[Kinetics of prothrombin activation by factor Xa]

It is shown that activation of prothrombin by factor Xa with the presence of the substrate low concentrations occurs as two-stage reaction described by Michaelis-Mentene equation for stationary conditions. The rate constant of the complex formation is 2 and 3 order higher than the constant of dissociation and decay, respectively. Judging from a two-stage character of the process, it is possible to conclude that the second intermediate product (enzyme-modified substrate) in not form in the reaction: the both links in prothrombin sensitive to factor Xa might be attacked simultaneously. In the presence of the substrate high concentrations an inhibition may be observed due to formation of a complex, which involves one enzyme molecule and 1 + n substrate molecules. This complex preserves the enzymic activity because the constant of changes in its decay (beta) rate is larger than 0, but less than 1. The inhibition of factor Xa by the prothrombin surplus may be considered as a self-regulation element in the coagulation system at the level of molecular interactions.