Vitamin K is used in which step of clotting factor activation-

Ans. is 'b' i.e., Carboxylation * Vitamin K functions as a cofactor for an enzyme carboxylase that catalyzes post-translation modification by carboxylation of glutamic acid (glutamate) residues on vitamin K dependent proteins.* Vitamin K plays an important role in blood coagulation for it is required for the post-translational processing of several clotting factors (factor II, VII, IX and X).* Maturation of these clotting factors requires the conversion of glutamyl residues of precursor proteins into g-carboxyglutamate (Gla) residues by addition of carboxylate group. This carboxylation of glutamyl resdiue is vitamin K dependent.* The g-carboxyglutamate (Gla) residues so formed serve as binding site for calcium ions. Each Gla contains two negative charges which chelate the positive calcium ion.* After binding to Gla residue on activated clotting factor, calcium binds with negatively charged phospholipids present on the platelet cell membrane.* In this way, bridging of the phospholipids the Gla residue ofprothromin occurs via calcium ion.