Mitochondrial electron transport – Complex III donor in isolation If Complex III (cytochrome bc1) is reconstituted into an artificial lipid vesicle to test its proton-translocating activity, which electron donor should be supplied to feed electrons into Complex III?

Introduction / Context:
The mitochondrial electron transport chain transfers electrons from reduced substrates to oxygen while pumping protons to create a proton-motive force. Complex III (cytochrome bc1) occupies a central position, oxidizing ubiquinol (QH2) and reducing cytochrome c. When Complex III is isolated in a model membrane, choosing the correct electron donor is crucial to observe its native quinol oxidation and proton translocation (Q-cycle).

Given Data / Assumptions:

• Only Complex III is present in the vesicle; other complexes are absent.
• The goal is to demonstrate proton translocation coupled to electron transfer.
• Appropriate electron carriers must interact directly with Complex III.

Concept / Approach:
Physiologically, Complex III accepts electrons from ubiquinol in the membrane and passes them to soluble cytochrome c in the intermembrane space. Therefore, to drive Complex III alone, the membrane-soluble donor must be ubiquinol (UQH2). Reduced cytochrome c is the product of Complex III, not its substrate. Enzymes such as cytochrome c oxidase (Complex IV) or cytochrome c peroxidase operate downstream or in different pathways and will not donate directly to Complex III in this setup.

Step-by-Step Solution:

Verification / Alternative check:
Observation of antimycin-sensitive proton pumping and reduction of added cytochrome c demonstrates that UQH2 is the correct donor. Spectral changes at cytochrome b and c1 peaks further validate canonical Complex III turnover.

Why Other Options Are Wrong:

• Reduced cytochrome c: it is an electron acceptor from Complex III, not a donor to it.
• Cytochrome c peroxidase: peroxisomal/mitochondrial intermembrane enzyme using peroxides; not part of the standard respiratory chain feeding Complex III.
• Cytochrome c oxidase (Complex IV): downstream oxidase that accepts electrons from cytochrome c; it does not donate to Complex III.
• NADH directly: requires Complex I or other dehydrogenases to reduce ubiquinone; NADH alone cannot feed Complex III.

Common Pitfalls:
Confusing electron donors and acceptors around Complex III. Remember: UQH2 → Complex III → cytochrome c → Complex IV → O2 is the physiological sequence.

Final Answer:
Ubiquinol (UQH2)