GGCX (gamma glutamyl carboxylase) in the endoplasmic reticulum gamma-carboxylates thirteen glutamate residues on PROZ(24-400) (pro-protein Z). MK4 (vitamin K hydroquinone) is oxidized to MK4 epoxide in the process (Berkner 2000; Furie et al. 1999; Stenina et al. 2001; Morris et al. 1995).