Acetylmuramic acid

Listowsky and coworkers showed that the c.d. of this sugar derivative is due entirely to lactic acid, and confirmed that this chromophore is in the D configuration for muramic acid. N-Acetylmuramic acid, in which the amino group is replaced by an amido group at C-2, has a c.d. spectrum that is roughly a linear combination of the lactic acid in muramic acid and the amide in 2-acetamido-2-deoxy-D-glucose. This indicates that the amide chromophore and the lactic acid chromophore in N-acetylmuramic acid behave independently. 

Fig. 1.2 A, peptidoglycan of Escherichia coli. , A -acetylmuramic acid , Af-acetylglucosamine. B, repeating unit of peptidoglycan ofE. coli. L-ala, L-alanine D-glu, D-glutamine DAP, diaminopimelic acid D-aia, D-alanine.

Fig. 8.1 Biosynthesis of peptidoglycan. The large circles represent A -acetylglucosamine orN-acetylmuramic acid to the latter is linked initially a pentapeptide chain comprising L-alanine, D-glutamic acid and meso-diaminopiraelic acid (small circles) terminating in two D-alanine residues (small, darker circles). The lipid molecule is undecaprenyl phosphate. In the initial (cytoplasm) stage where inhibition by the antibiotic D-cycloserine is shown, two molecules of Dalanine (small circles) are converted by an isomerase to the D-forms (small, darker circles), alter which a ligase joins the two D-alanines together to produce a D-alanyl-D-alanine dipeptide.

Figure 2.7. The outer membrane of Gram-negative bacteria. Abbreviations LB, LamB protein LP, lipoprotein O, OmpA protein MP, membrane protein. The peptidoglycan backbone consists of alternating residues of A-acetylglucosamine and /V-acetylmuramic acid, which are cross-linked via short peptides.

A -acetylmuramic acid) residues. These chains are cross-linked via peptide structures. Part of the peptidoglycan of Staphylococcus aureus is shown here. Amino acid abbreviations are given in Chapter 13. 

This shows the involvement of the lactyl group of the A -acetylmuramic acid in linking the peptide with the carbohydrate via an amide teptide bond. The biological activities of the P-lactam antibiotics, e.g. penicillins and cephalosporins (see Box 13.11), stem from an... 

At the Start of the cross-linking process, the peptide chains from the A -acetylmuramic acid residues have a terminal -Lys-o-Ala-o-Ala sequence. The lysine from one chain then becomes bonded to the penultimate d-alanine of another chain through five glycine residues, at the same time displacing the terminal o-alanine. The mechanism involves a serine residue at the active site of the enzyme. This residue is used to convert an amide linkage into an ester, and a reversal of this sequence provides the new peptide bond. 

This enzyme [EC 3.2.1.17], also called muramidase, catalyzes the hydrolysis of the l,4-/3-linkages between N-acetyl-D-glucosamine and A-acetylmuramic acid in pep-tidoglycan heteropolymers of prokaryotic cell walls. Some chitinases [EC 3.2.1.14] also exhibit this activity. See Kinetic Isotope Effect... 

The glycopeptides are inhibitors of cell wall synthesis. They bind to the terminal carboxyl group on the d-alanyl-D-alanine terminus of the A-acetylglucosamine-A-acetylmuramic acid peptide and prevent polymerization of the linear peptidoglycan by peptidoglycan synthase. They are bactericidal in vitro. 

Vancomycin acts by inhibiting the correct synthesis of cell walls in gram-positive bacteria by specifically inhibiting the incorporation of V-acetylmuramic acid (NAM) and A-acetylglucosamine (NAG), two important peptide subunits that are present in the peptidoglycan layer of these types of bacteria. 

The main function of the ester 34 in bacterial cells seems to be its participation in the biosynthesis of the glycopeptide cell-wall polymer. If this process is blocked, there results the accumulation of a high concentration of sugar nucleotide precursors in the cell. A number of these compounds have been isolated the simplest one is the ester of uridine 5 -pyrophosphate with N-acetylmuramic acid [2-acetamido-3-0-(D-l-carboxyethyl)-2-deoxy-D-glucose] (37), first obtained from Staphylococcus aureus cells that had been treated with penicillin7,151 or Gentian Violet.144 An intermediate in the biosynthesis of 37 was isolated and shown to be the 3 -enolpyruvate ether152,153 (38). 

Three of these saccharides have to be N-acetylmuramic acid, and are either A, C, and E, or B, D, and F. The choice is easy, because the hydroxyl group on C-3 of carbohydrate C points toward the cleft and is hydrogen-bonded to the ring-nitrogen atom of L-tryptophan at position No. 63 of the protein core hence, there is no space for the lactoyl side-chain of N-acetylmuramic acid. Consequently, B, D, and F are N-acetylmuramic acid residues, and this identification is consistent with the low-resolution results. The lactoyl side-chains point out of the cleft, so that there is room for a peptide tail, such as occurs in the cross-linking of the cell walls of Micrococcus lysodeikti-cus.Ui... 

The biosynthesis of cell wall peptidoglycan, showing the sites of action of five antibiotics (shaded bars 1 = fosfomycin, 2 = cycloserine, 3 = bacitracin, 4 = vancomycin, 5 = 3-lactam antibiotics). Bactoprenol (BP) is the lipid membrane carrier that transports building blocks across the cytoplasmic membrane M, N-acetylmuramic acid Glc, glucose NAcGIc or G, /V-acetylglucosamine. 

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