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Related Experiment Video

Updated: May 8, 2026

Inner Mitochondrial Membrane Sensitivity to Na+ Reveals Partially Segmented Functional CoQ Pools

Inner Mitochondrial Membrane Sensitivity to Na⁺ Reveals Partially Segmented Functional CoQ Pools

Published on: July 20, 2022

A caged, destabilized, free radical intermediate in the q-cycle.

Preethi R Vennam¹, Nicholas Fisher, Matthew D Krzyaniak

¹Chemistry Department, University of Alabama, Box 870336, Tuscaloosa, AL 35487 (USA).

Chembiochem : a European Journal of Chemical Biology

|September 7, 2013

Summary

Researchers studied the cytochrome bc complex

Keywords:

Q-cycle bc complex complex III paramagnetic relaxation enhancement pulsed EPR

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Area of Science:

Biochemistry
Bioenergetics
Enzyme kinetics

Background:

Cytochrome bc complexes are crucial for energy storage via the Q-cycle.
A semiquinone intermediate can produce harmful superoxide, but its control mechanism is unclear.

Purpose of the Study:

To determine the location and properties of the Q-cycle semiquinone anion intermediate (SQo) in the mitochondrial complex.
To elucidate the stabilization and kinetic trapping mechanisms of SQo.

Main Methods:

Pulsed-EPR spectroscopy was used to analyze the SQo intermediate.
Rapid freeze-quenching was employed to trap the intermediate.

Main Results:

SQo is kinetically trapped in the quinol oxidase (Qo) site, not thermodynamically stabilized.
The SQo binding site is distinct from known inhibitor-binding sites and close to cytochrome bL.
SQo formation involves proton stripping and conformational changes, not protein hydrogen bonds.

Conclusions:

SQo is kinetically controlled, conserving redox energy for electron transfer to cytochrome bL.
This mechanism minimizes Q-cycle bypass reactions, including O2 reduction.
Understanding SQo's unique stabilization is key to Q-cycle efficiency and preventing oxidative damage.