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). Expressed at highest levels in fetal and adult liver, followed by fetal heart, kidney, and lung, adult heart, and pancreas. The number of transmembrane domains and the membrane topology are controversial; supporting evidence is available both for models with three transmembrane domains (PubMed:15716279 and PubMed:22923610) and four transmembrane domains (PubMed:20696932 and PubMed:20978134). According to PubMed:15716279 and PubMed:22923610 the N-terminus of the protein is in the endoplasmic reticulum lumen, while the C-terminus is in the cytosol, which is in favor of three transmembrane domains. According to PubMed:20696932, both N-terminus and C-terminus are in the cytosol, indicating the presence of four transmembrane domains. Besides, the 3D-structure of a bacterial ortholog shows four transmembrane domains. Moreover, proteins that reside in the endoplasmic reticulum lumen can catalyze the reduction of the active site cysteines, possibly via Cys-43 and Cys-51 (PubMed:20696932 and PubMed:20978134), but less efficiently than the synthetic compound dithiothreitol (in vitro). Location of Cys-43 and Cys-51 in the endoplasmic reticulum lumen would be in agreement with four transmembrane domains. Again, these data are controversial, and papers do not agree on the effects of mutating Cys-43 and Cys-51, probably because of differences in the assay systems. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. 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. Truncated N-terminus.