Evidence for electron equilibrium between the two hemes bL in the dimeric cytochrome bc1 complex

Area of Science:

• Biochemistry
• Molecular Biology
• Bioenergetics

Background:

• The mitochondrial cytochrome bc1 complex is crucial for cellular respiration and energy production.
• Electron transfer pathways within the complex are complex and not fully elucidated.
• Structural data suggests aromatic residues between bL hemes may mediate inter-monomer electron transfer.

Purpose of the Study:

• To investigate the role of aromatic residues between bL hemes in cytochrome bc1 complex function.
• To determine if inter-monomer electron transfer impacts enzyme activity and electron leakage.

Main Methods:

• Site-directed mutagenesis of aromatic residues (Phe-195, Tyr-199, Phe-203) in the R. sphaeroides cytochrome bc1 complex.
• Characterization of photosynthetic growth rates in mutant strains.
• Assay of ubiquinol-cytochrome c reductase activity in purified bc1 complexes.
• Measurement of superoxide anion generation during ubiquinol oxidation.

Main Results:

• Mutations at Tyr-199 and Phe-203 did not significantly affect enzyme activity.
• Mutation at Phe-195 (F195A) reduced ubiquinol-cytochrome c reductase activity to 78% of wild-type.
• Replacing Phe-195 with other aromatic residues (Tyr, His, Trp) restored wild-type activity.
• Superoxide anion generation increased threefold in the F195A mutant, indicating enhanced electron leakage.

Conclusions:

• The aromatic residue at position 195 of cytochrome b plays a critical role in facilitating electron transfer within the bc1 complex.
• Interruption of electron transfer between bL hemes leads to decreased enzyme efficiency and increased reactive oxygen species production.