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). Detected in liver. 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. Mice are born at the expected Mendelian rate and appear normal, but die between one and twenty days after birth, due to severe bleeding. In about 75% of the cases, subdural bleeding is observed, in addition to intracerebral and intramuscular bleeding. Daily oral administration of vitamin K to the mutant mice leads to normal survival, but the mice die within seven days after the cessation of vitamin K administration. Besides, both homozygous and heterozygous mutant mice display defects in bone development with reduced length of the calcified part of the long bones in front and hind limbs. 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.