The Possible Model for 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) 

The latter simply implies that bound nucleotides break down the contact of acidic groups with the interior volume of the vesicle ( a is locked). Thus, after the substrates bind, the groups AH eventually become ionized due to the efflux of protons into the alkaline outer aqueous phase (pK pHJ, i.e., b is open (states 2-4). In accordance with well-known experimental fact [197-201], that the process of ADP phosphorylation accelerates the efflux of protons via the CF -CFo complex, the model suggests that the substrates binding stimulates the liberation of protons into the external medium under phosphorylating conditions. 

It is easy to extend this model in order to interpret any kind of experimental data on multiple reiterative ATP synthesis initiated by the creation of an artificial transmembrane difference of the electrical potentials, A , generated across the closed vesicles, e.g., liposomes encrusted with membrane-embedded ATPases. In this case, when the reiterative acts of ATP formation are driven by Aq in the lack of ApH, the protonation of the ionized acid group A is evidently provided by the field-induced decrease in the pK value of the ionizable group AH (Fig. 5.26). The Acp difference imposed across the coupling membrane can also lead to the occurrence of the difference in the proton concentrations related to the local regions in the vicinities of gates a and b. This difference, induced by the electric field, can exist even without any pH difference between the bulk phases on both sides of the coupling membrane. 

It is quite clear that for mitochondria, where is localized within the vesicle, a jump-like pH increase can lead to only one elementary act, that indeed was observed in [182]. In this case the component of the ATP-synthase is faced inside the vesicle and, thus, according to Fig. 5.27, the state 5 state 1 transition becomes impossible. Only a single act of the AH ionization would take place after the pH jump due to a passive leakage of protons via the mitochondrial membrane. Otherwise, for submitochondrial particles turned inside-out [21] a multiple ATP synthesis after the pH increase is achieved by means of the reiteration of the proton cycle. According to the model considered above, the transmembrane electrochemical gradient of protons plays the role of an extensive but not intensive factor [190,191]. Its increase (or decrease) changes the number of elementary acts of ATP synthesis which can be performed, until the condition formulated above for the elementary act ceases to be fulfilled. 

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