Identification of vitamin K-dependent proteins in mouse liver by LC-MS/MS

Vitamin K (VK), a fat-soluble vitamin, functions as a co-factor during the gamma-carboxylation reaction that converts glutamic acid residues to gamma-carboxyglutamic acid residues in proteins transiting through the endoplasmic reticulum. Two enzymes are involved which together form the VK cycle: gamma-glutamyl carboxylase (GGCX), and vitamin K oxidoreductase (VKORC1). GGCX requires reduced VK (VKH2) as an essential cofactor, which upon carboxylation, is oxidized to VK epoxide and then reconverted to VKH2 by VKORC1. Physiologically, gamma-carboxylation is essential in the liver for the activity of several coagulation factors (e.g., prothrombin, factor IX, etc.), and in arteries and cartilage to modulate the activity of Matrix Gla Protein (MGP) which prevents extra-osseous tissue mineralization. Gamma-carboxylation also negatively regulates the function of osteocalcin, a bone-derived hormone that promotes glucose homeostasis. Our objective is to identify unrecognized gamma-carboxylated proteins.