Electron transfer between complexes III and IV in S. cerevisiae mitochondrial membranes

Mitochondrial oxidative phosphorylation relies on cytochrome c transferring electrons between complexes III and IV. Earlier studies using detergent-purified complex III-IV supercomplexes from S. cerevisiae showed that this transfer is limited by two-dimensional cytochrome c diffusion. This study investigates this process in membrane-embedded mitoplasts. The results show that membrane embedment shifts the rate-limiting step from cytochrome c-mediated electron transfer to the catalytic activity of the supercomplex itself. Up to a cytochrome c : supercomplex ratio of unity, turnover increases sharply regardless of ionic strength. At higher ratios, the rate levels out at 15-20 s^-1, indicating that the process is no longer limited by salinity-dependent electron transfer, but rather by the catalytic capacity of complex IV.