Prothrombin thrombin

The clear serum of this example is an amber liquid free from prothrombin, thrombin, fibrinogen and fibrin. It contains profibrinolysin and is excellently suited to further purification by salt precipitation fractionation, as given below. 

In blood coagulation in the mammal, a key final reaction is Prothrombin —> Thrombin... 

Fig. 31.2. Structure of prothrombin. Thrombin is liberated through the cleavage of the Arg 274-Thr 275 and Arg 323-lle 324 peptide bonds (indicated by the stars). The y-carboxyglutamate residues are in the released N-terminal portion of prothrombin and are not part of thrombin. The A and B chains of thrombin are joined by a disulfide bond.

Circulating blood contains practically no thrombin, only its precursor prothrombin. The conversion of prothrombin to thrombin is usually considered to be the first phase of blood clotting. Numerous factors are involved in this preliminary phase First, thrombokinase from tissue with the accelerators and Ca++ second, thrombokinase from blood, which is formed by the thrombocyte factor in presence of at least four plasma factors and Ca++. The conversion prothrombin — thrombin proceeds autocatalytically the process is accelerated by the action of newly formed thrombin on factor V proaccelerin) by converting this factor into the active factor... 

Clotting may be inhibited or disturbed in several ways. The well-known bleeding condition hemophilia is caused by the absence (or inactivity) of any one component of the thrombokinase system from blood, i.e. of some plasma factor. In classical hemophilia A it is factor VIII (antihemophilic globulin) in hemophilia B, factor IX (Christmas factor). Due to a deficiency in these factors insufficient amounts of plasma thrombokinase are formed, and consequently the transition prothrombin —> thrombin is delayed greatly or even prevented. 

Heparin acts as an anticoagulant. It prevents blood clotting by inhibiting the prothrombin-thrombin conversion and thus eliminating the thrombin effect on fibrinogen. Heparin is used therapeutically. 

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