Conservation of structure in proton-translocating ATPases of Escherichia coli and mitochondria.

arriving at (c ,+c ,+P) in the absence of antimycin must represent the oxidation of a second equivalent of QH, by the cyt. cI which remained oxidized after the first cycle of the complex. This second electron transfer presumably also occurs via the FeS centre, so that effectively the ‘invisible’ electron on the FeS centre gets replaced and pushed over on to cyt. c, in a second turnover of the complex. It is important to realize that this second turnover of the complex is a necessary consequence of the relative stoichiometries of the complex and P. Every time the reaction centre turns over, the complex has to turn over twice to return things to the starting state. (9) Because of the constraints discussed above, it is clear that the electrogenic process is the oxidation of cyt. b,,]. This suggests that the redox drop in this reaction must be substantial (>200mV). (10) The antimycin-sensitive reduction of the c-cytochromes must reflect the regeneration of Q,H, in the uninhibited cycle from an oxidized entity (presumably Q’H or Q-*) which receives electrons from cyt. b,,, and which must be available to the cycle in a time less than, or equal to, the 500ps lag observed for these processes. The lag could reasonably be viewed as arising from the time taken to reduce cyt. b,,,. One area in which we have insufficient information to define the cycle is the pathway by which the entity above is generated. It has been suggested by Slater and colleagues (Berden et al., 1981 ; de Vries et al., 1981), that the stable semiquinone, Qc-* (Ohnishi & Trumpower, 1980) is formed at a binding site in equilibrium with the pool, by simple disproportionation, and that this is the acceptor of electrons from cyt. b,,, in mitochondria. (1 1) To complete the cycle, it would be necessary to regenerate the QC-* used in the first turn and to reoxidize cyt. bS6,. The simplest assumption would be that the oxidant is Q. either from the pool, or from the cycle. This would mean that cyt. b,,, could donate electrons to Q or Q-’ at the site on the cytoplasmic side of the bacteria. Effectively this site would then have the same mechanism as the ‘two-electron gate’ of the reaction-centre secondary quinone. It would also be necessary to replace the quinone at the Q, site with a quinol. For ‘book-keeping’ purposes, this would have to be the quinol generated in the photoreactions, though in practice it might well come from the pool. We have attempted to summarize in Scheme 2 the electron distribution, and the fates of the electrons, as the cycle goes through a succession of states. Obviously, these are highly idealized, but they provide a compact and comprehensible description of the Q-cycle in terms of reactions which can be written out in conventional chemical form. Space does not permit a more detailed consideration of mechanistic aspects which follow from the parameters we have measured, and the constraints these put on a Q-cycle. The cycle explains with beautiful economy many of the phenomena which we have measured over the past few years-an economy which complements the set of constraints provided by the measurements themselves.