Main Fates of the Carbon Skeleton
Once an amino acid loses its amino group, the remaining carbon moiety (the $\alpha$-keto acid) enters various metabolic pathways. There are five main directions for its utilization:
- Energy production. Oxidation of the carbon skeleton to carbon dioxide ($CO_2$) and water ($H_2O$) with the release of energy.
- Synthesis of non-essential amino acids. The carbon skeleton can re-acquire an amino group through transamination reactions, turning into another amino acid.
- Anaplerotic reactions. Carbon skeletons are used to replenish the pool of central catabolic pathway metabolites (specifically the citric acid cycle) or to generate other vital compounds.
- Gluconeogenesis. The process of forming glucose molecules from non-carbohydrate precursors.
- Ketogenesis. The synthesis of ketone bodies.
Final Catabolic Products and Glucogenic Amino Acids
Regardless of structural complexity, the catabolism of the carbon skeleton of any amino acid ultimately converges into just six basic compounds. All of them participate in the central pathways of catabolism or the tricarboxylic acid (TCA) cycle:
- Pyruvate;
- Acetyl-CoA;
- $\alpha$-Ketoglutarate;
- Succinyl-CoA;
- Fumarate;
- Oxaloacetate.
A special group consists of glucogenic amino acids. These are molecules whose carbon skeletons are converted during breakdown into pyruvate or TCA cycle intermediates (such as $\alpha$-ketoglutarate, succinyl-CoA, and fumarate). Ultimately, all these metabolites are transformed into oxaloacetate, which is a direct substrate for gluconeogenesis (glucose synthesis).
Synthesis of Non-Essential Amino Acids via Transamination
The carbon skeletons for eight non-essential amino acids (alanine, aspartate, asparagine, serine, glycine, proline, glutamate, glutamine) and cysteine can be synthesized entirely from glucose. The central mechanism here is transamination—the transfer of an $\alpha$-amino group to an $\alpha$-keto acid.
The universal amino group donor in these reactions is glutamate. The process is catalyzed by aminotransferases (transaminases), which strictly require the cofactor pyridoxal phosphate (the active form of vitamin $B_6$).
- Alanine formation occurs from pyruvate via alanine aminotransferase (ALT). Pyruvate accepts an amino group from glutamate.
- Aspartate formation proceeds from oxaloacetate via aspartate aminotransferase (AST).
- Glutamate synthesis itself is carried out by transferring an amino group to $\alpha$-ketoglutarate.
These reactions are reversible, allowing the body to flexibly redistribute nitrogen depending on the current diet (e.g., in a plant-based diet rich in carbohydrates and glutamate).
Specific Pathways of Individual Amino Acid Synthesis
Aside from direct transamination, other mechanisms form amino acids:
- Amidation reactions. Glutamine and asparagine are formed from glutamate and aspartate, respectively. This process requires energy (ATP hydrolysis). During glutamine synthesis, the enzyme glutamine synthetase attaches free ammonia. During asparagine formation, the amide group donor is glutamine itself.
- Synthesis from glycolysis intermediates. The precursor of serine is 3-phosphoglycerate. In turn, serine serves as a substrate for glycine formation (a reaction requiring the cofactor tetrahydrofolate) and provides its carbon skeleton for cysteine synthesis.
- Conditionally essential amino acids. Tyrosine and cysteine require the presence of essential amino acids for their synthesis. Tyrosine is synthesized via the hydroxylation of phenylalanine, while cysteine synthesis requires methionine as a sulfur atom donor.
- Partially essential amino acids. Arginine and histidine are synthesized in the body, but in amounts insufficient to meet total demands (especially in children). Arginine is synthesized as an intermediate of the urea cycle, and histidine is derived from ribose and ATP.