<P> Some amino acids contain the opposite absolute chirality, chemicals that are not available from normal ribosomal translation / transcription machinery . Most bacterial cells walls are formed by peptidoglycan, a polymer composed of amino sugars crosslinked with short oligopeptides bridged between each other . The oligopeptide is non-ribosomally synthesised and contains several peculiarities including D - amino acids, generally D - alanine and D - glutamate . A further peculiarity is that the former is racemised by a PLP - binding enzymes (encoded by alr or the homologue dadX), whereas the latter is racemised by a cofactor independent enzyme (murI). Some variants are present, in Thermotoga spp . D - lysine is present and in certain vancomycin - resistant bacteria D - serine is present (vanT gene). </P> <P> In animals, some D - amino acids are neurotransmitters . </P> <P> All proteinogenic amino acids have at least one hydrogen on the α - carbon . Glycine has two hydrogens, and all others have one hydrogen and one side - chain . Replacement of the remaining hydrogen with a larger substituent, such as a methyl group, distorts the protein backbone . </P> <P> In some fungi α - amino isobutyric acid is produced as a precursor to peptides, some of which exhibit antibiotic properties . This compound is similar to alanine, but possesses an additional methyl group on the α - carbon instead of a hydrogen . It is therefore achiral . Another compound similar to alanine without an α - hydrogen is dehydroalanine, which possess a methylene sidechain . It is one of several naturally occurring dehydroamino acids . </P>

Choose the amino acid whose function is involved in the production of carnosine