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276 CUAITKR II: Ki-.SIMRATION AND ! Il’ll> MKTAT.Oi ISM

FIGURĘ 11.7. A proton electrochemica! gradient {A/7_H =) is established across the inner mitochondrial mernbrane (acidic outside) during electron transport as outlined in the text. The membranę potential component (A£_m) cl ihe gradient drives the eiectrogenic exchange of AGP in the cytosoi for ATP in the mitochondria via the adenine nucleotide transporter, and the ApK drives ihe 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 AGP and P. via the many F₀F,-ATP syntnase comp!exes that linę the inner mernbrane. The F_c component forms a channel for the movement of protons across the inner mernbrane. while the F, complex provides the catalytic sile for the condensation of AGP and P. to ATP Unccupiers provide a pathway for movement of protons across the inner mernbrane, preventing the buhdup of the proton electrochemical gradient and inhibiting ATP synthesis but not electron transfer {Modified from Oouce. iSS5T

consisrs of rwo major componcnts, Fj and F_lt (Fig. 11.7). F_t is a peripheral membranę protein compIex that is com-posed of ar least hve differenr subunits and conrains the catalytic siec for converting ADP and P_; to ATP (or hydrol y/.ing ATP to ADP and Pj when the ATPase actmry is be mg mcasnred). This complex is attached to the ma-trix side of the inner membranę. P„ is an integral membranę protein complex that consists of at least three different polypeptides that form the channel through which protons are able to cross the inner mernbrane. The passage of protons through the channel aetivates the

A 1 i⁵ synrhase to synthesize Al P and simuhaneously dis-sipates the AThe structuie and function of the mitochondrial ATP svnthase are similar to those of the CP₀-(.T, A l Pasę in photosynihetie phorophosphoryla-tion (Chapter 8).

Aceeptanee ot the generał features of a chemios-motie mechanism ol A FP syitthesis has sevcral tmjni-cations. First, the truć site of ATP formation on the mitochondrial inner membranę is the ATP s\ nthase com-plex, not complex 1, 111, or IV. The larter serve as siics of energy consenuition whereby electron transport is