The kinetics of reoxidation of XXXX complex III. An evaluation of the Q-cycle.

@entity3276 @entity2351 @entity22951 @entity11382 @entity375 @entity5985 @entity1446
The reoxidation of reduced @entity375 Complex III by oxidants believed to react with cytochrome c1 exhibited multiple phases for both cytochrome c1 and the cytochromes b; the reoxidation of @entity2351 , but not cytochrome c1, was markedly slowed by the presence of @entity11382 . The data are consistent with the Q-cycle or any other scheme which proposes a branched path for electron transport between the @entity2351 centers and the endogenous Q6, provided certain constraints are relaxed. The reoxidation of the endogenous @entity3276 proceeded at a rate comparable to that of the rapidly reacting @entity2351 and appeared to be complete within 100 ms. Removal of the endogenous @entity3276 did not change the rate or extent of reoxidation of any of the @entity1446 centers, demonstrating that @entity3276 is not required for electron transport between cytochromes b and the @entity5985 cluster. This result is inconsistent with the requirements of the Q-cycle. Funiculosin completely inhibited the reoxidation of @entity2351 whereas the reoxidation of cytochrome c1 exhibited simple first-order kinetics in the presence of this inhibitor, implying that the @entity5985 cluster is on the direct path of electron transfer from @entity2351 to cytochrome c1. Potent inhibition of @entity2351 oxidation was also observed with @entity22951 and mucidin. The reaction of reduced Complex III with Q1 also exhibited multiple phases in the oxidation of the @entity2351 centers; these phases were unaffected by the presence of @entity22951 . Addition of antimycin, or removal of the endogenous @entity3276 , eliminated the rapid phases; only one of the @entity2351 centers was oxidized under these conditions. Epr showed that it is the low-potential @entity2351 that is the species rapidly oxidized.
