Rapid Electron Transfer within the III-IV Supercomplex in Corynebacterium glutamicum.

Complex III in C. glutamicum has an unusual di-heme cyt. c and it co-purifies with complex IV in a supercomplex. Here, we investigated the kinetics of electron transfer within this supercomplex and in the cyt. aa alone (cyt. bc was removed genetically). In the reaction of the reduced cyt. aa with O, we identified the same sequence of events as with other A-type oxidases. However, even though this reaction is associated with proton uptake, no pH dependence was observed in the kinetics. For the cyt. bc-cyt. aa supercomplex, we observed that electrons from the c-hemes were transferred to Cu with time constants 0.1-1 ms. The b-hemes were oxidized with a time constant of 6.5 ms, indicating that this electron transfer is rate-limiting for the overall quinol oxidation/O reduction activity (~210 e/s). Furthermore, electron transfer from externally added cyt. c to cyt. aa was significantly faster upon removal of cyt. bc from the supercomplex, suggesting that one of the c-hemes occupies a position near Cu. In conclusion, isolation of the III-IV-supercomplex allowed us to investigate the kinetics of electron transfer from the b-hemes, via the di-heme cyt. c and heme a to the heme a-Cu catalytic site of cyt. aa.