Which statement best explains why proteins are not a good rapid-energy source?

Question

BlackTom AI · Accepted Answer

Deamination of amino acids produces toxic ammonia, so the body must expend time and energy to remove nitrogen before using protein for fuel.

1) Relevant knowledge points and concepts
- Proteins are broken down to amino acids, which must be deaminated (remove the amino group) before their carbon skeletons can be used for energy (glycolysis / TCA intermediates).
- Ammonia (NH3/NH4+) is toxic; the liver converts it to urea via the urea cycle for safe excretion.
- The urea cycle is energy-consuming (uses ATP equivalents), and deamination/transport processes are relatively slow compared with carbohydrate catabolism.
- Proteins are primarily functional/structural molecules; catabolizing them impairs tissue function (muscle loss).

2) Step-by-step reasoning
- Dietary or tissue proteins → amino acids (fast).
- Amino acids undergo transamination/oxidative deamination to release an amino group and produce ammonia.
- Ammonia is converted to carbamoyl phosphate and then to urea in the urea cycle; only after deamination can the carbon skeleton be oxidized for ATP.
- Because deamination + urea synthesis are required, using protein as immediate fuel imposes an extra biochemical bottleneck and energetic cost versus using carbohydrates or fats that can be rapidly oxidized.

3) Calculations / intermediate steps (energy cost of urea synthesis)
- Carbamoyl phosphate synthetase I: consumes 2 ATP → 2 ADP (2 high-energy phosphates).
- Argininosuccinate synthetase: consumes 1 ATP → AMP + PPi (equivalent to 2 high-energy phosphates because ATP→AMP uses two bonds).
- Total cost ≈ 4 high-energy phosphate bonds per urea produced.
- That ATP expenditure reduces the net ATP gain from amino-acid oxidation, making protein a less efficient rapid source.

4) Why the correct answer is correct
- The central limiting factor for rapid use of protein as fuel is the need to remove nitrogen (toxic ammonia) and convert it to urea — a process that is both slow and metabolically costly in ATP — so proteins are poor rapid-energy sources.

5) Why the other options are incorrect
- "Amino acids cannot enter mitochondria.": Incorrect — many deamination and oxidation steps occur in mitochondria, and amino-acid carbon skeletons are transported into mitochondrial pathways.
- "Proteins are stored as glycogen.": Incorrect — glycogen is a carbohydrate storage form; proteins are not stored as glycogen.
- "Protein catabolism produces too little ATP.": Incorrect — oxidation of amino-acid carbon skeletons can yield significant ATP; the problem is the extra cost and time of nitrogen removal, not inherently low ATP yield.

6) Conclusion
The best explanation is: Nitrogen removal is slow and metabolically costly. This matches the provided correct answer.

Which statement best explains why proteins are not a good rapid-energy source?Single choice

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