Prothrombin zymogen form

Most of the proteases involved in blood clotting are zymogens, such as fibrinogen and prothrombin, which circulate in blood in the inactive form. They are cleaved to the active form (fibrin and thrombin, respectively) by other proteases, which have been activated by their attachment to the site of injury in a blood vessel wall. Thus, clots form at the site of injury and not randomly in circulation. 

The zymogen prothrombin is cleaved to give the active enzyme thrombin. The thrombin then cleaves a soluble molecule, fibrinogen, into an insoluble molecule, fibrin. Fibrin is a protein that forms part of the blood clot... 

Experiments were also conducted using pooled human plasma. The predominant, stable form of the protein in plasma is prothrombin, a zymogen that is cleaved to yield the activated thrombin upon appropriate signaling (Blomback and Hanson, 1979). The presence of any detectable thrombin in its active (i.e., cleaved) form is probably due to stabilizers in the pooled plasma preparation. The plasma concentration of prothrombin is about 90mg/L (Blomback and Hanson, 1979), which corresponds to about 2.5 p.M, or 7.5 pmol on the aptamer spot. 

Release of thrombin fi-om its inactive zymogen. Prothrombin (see), is the penultimate step in a series of reactions, each of which releases an active serine protease from an inactive precursor in the blood. It is an example of cascade regulation, in which each activated protease activates the next precursor down the line, increasing amounts of material being involved at each step (i.e. there is amplification). The clotting factors are listed in the Table. The assigned Roman numerals are historical, and the activated form is indicated by the subscript a (e.g. XII,). 

---