Prothrombin is the vitamin K-dependent zymogen of thrombin. As a single chain protein (Mr 72,000) it contains 10 Gla residues, 2 kringle domains and three N-linked carbohydrate units. Prothrombin is activated to thrombin by the prothrombinase complex consisting of enzyme factor Xa, cofactor factor Va, phospholipid and Ca2+.