ATPsynthase cyclic functioning

Let us now consider a feasible sequence of events during the cyclic functioning of membrane-bound ATPsynthase in the presence of a transmembrane pH difference. Position 1 in Fig. 5.27 corresponds to the enzyme quasiequilibrium state there are no phosphorylation substrates in the active center, the functional acid groups are protonated due to their contact with the acidic interior of a vesicle (pHj < pK ), and a is open a fast leakage of protons into the external aqueous phase via ATPsynthase is prevented by the barrier hindering the contact of the AH group with the exterior, symbolized in Fig. 5.27 by key b in the locked position. The attachment of phosphorylation substrates to the active center of the coupling factor (transition 1 2)... 

On the Nature of the Threshold Response of ATPsynthases. We have noted above that closed membrane vesicles are necessary to provide the cyclic functioning of ATPsynthases under steady state conditions, and that the transmembrane difference in proton electrochemical potentials plays the role of an extensive rather than intensive factor of ATP synthesis. According to the data discussed in Section 5.2, in chloroplasts the value of ApH = 1.0... 

Summing up, we can conclude that ATPsynthases may be considered as enthalpic molecular machines which operate in essentially nonequilibrium states formed after the ionization of several acid groups of the enzyme. Closed membranes are necessary to provide a cyclic functioning of ATFsynthases under steady state conditions. We hope that this approach would appear to be fruitful for the explanation of other experimental data concerning energy transduction in the living cell. 

A similar conclusion follows from the work of de Kouchkovsky s group who measured the rate of ATP formation driven by a cyclic (PSl) or non-cyclic (PSl -h PS2) electron transport chain as a function of the apparent ApH value [99]. The hypothesis of the direct interaction of PS2 protonic domains with ATPsynthase was also considered by Yaguzhinsky s group [124] who studied the effects of the uncoupler gramicidin D on the photophosphorylation rate in chloroplasts during cyclic or noncyclic electron transport. 

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