Biosynthetic pathway for vitamin K-dependent production of γ-carboxyglutamic acid. It has been hypothesized that a free cysteine residue in the carboxylase converts vitamin KH₂into a “strong base” of sufficient basicity to abstract a hydrogen from the γ-carbon of glutamic acid.17Subsequently, CO₂ is added to the γ-carbon of glutamic acid to form γ-carboxyglutamic acid. The activated vitamin K species collapses into vitamin K epoxide and is recycled back to vitamin KH₂, following the action of two vitamin K reductases, one of which is sensitive to warfarin.25