Electron Transport Chain—Identify the Non-ETC Protein Which of the following is NOT a principal protein complex of the mitochondrial electron transport chain (ETC)?

Introduction / Context:
The mitochondrial ETC comprises membrane-embedded complexes that transfer electrons from NADH/FADH2 to oxygen, pumping protons to drive ATP synthesis. Distinguishing ETC components from TCA enzymes is critical for mapping oxidative metabolism.

Given Data / Assumptions:

• ETC complexes I–IV reside in the inner mitochondrial membrane.
• Succinate dehydrogenase is both a TCA enzyme and Complex II in the ETC.
• Citrate synthase functions in the TCA cycle.

Concept / Approach:
Identify which listed protein is not part of the electron transfer machinery. Complexes I, III, and IV are core ETC components; Complex II also participates directly. Citrate synthase, however, catalyzes the condensation of acetyl-CoA with oxaloacetate to form citrate and is not an electron carrier complex.

Step-by-Step Solution:

Verification / Alternative check:
Biochemical fractionation shows citrate synthase in the matrix; ETC complexes co-purify with inner membranes.

Why Other Options Are Wrong:

• Complexes I–IV are definitive electron carriers/pumps in the chain.

Common Pitfalls:
Assuming all mitochondrial enzymes are part of the ETC; many are purely metabolic (e.g., TCA enzymes) without electron transfer roles.

Final Answer:
Citrate synthase