This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: AIIMS 2018, Subject name: Biochemistry
Vitamin K in its coenzyme form is regenerated by which enzyme? 
Select and explain. 
A. Glutathione reductase, B. Pyruvate carboxylase, C. Dihydrofolate reductase, D. Epoxide reductase
D: Epoxide reductase
Answer: D: Epoxide reductase 
Explanation: Vitamin K - Vitamin K hydroquinone is oxidized to the epoxide Epoxide activates a glutamate residue in the protein substrate to a carbanion, which reacts nonenzymically with carbon dioxide to form g-carboxyglutamate. Vitamin K epoxide is reduced to the quinone by an epoxide reductase, and the quinone is reduced to the active hydroquinone by Vitamin K reductase. Dicumarol & warfarin inhibits the enzyme (reductase) that conves epoxide to active Vitamin K (hydroquinone).