Apart from the 20 standard amino acids, all the rest amino acids are called as ‘non-standard’ amino acids. These include both proteinogenic as well as non-proteinogenic amino acids.
Amino acid derivatives as proteins (Proteinogenic)
4-hydroxyproline = found in plant cell wall proteins, also in collagen.
5-hydroxylysine= found in collagen
6-N-methyllysine= found in myosin, a muscle protein
γ-carboxyglutamate = found in prothrombin and certain other proteins that bind Ca2+ ions.
Desmosine= found in fibrous protein elastin
N-Formylmethionine= It is initially the N-terminal residue of all prokaryotic proteins.
D-amino acid residues form polypeptides that are generally found to constitute bacterial cell walls. Apart from this, certain bacterially produced peptide antibiotics also contain D amino acids like valinomycin, gramicidin A and actinomycin D.
Other amino acids (Non-proteinogenic)
Besides their role in protein synthesis, amino acids also serve various other important functions.
Some amino acids act as neurotransmitters like γ-aminobutyric acid (GABA) that’s formed as a result of glutamate decarboxylation and dopamine that’s a tyrosine derivative.
Histamine which is formed as a result of decarboxylation of histidine is a local mediator of allergic reactions
Tyrosine derivative thyroxine is a thyroid hormone.
Ornithine and citrulline are intermediates in the biosynthesis of arginine and urea cycle.
Homocysteine is an intermediate in amino acid metabolism.
S-adenosylmethionine is a biological methylating reagent.
Azaserine is a medically useful antibiotic.
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