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. {ECO:0000269|PubMed:14765194, ECO:0000269|PubMed:14765195, ECO:0000269|PubMed:15879509, ECO:0000269|PubMed:16270630, ECO:0000269|PubMed:20978134, ECO:0000269|PubMed:22923610, ECO:0000269|PubMed:33154105}. This is the function of VKORC1 (vitamin K epoxide reductase complex subunit 1, ENSG00000167397).