The relationship of γ-carboxylation to the enzymes of the vitamin K cycle and vitamin K metabolism. Also shown is the inhibition of the thiol-dependent vitamin K epoxide reductase activity by warfarin. In the presence of warfarin, dietary vitamin K can enter the cycle via an NAD(P)H-dependent quinone reductase that is relatively insensitive to warfarin. Illustration by Marie Dauenheimer.