The peptide N alpha-(L-alanyl-D-isoglutaminyl)-N epsilon-(D-isoasparaginyl)-L-lysyl-D-alanine and the disaccharide N-acetylglucosaminyl-beta-1,4-N-acetylmuramic acid in cell wall peptidoglycan of Streptococcus faecalis strain ATCC 9790

A major portion of the cell wall peptidoglycan in Streptococcus faecalis is composed of the disaccharide tetrapeptide β-1,4-N-acetylglucosaminyl-N-acetylmuramyl-Nα-(L-alanyl-D-isoglutaminyl)-L-lysyl-D-alanine. The tetrapeptides are cross-linked through single D-isoasparaginyl residues extending from the C-terminal D-alanine of one tetrapeptide unit to the Nє-terminal L-lysine of another. It is the first time that the occurrence of an isoasparaginyl residue in a natural product has been described. The Streptomyces SA en-dopeptidase cleaves D-alanyl-D-isoasparaginyl linkages and is thus the first enzyme known to hydrolyze D-D peptide bonds. Treatment of the disaccharide Nα-( L-alanyl-D-isoglutaminyl)-N є-(D-isoasparaginyl)- L-lysil-D-alanine with 10 equiv of NaOH at 37° for 1 hr results in deamidation of the isoasparaginyl residue together with migration of the aspartyl-lysine peptide bond giving rise to a mixture of Nє-(β-aspartyl)- and N є-(α-aspartyl)lysyl peptides. Under the same alkaline treatment, the N-acetylmuramyl residue undergoes a lactyl elimination which results in the production of acyl peptides and a Morgan-Elson prochromogenic compound, without hydrolysis of the glycosidic linkage. This conversion, interpreted to be the result of a β elimination, also occurs in the other disaccharide peptide monomers previously isolated from Staphylococcus aureus, Micrococcus roseus, and Streptococcus pyogenes.