Structure of the Cell Walls of Micrococcus lysodeikticus. III. Isolation of a New Peptide Dimer, Nα-[L-Alanyl-γ-(α-D-glutamyl-glycine )]-L-lysyl-D-alanyl-Nα- [L-alanyl-γ-(α-D-glutamyl-glycine)]-L-lysyl-D-alanine

The pentapeptide monomer Nα-[L-aIanyl-γ-(α-D-glutamyl-glycine)]-L-lysyl-D-alanine and two isomeric peptide dimers have been quantitatively isolated from walls of Micrococcus lysodeikticus. In one of the peptide dimers, referred as to peptide dimer (Ala→Lys), two pentapeptide monomers are linked through Nє-(D-alanyl)-L-lysine linkages. This linkage is hydrolyzed by the Streptomyces ML endopeptidase but not by the Myxobacter AL I protease. In the second peptide dimer, referred as to peptide dimer (Ala→Ala), two pentapeptide monomers are linked through D-alanyl-L-alanine linkages. This linkage is hydrolyzed by the Myxobacter AL I protease but not by the Streptomyces ML endopeptidase. According to the type of enzymatic degradation used, the pentapeptide monomer has been obtained in the free form, or substituted at its N-L-alanine terminus either by a D-lactic acid residue or by an N-acetylmuramic acid residue. Similarly, the peptide dimer (Ala→Ala) has been obtained in the free form or as a lactyl derivative. The peptide dimmer (Ala→Lys) has only been obtained in the free form. A comprehensive structure for a major part of the wall peptidoglycan is proposed. This structure takes into account the yields with which the peptide fragments are produced by the various enzymatic degradations. It provides explanation for the existence of a large number of peptide unsubstituted N-acetylmuramic acid residues in the glycan moiety. The structural peculiarity of the peptide moiety in M. lysodeikticus walls is the occurrence of large oligopeptides in which several pentapeptide monomers are linked through the aforementioned D-alanyl-L-alanine linkages. The isolation of the N-acetylmuramyl pentapeptide monomer involves, in one of the steps of the wall degradation, the use of a Streptomyces exo-N-acetylhexosaminidase active on both β-1,4-N-acetylglucosaminyl-N-acetylrnuramic acid and β-1,4-N-acetylmuramyl-N-acetylglucosamine disaccharides.
This is the first known enzyme acting as an exo-β-N-acelylmuramidase.