[Study of the formation of N-glycolylmuramic acid from Nocardia asteroides (author's transl)]

Peptidoglycan compositional analysis of Mycobacterium smegmatis using high-resolution LC-MS

Peptidoglycan (PG) is the exoskeleton of bacterial cells and is required for their viability, growth, and cell division. Unlike most bacteria, mycobacteria possess an atypical PG characterized by a high degree of unique linkages and chemical modifications which most likely serve as important determinants of virulence and pathogenesis in mycobacterial diseases. Despite this important role, the chemical composition and molecular architecture of mycobacterial PG have yet to be fully determined. Here we determined the chemical composition of PG from Mycobacterium smegmatis using high-resolution liquid chromatography-mass spectrometry. Purified cell walls from the stationary phase were digested with mutanolysin and compositional analysis was performed on 130 muropeptide ions that were identified using an in silico PG library. The relative abundance for each muropeptide ion was measured by integrating the extracted-ion chromatogram. The percentage of crosslink per PG subunit was measured at 45%. While both 3→3 and 4→3 transpeptide cross-linkages were found in PG dimers, a high abundance of 3→3 linkages was found associated with the trimers. Approximately 43% of disaccharides in the PG of M. smegmatis showed modifications by acetylation or deacetylation. A significant number of PG trimers are found with a loss of 41.00 amu that is consistent with N-deacetylation, whereas the dimers show a gain of 42.01 amu corresponding to O-acetylation of the PG disaccharides. This suggests a possible role of PG acetylation in the regulation of cell wall homeostasis in M. smegmatis. Collectively, these data report important novel insights into the ultrastructure of mycobacterial PG.

The Mycobacterial Cell Wall--Peptidoglycan and Arabinogalactan

The mycobacterial bacillus is encompassed by a remarkably elaborate cell wall structure. The mycolyl-arabinogalactan-peptidoglycan (mAGP) complex is essential for the viability of Mycobacterium tuberculosis and maintains a robust basal structure supporting the upper "myco-membrane." M. tuberculosis peptidoglycan, although appearing to be unexceptional at first glance, contains a number of unique molecular subtleties that become particularly important as the TB-bacilli enters into nonreplicative growth during dormancy. Arabinogalactan, a highly branched polysaccharide, serves to connect peptidoglycan with the outer mycolic acid layer, and a variety of unique glycolsyltransferases are used for its assembly. In this review, we shall explore the microbial chemistry of this unique heteropolysacchride, examine the molecular genetics that underpins its fabrication, and discuss how the essential biosynthetic process might be exploited for the development of future anti-TB chemotherapies.

Structural variation in the glycan strands of bacterial peptidoglycan

The normal, unmodified glycan strands of bacterial peptidoglycan consist of alternating residues of beta-1,4-linked N-acetylmuramic acid and N-acetylglucosamine. In many species the glycan strands become modified after their insertion into the cell wall. This review describes the structure of secondary modifications and of attachment sites of surface polymers in the glycan strands of peptidoglycan. It also provides an overview of the occurrence of these modifications in various bacterial species. Recently, enzymes responsible for the N-deacetylation, N-glycolylation and O-acetylation of the glycan strands were identified. The presence of these modifications affects the hydrolysis of peptidoglycan and its enlargement during cell growth. Glycan strands are frequently deacetylated and/or O-acetylated in pathogenic species. These alterations affect the recognition of bacteria by host factors, and contribute to the resistance of bacteria to host defence factors such as lysozyme.

N Glycolylation of the nucleotide precursors of peptidoglycan biosynthesis of Mycobacterium spp. is altered by drug treatment

The peptidoglycan of Mycobacterium spp. reportedly has some unique features, including the occurrence of N-glycolylmuramic rather than N-acetylmuramic acid. However, very little is known of the actual biosynthesis of mycobacterial peptidoglycan, including the extent and origin of N glycolylation. In the present work, we have isolated and analyzed muramic acid residues located in peptidoglycan and UDP-linked precursors of peptidoglycan from Mycobacterium tuberculosis and Mycobacterium smegmatis. The muramic acid residues isolated from the mature peptidoglycan of both species were shown to be a mixture of the N-acetyl and N-glycolyl derivatives, not solely the N-glycolylated product as generally reported. The isolated UDP-linked N-acylmuramyl-pentapeptide precursor molecules also contain a mixture of N-acetyl and N-glycolyl muramyl residues in apparent contrast to previous observations in which the precursors isolated after treatment with d-cycloserine consisted entirely of N-glycolyl muropeptides. However, nucleotide-linked peptidoglycan precursors isolated from M. tuberculosis treated with d-cycloserine contained only N-glycolylmuramyl-tripeptide precursors, whereas those from similarly treated M. smegmatis consisted of a mixture of N-glycolylated and N-acetylated residues. The full pentapeptide intermediate, isolated following vancomycin treatment of M. smegmatis, consisted of the N-glycolyl derivative only, whereas the corresponding M. tuberculosis intermediate was a mixture of both the N-glycolyl and N-acetyl products. Thus, treatment with vancomycin and d-cylcoserine not only caused an accumulation of nucleotide-linked intermediate compounds but also altered their glycolylation status, possibly by altering the normal equilibrium maintained by de novo biosynthesis and peptidoglycan recycling.

