Unique structural features of the peptidoglycan of Mycobacterium leprae

The peptidoglycan structure of Mycobacterium spp. has been investigated primarily with the readily cultivable Mycobacterium smegmatis and Mycobacterium tuberculosis and has been shown to contain unusual features, including the occurrence of N-glycolylated, in addition to N-acetylated, muramic acid residues and direct cross-linkage between meso-diaminopimelic acid residues. Based on results from earlier studies, peptidoglycan from in vivo-derived noncultivable Mycobacterium leprae was assumed to possess the basic structural features of peptidoglycans from other mycobacteria, other than the reported replacement of l-alanine by glycine in the peptide side chains. In the present study, we have analyzed the structure of M. leprae peptidoglycan in detail by combined liquid chromatography and mass spectrometry. In contrast to earlier reports, and to the peptidoglycans in M. tuberculosis and M. smegmatis, the muramic acid residues of M. leprae peptidoglycan are exclusively N acetylated. The un-cross-linked peptide side chains of M. leprae consist of tetra- and tripeptides, some of which contain additional glycine residues. Based on these findings and genome comparisons, it can be concluded that the massive genome decay in M. leprae does not markedly affect the peptidoglycan biosynthesis pathway, with the exception of the nonfunctional namH gene responsible for N-glycolylmuramic acid biosynthesis.