The vitamin (Vit.) K cycle, resulting in the posttranslational conversion of specific glutamate residues on certain target proteins into Gla. This conversion is catalyzed by the endoplasmic reticulum enzyme, GGCX, which utilizes the fully reduced form of vitamin K (KH₂) as a cosubstrate. An oxidized form of vitamin K (epoxide), produced as a byproduct of this reaction, is recycled back into fully reduced KH₂ by other enzymes (reductases). This recycling of oxidized vitamin K is the portion of the cycle inhibited by warfarin and related drugs. The figure has been adapted from Figure 2B in the article by Tie et al that begins on page 1847.