Questions: Fatty Acid Oxidation (β-Oxidation)

Question 1

Each complete round of β-oxidation of a saturated fatty acid produces which combination of products?

• A. One acetyl-CoA and one NADH only
• B. One acetyl-CoA, one FADH₂, and one NADH
• C. Two acetyl-CoA and one FADH₂
• D. One acetyl-CoA and two FADH₂

Answer: (B)

Each round of β-oxidation produces exactly one acetyl-CoA (from the thiolysis step), one FADH₂ (from the first oxidation by acyl-CoA dehydrogenase), and one NADH (from the second oxidation by L-3-hydroxyacyl-CoA dehydrogenase). The four steps are: oxidation, hydration, oxidation, thiolysis.

Question 2

Activating a fatty acid for β-oxidation is energetically free — the acyl-CoA synthetase reaction does not consume any ATP equivalents.

• T. True
• F. False

Answer: False

Activation by acyl-CoA synthetase consumes ATP by converting it to AMP + PPi (pyrophosphate). Because pyrophosphate is rapidly hydrolyzed, this is equivalent to consuming 2 ATP equivalents. This activation cost must be subtracted when calculating the net ATP yield from β-oxidation.

Question 3

Why do fatty acids yield significantly more ATP per gram than carbohydrates such as glucose?

Model answer: Fatty acids are far more reduced (higher C–H bond density) than carbohydrates, which are partially oxidized (containing many C–OH and C=O groups). More reduced carbons donate more electrons to NADH and FADH₂, which in turn drive more ATP synthesis through the electron transport chain.

The key insight is the degree of reduction. Glucose has an oxidation state near zero for its carbons (due to oxygen substituents), while fatty acid carbons are almost fully reduced. More electrons per carbon means more NADH/FADH₂ per gram, and therefore more ATP. This is why fats store roughly twice as much energy per gram as carbohydrates.