Vitamin K is required for carboxylation of glutamate residues in clotting factors. Which enzyme catalyses this reaction, and what happens to vitamin K in the process?

• A Gamma-glutamyl carboxylase carboxylates Glu to Gla, and vitamin K hydroquinone is oxidised to vitamin K epoxide; vitamin K epoxide reductase (VKORC1) then recycles it ✓
• B Prothrombin activator carboxylates prothrombin using vitamin K as a direct acyl donor, generating vitamin K aldehyde
• C ATP-dependent carboxylation uses vitamin K as a carbon donor, similar to biotin-dependent carboxylations
• D Vitamin K undergoes no chemical change itself; it acts as a structural cofactor holding the carboxylase substrate in position

Explanation

The gamma-glutamyl carboxylase in the ER uses the reduced form of vitamin K (vitamin K hydroquinone, KH2) as an electron donor for the carboxylation of specific glutamate residues to gamma-carboxyglutamate (Gla). In this reaction, KH2 is oxidised to vitamin K epoxide (KO). VKORC1 (vitamin K epoxide reductase complex subunit 1) regenerates KH2 from KO, recycling the vitamin (the 'vitamin K cycle'). Warfarin inhibits VKORC1, depleting KH2 and impairing Gla formation. Gla domains chelate Ca²⁺ and anchor clotting factors to phospholipid membranes.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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