to formation of
β-oxidation breaks down fatty acids into acetyl-CoA units for energy production. A 16-carbon even-chain fatty acid fully yields acetyl-CoA, while a 17-carbon odd-chain leaves a propionyl-CoA remnant. The correct answer is option (D).
Correct Answer
(D) (8 Acetyl CoA) and (7 Acetyl CoA + 1 Propionyl CoA), respectively
This matches standard β-oxidation outcomes: even-chain fatty acids produce only 2-carbon acetyl-CoA (CH3COSCoA), while odd-chain end with a 3-carbon propionyl-CoA (CH3CH2COSCoA).
β-Oxidation Process Overview
Each cycle removes 2 carbons as acetyl-CoA via four steps: dehydrogenation, hydration, oxidation, and thiolysis. For an n-carbon chain, cycles = (n/2) – 1, yielding (n/2) acetyl-CoA for even n.
- 16C fatty acyl-CoA (e.g., palmitoyl-CoA) undergoes 7 cycles, producing 8 acetyl-CoA.
- 17C undergoes 7 cycles, yielding 7 acetyl-CoA + 1 propionyl-CoA (final 5C → 2C acetyl + 3C propionyl).
Propionyl-CoA converts to succinyl-CoA via carboxylation, methylmalonyl-CoA mutase (vitamin B12-dependent), but the question focuses on direct β-oxidation products.
Option-by-Option Analysis
| Option | 16C Products | 17C Products | Why Incorrect/Correct |
|---|---|---|---|
| (A) | 8 Acetyl CoA | 8 Acetyl CoA + CO2 | Wrong for 17C: odd-chain yields propionyl-CoA, no CO2 in β-oxidation. |
| (B) | 5 Propionyl CoA + 1 CO2 | 5 Propionyl CoA + 1 Acetyl CoA | Wrong: even-chain never produces propionyl-CoA; illogical counts. |
| (C) | 5 Propionyl CoA + 1 CO2 | 5 Propionyl CoA + 2 CO2 | Wrong: confuses chains; no propionyl from 16C, CO2 not a product. |
| (D) | 8 Acetyl CoA | 7 Acetyl CoA + 1 Propionyl CoA | Correct: Matches cycle math for even/odd chains. |
Key Takeaways for Exams
- Even-chain (like C16 palmitic acid): purely acetyl-CoA.
- Odd-chain (C17): acetyl-CoA + propionyl-CoA, which feeds gluconeogenesis via succinyl-CoA.
- No CO2 in core β-oxidation pathway.
