thrombin A proteolytic enzyme, involved in the clotting of blood, that is generated by the conversion of its zymogen prothrombin, the process being activated by the enzyme thromboplastin, in the presence of calcium ions, and other factors. Thrombin acts upon the soluble fibrinogen in blood to produce insoluble fibrin by catalysing the removal of the fibrinopeptides A and B from fibrinogen. See also BLOOD CLOTTING; BLOOD PLASMA.

thrombin An enzyme that catalyses the conversion of fibrinogen to fibrin. See blood clotting; prothrombin.

thrombin (throm-bin) n. a substance (coagulation factor) that acts as an enzyme, converting the soluble protein fibrinogen to the insoluble protein fibrin in the final stage of blood coagulation. Thrombin is derived from the inactive substance prothrombin.

thrombin Plasma protein involved in the coagulation of blood.