Fibrin Meshwork Formation : Formation of γ-carboxyglutamate residues

γ-Carboxylation is a posttranslational modification in which 9–12 glutamate residues (at the amino [N]-terminus of the target protein) get carboxylated at the γ carbon, thereby forming Gla residues. The process occurs in the rough endoplasmic reticulum (RER) of the liver.
1. γ-Carboxylation: This carboxylation reaction requires a protein substrate, oxygen (O2), carbon dioxide (CO2), γ-glutamyl carboxylase, and the hydroquinone form of vitamin K as a coenzyme (Fig. 1). In the reaction, the hydroquinone form of vitamin K gets oxidized to its epoxide form as O2 is reduced to water. [Note: Dietary vitamin K, a fat-soluble vitamin , is reduced from the quinone form to the hydroquinone coenzyme form by vitamin K reductase (Fig. 2).]

Figure 1:  γ-Carboxylation of a glutamate (Glu) residue to γ-carboxyglutamate (Gla) by vitamin K–requiring γ-glutamyl carboxylase. The γ carbon is shown in blue. O2 = oxygen; CO2 = carbon dioxide.

Figure 2:  The vitamin K cycle. VKOR = vitamin K epoxide reductase.
2. Inhibition by warfarin: The formation of Gla residues is sensitive to inhibition by warfarin, a synthetic analog of vitamin K that inhibits the enzyme vitamin K epoxide reductase (VKOR). The reductase, an integral protein of the RER membrane, is required to regenerate the functional hydroquinone form of vitamin K from the epoxide form generated in the γ-carboxylation reaction. Thus, warfarin is an anticoagulant that inhibits clotting by functioning as a vitamin K antagonist. Warfarin salts are used therapeutically to limit clot formation.

[Note: Warfarin is used commercially as a pest control agent such as in rat poison. It was developed by the Wisconsin Alumni Research Foundation, hence the name.]
Genetic differences (genotypes) in the gene for catalytic subunit 1 of VKOR (VKORC1) influence patient response to warfarin. For example, a polymorphism  in the promoter region of the gene decreases gene expression, resulting in less VKOR being made, thereby necessitating a lower dose of warfarin to achieve a therapeutic level. Polymorphisms in the cytochrome P450 enzyme (CYP2C9) that metabolizes warfarin are also known. In 2010, the U.S. Food and Drug Administration added a genotypebased dose table to the warfarin label (package insert). The influence of genetics on an individual’s response to drugs is known as pharmacogenetics.