NON STANDARD AMINO ACIDS

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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