Biochemistry, Vol. 1 - A. Lehninger 1985

Biomolecules
Vitamins and Trace Elements: Their Role in Enzyme Function
Vitamin K as a Component of the Carboxylating Enzyme

The two main forms of vitamin K—Vitamins K₁ and K₂—are found in significant amounts in most higher plants. Both forms are naphthoquinones with isoprenoid side chains of varying lengths (Fig. 10-24). Vitamin K deficiency in chicks and other animals leads to impaired Blood clotting.

Relatively recently, The biochemical function of vitamin K in the blood clotting mechanism was successfully established. Vitamin K is essential for the normal production of prothrombin, a Blood Plasma protein that serves as the inactive precursor of Thrombin—the enzyme that converts the plasma protein fibrinogen into fibrin, an insoluble, fibrous protein that promotes blood clot formation. For prothrombin to be activated and converted into thrombin, it must bind Ca²⁺ ions. In animals with a vitamin K deficiency, defective prothrombin molecules are synthesized that are unable to properly bind Ca²⁺ ions. A normal prothrombin molecule contains several residues of a unique amino acid, γ-carboxyglutamic acid, which is responsible for binding Ca²⁺ ions. In vitamin K deficiency, glutamic acid residues are incorporated into the prothrombin molecule instead of γ-carboxyglutamic acid residues. John Suttie of the University of Wisconsin discovered an enzymatic system capable of converting glutamic acid residues in prothrombin—isolated from the blood of vitamin K-deficient animals—into γ-carboxyglutamic acid residues. Vitamin K is required for this enzyme's activity (Fig. 10-25). Several other Ca²⁺-binding Proteins in the body also contain γ-carboxyglutamic acid residues.

Class="center">

Fig. 10-24. Different forms of vitamin K.

Fig. 10-25. The function of vitamin K as a cofactor in The formation of γ-carboxyglutamic acid residues in prothrombin and other proteins.