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Vitamin K Epoxide
Figure 1. Vitamin K Epoxide structure
As mentioned above, Vitamin K epoxide is a part of the Vitamin K cycle, necessary for blood coagulation. In the cycle, Vitamin K epoxide reductase (VKOR) reduces Vitamin K epoxide to quinone, or the active form of Vitamin K. What is occurring is VKOR donated electrons to Vitamin K epoxide, and those electrons come from the S-H of one of the cysteine pairs discussed above. The one cysteine pair has to be reduced for the transfer of electrons to the substrate can occur.
Binding
To start, VKOR is in its open conformation. The Vitamin K epoxide enters. The oxygens of the ketones bind to . With Vitamin K epoxide in its place, the conformation of VKOR is partially oxidized in regards to the cysteine pairs, which overall leads to the reduction of the substrate. A disulfide bond forms between Cys51 and Cys132, resulting in the closed conformation. This leaves the sulfur on Cys43 and the sulfur on Cys135 protonated. The available hydrogens on these cysteines are utilized in reducing the epoxide. First, the sulfur on Cys51 and Cys43 form a new bond. The hydrogen from Cys43 binds to the oxygen in the epoxide. The sulfur on Cys132 and the sulfur on Cys15 then form a new disulfide bond. The hydrogen that was present on Cys135 forms a new bond with the oxygen of the epoxide. With these cysteine pairs formed, VKOR is left in an open conformation. The end products are the Vitamin K/quinone and water.
Warfarin
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