Carboxylase function and disruption in PXE-like disease. (A) The carboxylase performs 2 reactions: epoxidation of reduced vitamin K (KH₂) to vitamin K epoxide (KO) and carboxylation where CO₂ addition to Glu generates carboxylated Glu (Gla). (B) VKD proteins contain an exosite-binding domain (EBD) that mediates high-affinity binding through the carboxylase exosite and also activates Glu catalysis (red arrow). Multiple Glu residues are converted to Gla by a processive mechanism in which VKD proteins remain bound to the carboxylase until the Gla domain is fully carboxylated.^27,28 (C) Known functional regions of the carboxylase include those facilitating catalysis (CAT), VKD protein binding (VKS, EXO), Glu binding (Glu), and the carboxylase Gla domain (Gla). Most of the residues whose mutations cause disease reside in regions where the function of the carboxylase is unknown. This study shows that impaired processivity in the V255M mutant results in PXE-like disease.