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)?
-
ANADH dehydrogenase (Complex I)
-
BCytochrome bc1 complex (Complex III)
-
CCytochrome c oxidase (Complex IV)
-
DCitrate synthase
-
ESuccinate dehydrogenase (Complex II)
Answer
Correct Answer: Citrate synthase
Explanation
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:
Match each name to ETC roles: I (NADH dehydrogenase), II (succinate dehydrogenase), III (bc1), IV (cytochrome c oxidase).Identify citrate synthase as a soluble matrix enzyme in TCA, not in the inner membrane ETC chain.Select citrate synthase as the non-ETC protein.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