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276 CIIAITFR II: RF.SRIRATION AND 1.11'ID MKTAROI.ISM

FIGURĘ 11.7. A proton electrochemicai gradient (A//_H.) is established across the inner mitochondrial mernbrane (acidic outside) during electron transport as outlined in the text. The membranę potential component (A£_m) cf the gradient drives the electrogenic exchange of ADP in the cytosoi for ATP in the mitochondria vig the adenine nucteotide transporter, and the ApH arives the electroneutrai uptake of P, through the phosphate transporter. The free energy stored in the proton gradient is then coupled to the synthesis of ATP from ADP and P, via the many F₀F,-ATP synthase comp!exes that linę the inner mernbrane. The F₀ component forms a channel for the movement of protons across the inner mernbrane. while the F, compiex provides the catalytic site for the condensation of ADP and P. to ATP Uncoupiers provide a pathway for movement of protons across. the inner mernbrane, preventing the buiidup of the proton electrochemicai gradient and inhibiting ATP synthesis but not electron transfer. {Modified from Douce, 1SS5.)

consisrs of rwo major componcnts, F, and F„ (Fig. 11.7). F, is a pcripheral membranę protein complex rhat is com-posed of ar leasr five differenr subunits and conrains rhe catalytic site for ęonvcrring ADP and P_s to ATP (or hydrol y/.ing ATP ro ADP and P_s when rhe ATPase icrivirv is being measured). This complex is attached to rhe ma-trix sidc of rhe inner mernbrane. Ę, is an inrcgral membranę protein comp!ex rhat consisrs of at least threc differenr polypepridcs rhar form rhe channel through which protons are able ro cross the inner membranę. The passage of protons through the channel acrivates rhe

A TP synthase to synihesize Al P and simulrancously dis-sipates rhe A/t,,.. The struciurc and funerion of rhe mitocltónsrial ATP synrhase are similar to rhose of rhe CF„-0-, A 1 Pasę in phorosyiuhetic photophosphoryla-tion (Chaptor S’}.

Acceptance of the generał features of a chcmios-motic mechanism of A TP synthesis has several impli-carions. First, the true site of Ali’ formation on the miiochondrial inner membranę is rhe A TP symhasc cum-plcx, not comples !, III, or IV. The larter sorve as sites of cncry,}' conseruiition whereby electron transport is