Amino acids are metabolically classified as glucogenic, ketogenic, or both (glucogenic and ketogenic), depending on the metabolic fate of their carbon skeletons during catabolism.
Detailed Explanation:
During amino acid catabolism, the carbon skeleton of each amino acid is broken down into one of seven common metabolic intermediates. These intermediates then enter various metabolic pathways. The specific intermediate that an amino acid's carbon skeleton is converted into determines its metabolic classification.
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Glucogenic Amino Acids: These amino acids are catabolized to pyruvate or one of the intermediates of the citric acid cycle (α-ketoglutarate, succinyl CoA, fumarate, or oxaloacetate). These intermediates can be used for gluconeogenesis, the synthesis of glucose.
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Ketogenic Amino Acids: These amino acids are catabolized to acetyl CoA or acetoacetyl CoA. These products can be used for ketogenesis, the synthesis of ketone bodies, or for the synthesis of fatty acids. Acetyl CoA cannot be used for net glucose synthesis in animals.
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Glucogenic and Ketogenic Amino Acids: Some amino acids can be broken down into precursors for both glucose and ketone bodies, and are therefore classified as both glucogenic and ketogenic.
Summary Table:
| Classification | Metabolic Fate | Examples |
|---|---|---|
| Glucogenic | Converted to pyruvate or citric acid cycle intermediates | Alanine, Arginine, Asparagine, Aspartate, Cysteine, Glutamate, Glutamine, Glycine, Histidine, Methionine, Proline, Serine, Threonine, Valine |
| Ketogenic | Converted to acetyl CoA or acetoacetyl CoA | Leucine, Lysine |
| Glucogenic and Ketogenic | Converted to precursors for both glucose and ketone bodies | Isoleucine, Phenylalanine, Tryptophan, Tyrosine |
Therefore, amino acids are classified based on the end products of their catabolism and whether those products can contribute to glucose or ketone body synthesis.
