glutamate residues in coagulation factors II, VII, IX, X, and proteins C and S. The reaction requires which form of vitamin K as the active carboxylase cofactor?

• A Vitamin K3 (menadione)
• B Vitamin K hydroquinone (KH2, reduced form) ✓
• C Vitamin K epoxide
• D Vitamin K1 (phylloquinone) before reduction

Explanation

Vitamin K carboxylase uses the reduced hydroquinone form of vitamin K (KH2) as cofactor — KH2 is oxidized to vitamin K epoxide while simultaneously driving gamma-carboxylation of Glu to Gla residues. The Gla residues bind Ca2+ and enable coagulation factor binding to phospholipid membrane surfaces. Vitamin K epoxide is then recycled back to KH2 by vitamin K epoxide reductase (VKOR), the target of warfarin (which blocks VKOR). Vitamin K3 (menadione, synthetic) lacks the side chain and can cause hemolytic anemia. Measuring PIVKA-II (proteins induced by vitamin K absence) confirms functional deficiency.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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