Involved in vitamin K metabolism. Catalytic subunit of the vitamin K epoxide reductase (VKOR) complex which reduces inactive vitamin K 2,3-epoxide to active vitamin K. Vitamin K is required for the gamma-carboxylation of various proteins, including clotting factors, and is required for normal blood coagulation, but also for normal bone development. [protein]-disulfide + H2O + phylloquinone = 2,3-epoxyphylloquinone + [protein]-dithiol [protein]-disulfide + phylloquinol = [protein]-dithiol + phylloquinone Inhibited by warfarin (coumadin). Highly expressed in liver. Detected at lower levels in lung, kidney and testis. The number of transmembrane domains and the membrane topology are controversial; supporting evidence is available both for models with three transmembrane domains and four transmembrane domains. The location of two cysteine active-site residues within a proposed transmembrane is consistent both with the known hydrophobic environment of the thiol redox site of the enzyme and with the lipophilicity of vitamin K and warfarin (coumadin). Belongs to the VKOR family.