Electron Transport Chain—Classes of Electron Carriers Which of the following is <em>not</em> an electron carrier class in the mitochondrial electron transport chain?

Introduction / Context:
The electron transport chain (ETC) employs several classes of carriers to pass electrons from reduced substrates to oxygen. Recognizing carrier classes versus specific multi-subunit enzymes helps avoid category errors.

Given Data / Assumptions:

• Main carrier classes include flavoproteins (FAD/FMN), iron–sulfur proteins (Fe–S clusters), cytochromes (heme proteins), and quinones (lipid-soluble, e.g., CoQ).
• Complex IV (cytochrome c oxidase) is a specific enzyme complex containing heme and copper centers.

Concept / Approach:
Carrier classes are broad categories of redox cofactors. In contrast, cytochrome c oxidase is a particular terminal oxidase enzyme (Complex IV) that contains cytochromes but is not a class itself. Therefore, it does not belong alongside classes like cytochromes or quinones.

Step-by-Step Solution:

Verification / Alternative check:
Biochemistry texts group carriers by cofactor type; Complex IV is treated as an enzyme complex within the chain, not as a class.

Why Other Options Are Wrong:

• Flavoproteins, cytochromes, Fe–S proteins, and quinones are standard carrier classes in the ETC.

Common Pitfalls:
Confusing a named complex (e.g., Complex IV) with the broader class “cytochromes.”

Final Answer:
Cytochrome c oxidase