The Q cycle of cytochrome bc complexes: a structure perspective

Area of Science:

• Biochemistry and structural biology
• Membrane protein complexes
• Electron transport chains

Background:

• Cytochrome bc(1) and b(6)f complexes are crucial for cellular respiration and photosynthesis, respectively.
• Their dimeric structures coordinate redox centers including hemes and iron-sulfur clusters.
• Understanding their crystal structures is key to elucidating quinone redox reactions and proton transfer.

Purpose of the Study:

• To discuss crystal structure aspects of cytochrome bc(1) and b(6)f complexes.
• To highlight differences relevant to electron/proton transfer and quinone redox reactions.
• To compare the structural divergence from a common core in these hetero-oligomeric complexes.

Main Methods:

• Analysis of crystal structures of multiple cytochrome bc complexes from diverse sources (mitochondrial, cyanobacterial, algal).
• Comparison of subunit composition and prosthetic group coordination.
• Structure-based evaluation of inter-heme distances and potential electron transfer pathways.

Main Results:

• Both complexes share a common core with redox groups but diverge in peripheral subunits and prosthetic groups (e.g., chlorophyll in b(6)f).
• A conserved inter-monomer cavity facilitates quinone/quinol exchange with the membrane bilayer.
• Intra-monomer electron transfer pathways are kinetically favored over inter-monomer pathways based on heme distances.

Conclusions:

• The dimeric structure and spatial separation of quinone binding sites minimize steric interference.
• Distinct structural features, including portals and pathways, influence trans-membrane quinone/quinol transfer.
• Differences in structure, particularly the presence of additional prosthetic groups in b(6)f, underscore functional specializations.