How are proteins used for energy within the context of the Citric Acid Cycle?
The correct answer and explanation is:
Correct Answer:
Proteins are broken down into amino acids, which are deaminated to remove the amino group. The remaining carbon skeletons are converted into intermediates of the Citric Acid Cycle (also known as the Krebs Cycle), such as pyruvate, acetyl-CoA, or other cycle intermediates, which can then be used to produce ATP.
300-Word Explanation:
Proteins serve primarily as building blocks for tissues and enzymes, but they can also be used as an energy source, especially during prolonged fasting, starvation, or intense exercise when carbohydrates and fats are depleted. This energy utilization occurs through the breakdown of proteins into their constituent amino acids, followed by their entry into the Citric Acid Cycle (Krebs Cycle) — a central metabolic pathway in cellular respiration.
The process begins with deamination, the removal of the amino (-NH₂) group from the amino acids. This takes place mainly in the liver. The removed amino group is converted into ammonia and then into urea, which is excreted in urine. What remains is the carbon skeleton, which is a molecule that can be reshaped into various metabolic intermediates.
Depending on the structure, these carbon skeletons are converted into:
- Pyruvate (which can become acetyl-CoA or enter gluconeogenesis),
- Acetyl-CoA, or
- Direct intermediates of the Citric Acid Cycle (such as α-ketoglutarate, succinyl-CoA, fumarate, or oxaloacetate).
Once inside the Citric Acid Cycle, these intermediates undergo a series of reactions that generate high-energy molecules: NADH and FADH₂. These molecules then donate electrons to the Electron Transport Chain, leading to the generation of ATP, the cell’s energy currency.
While protein metabolism is less efficient than carbohydrate or fat metabolism for energy production, it becomes vital during times of energy scarcity. However, prolonged use of protein for energy can lead to muscle wasting, since body proteins are broken down to fuel this process.