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Amino acids are chiral compounds that are present in living organisms as free compounds or as polymers such as peptides or proteins. The 20 L-Amino acids present in proteins and peptides are defined as proteinogenic amino acid. About 500 amino acids are known and have been classified in many ways base on their chemical/physical properties.
All α-amino acids (AA) but glycine exist in either of the two enantiomers, which are mirror images of each other (the so called D- and L-enantiomers).
Although present in a less concentration in mammals, free D-amino acids play central roles: as an example D-serine is one of the main neuromodulators of the glutamatergic neurotransmission and D-aspartate is fundamental for brain development in fetus.  
The major sources of D-amino acids in Nature are bacteria. In these microorganisms, D-amino acids are part of peptidoglycan (a major component of the bacterial cell wall). In addition, also the compounds teichoic acids and poly-γ-glutamate contain D-amino acids.
In peptidoglycan, D-alanine or D-glutamate contribute to antibiotic resistance in some bacteria. Many diverse bacteria synthesize D-amino acids such as, D-methionine and D-leucine.
D-amino acids are also present in many venoms from platypus, funnel web spider, and cone snail produced by the action of isomerases. 
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