boxylated VKD proteins. Their localization at/near sites of vascular injury is crucial for proper hemostatic function. The hemostatic system, in response to vascular injury, triggers platelet aggregation and initiation of the coagulation cascade, primary and secondary hemostasis, respectively, to prevent bleeding. The coagulation cascade initiated via tissue factor - the extrinsic pathway, or contact - the intrinsic pathway, converges into a common pathway leading to formation of insoluble fibrin clots. A fibrinolytic cascade follows to lyse fibrin and protect against blood clotting. Several anticoagulant systems are in place to tightly regulate coagulation. Major blood coagulation proteases - F2, F7, F9 and F10, components of the protein C anticoagulant pathway - protein C (Proc) and protein S (Pros), and the protein Z (Proz) cofactor of Serpina10 (protein Z-dependent protease inhibitor, ZPI), are vitamin K-dependent proteins (VKD). In clotting factors, 10 to 12 Gla residues are located in a homologous amino-terminal region known as the Gla domain(click to access the PFAM entry). Gla domains are also found in several other proteins such as osteocalcin, matrix Gla, transmembrane Gla or proline-rich Gla proteins (MGP, TMGs, PRGPs). With each modified residue, a reduced form of vitamin K is oxidized and is then converted back, to provide the necessary cofactor for the next reaction - the metabolic pathway is known as the vitamin K cycle. Vitamin K and its cycle in the gamma-carboxylation reaction are described in more detail.

Vitamin K is a fat-soluble vitamin derived from diet that can also be synthesized by gut microflora. There are several forms of vitamin K, they contain a common ring structure - menadione (2-methyl-1,4-naphtoquinone) and a hydrophobic polyisoprenoid side chain that varies in length and degree of saturation. Vitamin K₁, known as phylloquinone, is the biological important form. Its highest content is found in green leafy vegetables, followed by certain vegetable oils, then fruits, cereals, meats and dairy products. MK family or vitamin K₂ family members are synthesized by certain bacteria, of which some are present in the human gut microflora. Long-chain MKs can be found in animal livers and foods that use a bacterial fermentation stage. Relatively little is known about vitamin K cellular uptake and metabolism. Plasma concentration is rather low, and it is rapidly catabolized in the liver and excreted, primarily in the bile and also urine, mostly in the form of glucuronides.
The vitamin K cycle is the metabolic pathway whereby the oxidized vitamin, resulting from the gamma-carboxylation of VKD proteins in the endoplasmic reticulum, is converted back to its reduced form. Gamma-glutamyl carboxylase (Ggcx) carries out the Glu to Gla reaction using a reduced form of K₁ - K hydroquinone (KH₂), carbon dioxide and oxygen, as cofactors. The reaction converts glutamate residues to gamma-carboxyglutamate and KH₂ is oxidized to vitamin K 2,3-epoxide (KO). KO is converted back to KH₂ in a two-step reduction: first to vitamin K (K quinone) and then to KH₂. Vitamin K epoxide reductase (Vkorc1) carries out the first reduction, and is able to carry out the second. A putative reductase, specifically carrying out the second step, is still to be identified. Reduction is important not only for the regeneration of the necessary cofactor in the carboxylation reaction, but also for molecules entering the system from diet, where vitamin K is present in its oxidized form. Natural antagonists or drugs can interfere with the regeneration of vitamin K cofactor, mostly by targeting VKORC1. VKORC1 is the target of the widely-used oral anticoagulant, warfarin. The first reduction step in the cycle is warfarin-sensitive, the second is not and the search for the reductase referred to as the 'warfarin-resistant antidotal enzyme' is still on. More than 30 mutations in GGCX have been found in patients with vitamin K-related disorders. In addition to the bleeding phenotype associated with impaired coagulation, non-bleeding phenotypes are also documented. Mutations in VKORC1 lead to warfarin resistance. In addition to warfarin, which affects coagulation by targeting the vitamin K cycle, newer drugs are being developed that directly target the coagulation factors. To see the ontology report for GViewer, annotations, and download, click here...(less)