Amino acids can be used in the citric acid cycle by being converted into intermediates of the cycle or into pyruvate, which then enters the cycle.
Here's a breakdown:
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Glucogenic Amino Acids and the Citric Acid Cycle: Glucogenic amino acids are those that can be converted into glucose. Several of these amino acids can also feed into the citric acid cycle. After deamination (removal of the amino group), their carbon skeletons are transformed into various intermediates of the cycle or pyruvate.
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Entry Points:
- Pyruvate: Alanine, cysteine, glycine, serine, and threonine can be converted to pyruvate. Pyruvate can then be converted into acetyl-CoA (which enters the cycle directly) or oxaloacetate (via an anaplerotic reaction to replenish the cycle).
- Other Intermediates: Other amino acids are converted to alpha-ketoglutarate, succinyl-CoA, fumarate or oxaloacetate.
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Anaplerotic Reactions: When amino acids are converted into oxaloacetate, it replenishes the citric acid cycle intermediates. These replenishment reactions are known as anaplerotic reactions. This ensures the cycle continues to function even when intermediates are being drawn off for other metabolic processes.
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Catabolism and Energy Production: The citric acid cycle is central to energy production. When amino acids are broken down and enter the cycle, the carbon skeletons are ultimately oxidized to CO2, and the electrons released are used to generate ATP through oxidative phosphorylation.
In essence, certain amino acids can serve as fuel for the citric acid cycle by being converted into intermediates, allowing for the continued production of energy within the cell.
