The Vitamin K-dependent Carboxylase

Abstract

The only known biological function of Vitamin K (Fig. 1) in animals is as a required cofactor for the production of the unusual amino acid, -carboxyglutamate (Gla). This amino acid has a profound role in human blood coagulation. Several blood proteins require the presence of nine to thirteen Gla residues for normal function; these are the socalled Vitamin K-dependent (VKD) proteins. While some of the VKD blood proteins have a pro-coagulant function (prothrombin, and factors VII, IX, and X), others primarily serve anti-coagulant roles (proteins C, S, and Z). For all of the VKD blood proteins, however, the Gla residues are located in a homologous ≈ 45 residue amino-terminal “Gla” domain (1-3). The presence of multiple Gla residues in this domain allow it to adopt a calcium-dependent conformation that promotes binding to a membrane surface (4, 5), such as found on damaged vascular endothelial cells or activated platelets. This interaction allows the localization of the VKD blood protein near the site of vascular injury, where it participates in reactions that either promote or regulate clotting (6-8). The importance of Gla in the catalytic function of these proteins is emphasized by the effect that administration of Vitamin K antagonists (e. g. coumadins, such as warfarin sodium) has on humans. Warfarin decreases the concentration of vitamin K in the tissues, which, in turn, results in the production of VKD proteins that contain a decreased number (or a complete absence) of Gla. The Gla domain of these under-carboxylated blood proteins cannot adopt its natural conformation and, as a result, the VKD blood proteins have poor affinity for phospholipid surfaces. For this reason, the reactions that involve these proteins are significantly damped, and efficient clotting no longer occurs (9-11). Gla is synthesized in mammals by post-translational modification of glutamate. The enzyme that catalyzes the conversion of glutamate to Gla is the vitamin K-dependent -glutamyl carboxylase (12). In addition to a glutamate-containing substrate, this enzyme requires carbon dioxide, reduced Vitamin K, and molecular oxygen as reactants. The products of the carboxylase-catalyzed reaction are Gla, Vitamin K 2,3 epoxide, and water (13) (Fig. 2). The enzyme is found not only in liver (where the VKD blood proteins are produced and secreted), but also in a variety of other tissues, such as skin, lung, and kidney (13, 14). The widespread tissue distribution of carboxylase in humans suggests the presence of additional Gla-containing proteins of diverse function. To date, fourteen VKD proteins have been identified in humans, seven of which are the blood proteins mentioned above. Two other proteins (bone-gla and matrix-gla protein) are involved in bone metabolism (15), while another (Gas6) is involved in cell signaling (16, 17). Screenings of human nucleotide-based databases for sequences with homology to Gla domain sequences have identified four additional putative VKD proteins of unknown function: PRGP1, PRGP2, TMG3 and TMG4 (18, 19).