Pathways for proton release during ubihydroquinone oxidation by the bc(1) complex

Area of Science:

• Biochemistry
• Molecular Biology
• Bioenergetics

Background:

• The bc(1) complex is essential for cellular respiration, catalyzing quinol oxidation.
• Understanding the mechanism of quinol oxidation is key to comprehending electron transport chains.

Purpose of the Study:

• To elucidate the structural basis of quinol oxidation by the bc(1) complex.
• To investigate the role of specific amino acid residues and protein interactions in the catalytic mechanism.

Main Methods:

• X-ray crystallography of the bc(1) complex with various inhibitors.
• Site-directed mutagenesis of key residues in the iron-sulfur protein (ISP) and cytochrome b.
• Analysis of enzyme kinetics and proton transfer pathways.

Main Results:

• Identified a stabilized enzyme-substrate complex between quinol and the ISP at the Q(o) site.
• Revealed specific hydrogen bonds involving Glu-272 and His-161 that stabilize the complex and facilitate proton release.
• Demonstrated that mutations affecting ISP redox potential and pK support the proposed mechanism.
• Observed distinct orientations of Glu-272 in the presence of different inhibitors, suggesting its role in substrate binding and proton transfer.

Conclusions:

• The bc(1) complex utilizes a precisely structured enzyme-substrate complex for efficient quinol oxidation.
• Glu-272 and His-161 play critical roles in stabilizing the complex, facilitating proton transfer, and enabling rapid turnover.
• The dynamic movement of protein domains and conformational changes are integral to the catalytic cycle and proton release pathway.