Amino acids are organic compounds containing amine (-NH2) and carboxyl (-COOH) functional groups, along with a side chain (R) group specific to each amino acid. Many amino acids contain only carbon, hydrogen, oxygen and nitrogen, but other atoms may be present (e.g. sulphur in cystine, and iodine in thyroxin). As already mentioned, more than one amino group may be present (e.g. Lysine, diaminocaproic acid) and more than one carboxylic acid group (e.g. aspartic or amino succinic acid). Some amino acids are aromatics such as phenylalanine, or heterocyclic
such as proline (pyrolidine nucleus), tryptophan (indole nucleus) and histidine (imidazole nucleus).
|Essential amino acids||Non-essential amino acids|
Amino acid synthesis is the set of biochemical processes (metabolic pathways) by which the amino acids are produced. The substrates for these processes are various compounds in the organism’s diet or growth media. Not all organisms are able to synthesize all amino acids. For example, humans can only synthesize 11 of the 20 standard amino acids (a.k.a. non-essential amino acid), and in time of accelerated growth, histidine can be considered an essential amino acid.
Of the basic set of twenty amino acids (not counting selenocysteine), humans cannot synthesize eight. In addition, the amino acids arginine, cysteine, glycine, glutamine, histidine, proline, serine, and tyrosine are considered conditionally essential, meaning they are not normally required in the diet but must be supplied exogenously to specific populations that do not synthesize it in adequate amounts. For example, enough arginine is synthesized by the urea cycle to meet the needs of an adult but perhaps not those of a growing child.
Amino acids that must be obtained from the diet are called essential amino acids. Nonessential amino acids are produced in the body.
The pathways for the synthesis of nonessential amino acids are quite simple. Glutamate dehydrogenase catalyzes the reductive amination of α-ketoglutarate to glutamate. A transamination reaction takes place in the synthesis of most amino acids. At this step, the chirality of the amino acid is established. Alanine and aspartate are synthesized by the transamination of pyruvate and oxaloacetate, respectively.
Glutamine is synthesized from NH4+ and glutamate, and asparagine is synthesized similarly. Proline and arginine are derived from glutamate. Serine, formed from 3-phosphoglycerate, is the precursor of glycine and cysteine. Tyrosine is synthesized by the hydroxylation of phenylalanine, an essential amino acid.
The pathways for the biosynthesis of essential amino acids are much more complex than those for the nonessential ones.