Triphasic reduction of XXXX and the protonmotive Q cycle pathway of electron transfer in the cytochrome bc1 complex of the mitochondrial respiratory chain.

@entity22951 @entity15401 @entity12931 @entity4588 @entity4674 @entity11382 @entity13508 @entity599 @entity5985 @entity7447 @entity68
Reduction of @entity13508 in isolated succinate- @entity4674 reductase is a triphasic reaction. Initially, there is a relatively rapid, partial reduction of the @entity13508 , the rate of which matches the rate of reduction of cytochrome c1. This is followed by partial or complete reoxidation of the b, which is then followed by slow rereduction. At very low concentrations of @entity4588 , the initial partial reduction of b is followed by reoxidation, but the third (rereduction) phase is absent, owing to insufficient substrate to rereduce the cytochromes. If @entity11382 is added at various times during the triphasic reaction, it inhibits the reoxidation and also inhibits the rereduction phase. @entity11382 does not inhibit the initial phase of b reduction and, if added before or during this phase, it causes reduction of b to proceed to completion as a monophasic reaction. @entity22951 inhibits the first phase of b reduction and the subsequent reoxidation, but does not inhibit the third, slow phase of b reduction. The resulting monophasic reduction of b which is observed in the presence of @entity22951 is slower than that in the presence of @entity11382 . The combination of both inhibitors, whether added together or successively during the triphasic reaction, completely inhibits b reduction. The triphasic reduction of @entity13508 is consistent with electron transfer by a protonmotive Q cycle in which there are two pathways for @entity13508 reduction. One pathway allows the initial phase of @entity13508 reduction by a @entity22951 -sensitive reaction in which reduction of b by @entity7447 is linked to reduction of @entity68 - @entity599 protein and @entity4674 1 by @entity15401 . In the second phase of the triphasic reaction, the b cytochromes are reoxidized by @entity12931 or @entity7447 through an @entity11382 -sensitive reaction. If oxidation of @entity15401 by @entity5985 protein is blocked, either by @entity22951 or by reduction of @entity68 - @entity599 protein and cytochrome c1, the b cytochromes can be reduced by reversal of the @entity11382 -sensitive pathway, thus accounting for the third phase of b reduction.
