Chemistry will tell you that an amino acid is any molecule that has both carboxylic acid and amino functional groups. They are the basic building units of a protein. In biochemistry, the shorter and more general term is used to refer to alpha amino acids. Those are amino acids wherein the amino and carboxylate functionalities are attached to a common carbon. Amino acids' residue is what's left of an amino acid once a water molecule has been lost in the formation of a peptide bond. Peptides are polymer chains which form the protien in our bodies.
Twenty amino acids will be encoded by the standard genetic code. These are called proteinogenic or standard Amino acids. More complicated ones are produced by our bodies, and are called nonstandard (these are not as common). Proline is the only proteinogenic amino acid, whose side group links to the a-amino group and is cyclic. This forms a secondary amino group. Before, proline was called "imino", which was misleading and was changed. Protiens contain other amino acids which are usually formed by post-translational modification (modifications AFTER translation). These modifications are essential for the function of protein. At least two amino acids, other than the standard 20, are sometimes incorporated into proteins during the translation process.
Even though only 20 amino acids are genetically encoded, over one hundred have been found in nature. Some of these have been seen in meteorites, especially in a type called carbonaceous chondrites. Microorganisms and plants can often produce somewhat uncommon amino acids, which are located in peptidic antibiotics. Lanthionine is a sulfide-bridged alanine dimer found along with unsaturated amino acids in "lantibiotics", which are antibiotic peptides of microbial origin.
As well as protein synthesis, amino acids have some other biologically important jobs. Glycine, and glutamate, are used as neurotransmitters as well as standard amino acids in proteins. Many amino acids take on the role of synthesizing other molecules, such as tryptophan, which is a precursor of the neurotransmitter serotonin, and glycine, one of the few reactants in the synthesis of porphyrins such as heme. Heme is in "hemoglobin", which is also important in the makeup of protein. Numerous, non-standard amino acids, are biologically important: "GABA",another neurotransmitter, carnitine, which is used in lipid transport within the cell, citrulline, ornithine, hydroxyproline, hydroxylysine, homocysteine, and sarcosine.
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