Introduction
This topic covers which amino acids the body must obtain from food, how the non-essential ones are made, the 21st amino acid selenocysteine, and how arginine and serine are converted into specialised products. It follows the topics on protein digestion and transamination and comes before amino acid catabolism.
Essential and non-essential amino acids
Nine amino acids are essential for adults, because the body cannot make their carbon skeletons: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. The rest are non-essential, meaning they can be synthesised, not that they are unimportant. Several normally non-essential amino acids become conditionally essential in growth, illness or metabolic disease: arginine, cysteine, glutamine, glycine, proline and tyrosine.
Common reactions in the synthesis of non-essential amino acids
Most non-essential amino acids are made from intermediates of glycolysis and the citric acid cycle by a few common reactions:
- Transamination from an α-ketoacid, with glutamate as the usual amino donor and a pyridoxal phosphate-dependent aminotransferase. Alanine comes from pyruvate, aspartate from oxaloacetate and glutamate from α-ketoglutarate.
- Amidation: glutamine is made from glutamate and ammonia by glutamine synthetase, using ATP, and asparagine is made from aspartate by transfer of an amide group from glutamine.
- From 3-phosphoglycerate: serine is made by a three-step route involving oxidation, transamination and dephosphorylation. Glycine is made from serine by serine hydroxymethyltransferase, which needs tetrahydrofolate. Cysteine is made from serine and homocysteine through cystathionine.
- From other amino acids: proline is made from glutamate, and tyrosine is made by hydroxylation of phenylalanine, which needs tetrahydrobiopterin as a cofactor.