[Comparative theoretic analysis of an open reaction S1 goes to and comes from S2 E(R,T) in which the oligomeric enzyme E(R,T) is isosterically or allosterically activated by the product S2].

In connection with evolutionary aspects of the mechanisms of allosteric regulation of cell metabolism, a mathematical model of an open reaction leads to S1 E(R,T) in equilibrium S2 leads to involving product activation of the olygomeric enzyme E(R,T) whose protomers undergo the concerted conformational transitions R in equilibrium T, has been analysed. Two activation mechanisms, isosteric and allosteric, were considered. Both mechanisms produce qualitatively the same effects: the input characteristic of the reaction possesses hysteresis, which causes multiple steady states and self-oscillations. In the case of isosteric activation, only a small fraction of the maximum enzyme activity is utilized because of the strong competition between S1 and S2 for the active sites. In the case of allosteric regulation, this competition may almost completely be eliminated and the enzyme activity may be utilized with high efficiency, provided the affinity of S2 for the allosteric sites is at least an order of magnitude higher than for the active sites. Qualitative similarity between the two cases of product activation, in spite of their great quantitative discrepancy, favours the hypothesis that the mechanisms of isosteric regulation were the direct precursors of the homological allosteric regulatory mechanisms.