Possible proton relay pathways in cytochrome c oxidase

Area of Science:

• Biochemistry
• Molecular Biology
• Bioenergetics

Background:

• Cytochrome c oxidase (EC 1.9.3.1) is vital for cellular respiration, catalyzing oxygen reduction to water and generating proton gradients.
• Understanding proton pathways through this enzyme is key to elucidating energy transduction mechanisms.

Purpose of the Study:

• To investigate the role of specific conserved regions in subunit I of Rhodobacter sphaeroides cytochrome c oxidase in proton pumping.
• To identify key residues involved in proton loading, unloading, and transfer during enzyme activity.

Main Methods:

• Site-directed mutagenesis was employed to create mutants in conserved regions of subunit I.
• Mutants were characterized by assessing enzyme activity, proton pumping, electron transfer, CO binding, and spectral properties.
• Reconstitution assays with ionophores and uncouplers were performed to evaluate proton gradient response.

Main Results:

• Mutants at residue 132 (Asp132Asn/Ala) lost proton pumping activity but retained electron transfer.
• Spectroscopic analysis of D132 mutants showed no major structural changes, though CO binding was slightly reduced.
• Asp132 mutants exhibited inhibition by ionophores/uncouplers, suggesting altered proton gradient response.

Conclusions:

• The conserved II-III loop, particularly residue 132, plays a critical role in the proton pumping mechanism of cytochrome c oxidase.
• Results are consistent with potential involvement of residues in helix VIII and the IX-X loop in proton translocation.
• The study provides insights into the intricate process of proton translocation coupled to electron transfer in respiratory enzymes.