FIGURE 54-6 The vitamin K cycle: y-glutamyl carboxylation of vitamin K-dependent proteins. The enzyme ^-glutamyl carboxylase couples the oxidation of the reduced hydroquinone form (KH2) of vitamin Kx or K2, to )-carboxylation of Glu residues on vitamin K-dependent proteins, generating the epoxide of vitamin K (KO) and )-carboxyglutamate (Gla) residues in vitamin K-dependent precursor proteins in the endoplasmic reticulum. A 2,3-epoxide reductase regenerates vitamin KH2 and is the warfarin-sensitive step. The R on the vitamin K molecule represents a 20-carbon phytyl side chain in vitamin Kx and a 5- to 65-carbon prenyl side chain in vitamin K2.
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