olites. Metabolism by phase II enzymes is less well understood although glucuronidation of most metabolites, carried out by several (UDP)-glucuronosyltransferases (UGTs) with various specificities, is reported. The target of warfarin is vitamin K epoxide reductase (VKORC1), the enzyme that reduces the oxidized vitamin K and therefore regenerates the reduced form of the vitamin and essential cofactor for the carboxylation reaction of glutamate (Glu) to gamma-carboxyglutamate (Gla). The posttranslational modification of proteins known as 'vitamin K-dependent proteins' (VKDs) is an important modification that allows for the reversible, calcium-dependent, binding to negatively charged phospholipids (phosphatidylserine or phosphatidic acid) of membranes. This is particularly important for coagulation/clotting factors and natural anticoagulants, classic examples and better characterized systems of gamma-carboxylated VKD proteins. The reaction is carried out by gamma-glutamyl carboxylase (GGCX) and in the reaction the reduced K hydroxyquinone (KH₂) is oxidized to vitamin K 2,3-epoxide (KO). KO is converted back to KH₂ by VKORC1. The shuttling between the oxidized and reduced forms is known as the vitamin K cycle metabolic pathway. By inhibiting the cycle, warfarin precludes the modification of VKD proteins and therefore affects their function. As many of the important enzymes of the coagulation cascade are VKDs, warfarin inhibits blood clotting and is antithrombotic. Coagulation factor FVII (F7) of the extrinsic pathway, factors IX (F9) and X (F10) of the tenase and prothrombinase complexes, respectively and thrombin/F2, the last enzyme of the coagulation cascade, are VKD proteins. Several anticoagulation factors, such as protein C, protein S and protein Z, and a number of other proteins that include osteocalcin and matrix Gla protein, among others, are also VKDs. Inhibition of clotting can experience some delay or there may even be pro-coagulation effects, particularly at the beginning of drug treatment. Bleeding and thromboembolic effects are observed and can be rather severe. Relative dose of the drug, inter-individual variability, diet can all contribute. Warfarin can interact with drugs and foods and overall, dosing of warfarin is challenging. Foods, such as green leafy vegetables in particular but others too, are high in vitamin K. Variants in the enzymes of warfarin or vitamin K metabolism can affect the dosing of the drug and impact on its response. Side effects of warfarin include necrosis, particularly of the skin, osteoporosis, vascular calcification, warfarin-related nephropathy, some likely due to VKD proteins beyond coagulation. No structure of the human VKORC1 is available. The structure of a bacterial homolog shows the enzyme being composed of five transmembrane helices (TMs) and the quinone substrate close to the CXXC (Cys132 and Cys135, human protein) motif in the active site. Homologs of VKORC1 are found in bacteria, plants and mammals and possess the CXXC motif in the active site; the motif is essential for substrate reduction. Warfarin binds VKORC1 and the binding is believed to be irreversible. Mutations in VKORC1 largely result in resistance to warfarin; a mutation resulting in reduced activity of the enzyme is reported, but is very rare. More novel anticoagulant drugs are directly targeting important components of the coagulation cascade such as the activated factor X (F10a) or thrombin (F2a). There are both advantages and disadvantages in the use of these drugs, compared to the vitamin K antagonist, warfarin. To see the ontology report for annotations, GViewer and download, click here...(less)