Reversible redox energy coupling in electron transfer chains.

Area of Science:

• Biochemistry and Biophysics
• Bioenergetics
• Electron Transfer Systems

Background:

• Reversibility is crucial in biological energy transduction, particularly in respiratory and photosynthetic systems.
• The cytochrome bc1 complex is central to energy coupling, catalyzing electron transfer between quinone and cytochrome c.
• Understanding the mechanisms of efficient, reversible energy coupling is essential for comprehending cellular energy production.

Purpose of the Study:

• To investigate the reversibility of individual cofactors within the cytochrome bc1 complex.
• To elucidate the mechanisms enabling efficient and reversible energy coupling on a catalytic timescale.
• To challenge and refine existing models of quinone catalysis at the Q(o) site.

Main Methods:

• Progressive inactivation of individual cofactors within the cytochrome bc1 complex.
• Resolution of millisecond-timescale reversibility in electron tunneling and proton exchange.
• Analysis of charge-separating hydroquinone-quinone catalysis at the Q(o) site.

Main Results:

• Millisecond reversibility was observed across all electron-tunneling steps and coupled proton exchanges.
• Rapid reversibility at the Q(o) site indicates the relevance of redox equilibria on a catalytic timescale.
• Existing models based on semiquinone intermediates are challenged due to potential short-circuit failures.

Conclusions:

• Two distinct mechanisms—conformational gating of semiquinone or concerted two-electron quinone chemistry—enable reversible function.
• These mechanisms prevent short-circuiting by relegating it to slower, long-distance electron tunneling (seconds timescale).
• The findings provide critical insights into the dynamic and reversible nature of biological energy transduction.