Vitamin K undergoes a specific enzymatic regeneration cycle after each round of carboxylation. Which enzyme catalyses the reduction of vitamin K epoxide back to active vitamin K (hydroquinone form)?

• A Glutamate carboxylase (GGCX)
• B Vitamin K epoxide reductase complex 1 (VKORC1) — the target of warfarin ✓
• C Thioredoxin reductase using NADPH
• D DT-diaphorase (NQO1) using NADH

Explanation

In the vitamin K cycle: KH2 (hydroquinone) is the active form that serves as electron donor for gamma-glutamyl carboxylase (GGCX), converting glutamate to gamma-carboxyglutamate (Gla) in coagulation factors II, VII, IX, X, protein C, S, Z. KH2 is oxidised to vitamin K epoxide (KO) in this reaction. VKORC1 (vitamin K epoxide reductase complex subunit 1) then reduces KO → K → KH2, completing the cycle. Warfarin inhibits VKORC1, depleting KH2 and impairing Gla synthesis, anticoagulating the patient.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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