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.