Thrombin (Factor IIa) is an enzyme, a serine protease, which participates in blood clotting. It is formed when prothrombin (Factor II) is proteolytically cleaved to form thrombin, which is glycoprotein formed by two peptides chains of 36 and 259 amino acids. Thrombin converts soluble fibrinogen into insoluble strands of fibrin and is then neutralized by antithrombin in order to for a thrombin-antithrombin (TAT) complex.