Identification of the namH gene, encoding the hydroxylase responsible for the N-glycolylation of the mycobacterial peptidoglycan

The peptidoglycan of most bacteria consists of a repeating disaccharide unit of beta-1,4-linked N-acetylmuramic acid and N-acetylglucosamine. However, the muramic acid moieties of the mycobacterial peptidoglycan are N-glycolylated, not N-acetylated. This is a rare modification seen only in the peptidoglycan of mycobacteria and five other closely related genera of bacteria. The N-glycolylation of sialic acids is a unique carbohydrate modification that has been studied extensively in eukaryotes. However, the significance of the N-glycolylation of bacterial peptidoglycan is unknown. The goal of this project was to identify the gene encoding the hydroxylase responsible for the N-glycolylation of the mycobacterial peptidoglycan. We developed a novel assay for the mycobacterial UDP-N-acetylmuramic acid hydroxylation reaction and demonstrated that Mycobacterium smegmatis has an enzyme activity that can convert UDP-N-acetylmuramic acid to UDP-N-glycolylmuramic acid. We identified the gene namH encoding the mycobacterial UDP-N-acetylmuramic acid hydroxylase by computer data base searching and motif comparisons with the eukaryotic enzymes responsible for the N-glycolyation of sialic acids. The namH gene is not essential for in vitro growth as we were successful in deleting the gene in M. smegmatis. The M. smegmatis mutant is devoid of UDP-N-acetylmuramic acid hydroxylase activity and synthesizes only N-acetylated muropeptide precursors. Furthermore, the mutant exhibits increased susceptibility to beta-lactam antibiotics and lysozyme. Our studies suggest that the N-glycolylation of mycobacterial peptidoglycan may play a role in lysozyme resistance or may contribute to the structural stability of the cell wall architecture.

A method for the enzymatic synthesis and HPLC purification of the peptidoglycan precursor UDP-N-acetylmuramic acid

UDP-N-acetylmuramic acid (UDP-MurNAc) is a precursor for peptidoglycan biosynthesis in bacteria. A major difficulty in the study of this pathway is that UDP-MurNAc is not commercially available. We have developed an enzymatic synthesis scheme for UDP-MurNAc using two easily purified Escherichia coli polyhistidine tagged peptidoglycan biosynthesis enzymes, MurZ and MurB, followed by a single-step purification of UDP-MurNAc by high-performance liquid chromatography. The identity of the UDP-MurNAc synthesized by our method was confirmed by electrospray ionization mass spectrometry. Furthermore, we show that the UDP-MurNAc can support a UDP-MurNAc-L-alanine ligase reaction.

A comprehensive phylogenetic analysis of Rieske and Rieske-type iron-sulfur proteins

The Rieske iron-sulfur center consists of a [2Fe-2S] cluster liganded to a protein via two histidine and two cysteine residues present in conserved sequences called Rieske motifs. Two protein families possessing Rieske centers have been defined. The Rieske proteins occur as subunits in the cytochrome bc1 and cytochrome b6f complexes of prokaryotes and eukaryotes or form components of archaeal electron transport systems. The Rieske-type proteins encompass a group of bacterial oxygenases and ferredoxins. Recent studies have uncovered several new proteins containing Rieske centers, including archaeal Rieske proteins, bacterial oxygenases, bacterial ferredoxins, and, intriguingly, eukaryotic Rieske oxygenases. Since all these proteins contain a Rieske motif, they probably form a superfamily with one common ancestor. Phylogenetic analyses have, however, been generally limited to similar sequences, providing little information about relationships within the whole group of these proteins. The aim of this work is, therefore, to construct a dendrogram including representatives from all Rieske and Rieske-type protein classes in order to gain insight into their evolutionary relationships and to further define the phylogenetic niches occupied by the recently discovered proteins mentioned above.

The biosynthesis of N-glycoloylneuraminic acid occurs by hydroxylation of the CMP-glycoside of N-acetylneuraminic acid

The biosynthesis of N-glycoloylneuraminic acid in fractionated porcine submandibular glands was investigated. The following substrates: [3H]N-acetylmannosamine, free [14C]N-acetylneuraminic acid, CMP-[14C]N-acetylneuraminic acid, [14C]N-acetylneuraminic acid linked alpha(2----3) to galactose residues, or alpha(2----6) to Gal-beta(1----4)-GlcNAc residues of porcine submandibular mucin and [14C]N-acetylneuraminic acid linked alpha(2----6) to GalNAc residues of ovine submandibular gland mucin were incubated, in the presence of cofactors, with the soluble protein, heavy membrane and microsomal fractions of porcine submandibular glands. Radio thin-layer chromatographic analysis revealed that only one substrate, CMP-[14C]N-acetylneuraminic acid, was hydroxylated. The product was identified as CMP-[14C]N-glycoloylneuraminic acid by (i) co-chromatography with non-radioactive CMP-N-glycoloylneuraminic acid standard, (ii) acid hydrolysis to free [14C]N-glycoloylneuraminic acid, (iii) alkaline hydrolysis to yield N-glycoloylneuraminic acid and 2-deoxy-2,3-didehydro-N-glycoloylneuraminic acid and (iv) transfer of [14C]N-glycoloylneuraminic acid to asialo-fetuin by sialyltransferase. 85% of CMP-N-acetylneuraminic acid hydroxylase activity was present in the soluble protein fraction, with small amounts of activity in the two particulate fractions. The CMP-N-acetylneuraminic acid hydroxylase in the soluble protein fraction had an absolute requirement for Fe2+ ions and a reducing cofactor. NADPH and NADH were by far the most effective cofactors, smaller amounts of hydroxylation could, however, be supported by ascorbic acid and 6,7-dimethyl-5,6,7,8-tetrahydrobiopterin.

Evidence for the incorporation of molecular oxygen, a pathway in biosynthesis of N-glycolylmuramic acid in Mycobacterium phlei

The hypothesis that the biosynthesis of the glycolyl group of muramic acid in the peptidoglycan of Mycobacterium phlei is catalyzed by a N-acetyl hydroxylase is strongly supported by the experiments reported in this paper. 18O is incorporated into the N-substituent of muramic acid isolated from the peptidoglycan of M. phlei grown under pure oxygen enriched with the 18O isotope.