Energy-Requiring Step of ATP Formation

In aqueous solution the reaction of ATP formation from ADP and P is unprofitable for any reasonable concentration of ATP, ADP, and Pf in a cell. Within the pH interval from 6 to 9, and Mg concentrations up to 50 mM, the equilibrium constant, of the reaction ADPH -h H2PO4 ATPH -h H2O (this equation is written, for certainty, just for one state of the reagent s ionization, while there are other possible states depending on 

Let us consider the simplest formal scheme of the ATPsynthase reaction 

For the sequence of processes (5.8) the equilibrium constant, related to the reagents in aqueous solution, can be expressed using the Haldane relationship 

Direct evidence for the last statement was obtained in a number of very important experiments carried out in Boyer s laboratory. Studying the Pf-H2 0 isotopic exchange reaction in uncoupled mitochondria, Boyer and his collaborators had demonstrated that, in the catalytic center of the mitochondrial ATPsynthase, there occurred numerous acts of the formation and rupture of the covalent bond between ADP and P . Boyer proposed that ATP formation in the catalytic center is simply the reversion of the ATP hydrolysis reaction, ATP + H2O ADP -h Pj. This conclusion follows from the experimental fact that ATPase catalyzes the incorporation of the isotope from H2O into Pf that appears in the medium. The main features of Boyer s binding-change mechanism of ATP synthesis were clearly formulated in [167] as follows  

Energy promotes release of ATP tightly bound to a catalytic site. 

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