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Amino acids form peptide and protein polymers. metastream - amino acid : animation - peptide : image - primary structure protein : animation - protein : Over 90 amino acids are found in nature, while only 20 proteinogenic, standard amino acids are coded for by DNA. Of interest, 13 of the 21 amino acids found in cellular protein were generated in the Miller-Urey experiment, glycine, the smallest, being the commonest.
Several of the proteinogenic amino acids are termed ‘essential’ because they cannot be synthesized by the body’s metabolism and must be ingested in the diet. For adult humans isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine are essential, while children also require dietary histidine and arginine. Two other amino acids, selenocysteine and pyrrolysine, are sometimes incorporated during translation from RNA to protein.
In addition to their roles as substrates in peptide and protein synthesis, amino acids have other biologically important functions. Glycine, and glutamate, are both employed as neurotransmitters.
Non-standard amino acids are produced by post-translation modification, and some non-standard amino acids are produced only by plants and micro-organisms. Many amino acids are modified in the synthesis of other bio-active molecules: tryptophan is a precursor of the neurotransmitter serotonin, and glycine is one of the reactants in the synthesis of porphyrins such as heme. Numerous non-standard amino acids are also biologically important: GABA is another neurotransmitter, carnitine is employed in lipid transport within cells. Others non-standard amino acids include citrulline, homocysteine, hydroxyproline, hydroxylysine, ornithine, and sarcosine.
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