Mureins

Peptidoplycans (14,16) are the primary component of bacterial cell walls. They consist of a heteropolysaccharide called murein cross-linked with short peptide chains. 

Proteins identified by their ability to bind labelled (3-lactam antibiotics in vivo and in vitro. The intrinsic activities of PBPs include transglycosylase/transpepti-dase, carboxypeptidase and endopeptidase activities required for the formation of the bacterial murein sacculus forming the bacterial cell wall. The enzymes are located in the cytoplasmic membrane. 

The essential genetic material ofthe original vegetative bacterium is retained in the core or protoplast around this lies the thick cortex which contains the murein or peptidoglycan already encountered as a cell wall component (see Fig. 1.2). The outer coats which are protein in composition are distinguished by their high cysteine content. In this respect they resemble keratin, the protein of hair and horn. 

Figure 13.9 9.4 T MALDI-FTMS spectrum of E. coli JM109 whole cells showing both Braun s and Murein lipoproteins.

Although ribosomal proteins are readily observed as in Figures 13.7 and 13.8 altered matrix conditions can alter the relative ionization of bacterial whole-cell compounds. A systematic analysis involving laser power/fluence and sample preparation conditions reveals that if the concentrated trifluo-roacetic acid is added and the laser power increased above optimal conditions, ionization of bacterial surface compounds can be enhanced. Figure 13.9 is the resulting 9.4 T MALDI-FTMS, seen are both the Braun s lipoprotein56,57 and the Murein lipoprotein. Both of these compounds are complex combinations of hydrocarbon lipids attached to a protein base. This is the first MALDI-FTMS observation of surface proteins desorbed directly from whole cells by influencing ionization conditions. 

Braun, V. Bosch, V. Repetitive sequences in the murein-hpoprotein of the cell wall of Escherichia coli. Proc. Nat. Acad. Sci. USA 1972, 69, 970-974. 

Also in the PP, associated with the CM, one can find the murein sacculus (for a review see [8]). This network is formed by the macromolecule pepti-doglycan, which confers the characteristic cell shape and provides the cell with mechanical protection. Peptidoglycans are unique to prokaryotic organisms and consist of a glycan backbone of N-acetylated muramic acid and N-acetylated glucosamine and cross-linked peptide chains [9-13]. 

Figure 10. Schematic view of the uptake of ferric siderophores by Gram-positive and Gram-negative bacteria. Please note that the murein (peptidoglycan) network associated with the cytoplasmic membrane is not shown. For details see text...

Holtje, J. Y. (1998). Growth of the stress-bearing and shape-maintaining murein sacculus of Escherichia coli, Microbiol. Mol. Biol. Rev., 62, 181-203. 

Park, J. T. (1996). The convergence of murein recycling research with beta-lactamase research, Microb. Drug Resist. Springer, 2, 105-112. 

Braun, V. and Rehn, K. (1969). Chemical characterization, spatial distribution and function of a lipoprotein (murein-lipoprotein) of the E. coli cell wall. The specific effect of trypsin on the membrane structure, Eur. J. Biochem., 10, 426-438. 

Cell wall Murein, teichoic acid and lipopolysaccharide Chitin or cellulose... 

In addition to murein, bacterial polysaccharides include dextrans—glucose polymers that are mostly al 6-linked and al 3-branched. In water, dextrans form viscous slimes or gels that are used for chromatographic separation of macromolecules after chemical treatment (see p.78). Dextrans are also used as components of blood plasma substitutes (plasma expanders) and foodstuffs. 

The natural amino acids are mainly a-amino acids, in contrast to (3-amino acids such as p-alanine and taurine. Most a-amino acids have four different substituents at C-2 (Ca). The a atom therefore represents a chiral center—I e., there are two different enantiomers (L- and D-amino acids see p. 8). Among the proteinogenic amino acids, only glycine is not chiral (R = H). In nature, it is almost exclusively L-amino acids that are found. D-Amino acids occur in bacteria—e. g., in murein (see p.40)—and in peptide antibiotics. In animal metabolism, D-Amino acids would disturb the enzymatic reactions of L-amino acids and they are therefore broken down in the liver by the enzyme D-amino add oxidase. 

An attractive hypothesis is the independent evolution in bacteria of their diffusible individualites and the currently recognized secondary metabolic pathways, in parallel with their surface components and their biosynthesis. An indicator for this would be the use of the same gene pool. The theory would include all substances that play a role in the build-up of glycan and other modified surface layers, lipids, murein, (glyco-) proteins (e.g., S-layers), polysaccharides, teichoic... 

Alice et al studied the turnover kinetics of Listeria OTonocytogenex-secreted p60 protein (a murein hydrolase) by host cell cytosolic proteasomes. J774 cells, seeded in flasks and incubated overnight in culture medium, were infected with log-phase cultures of E. monocytogenes for 30 min, washed, and incubated in culture medium for 3 h, with gentamicin (50 tg/ml) added after the first 30 min to inhibit extracellular bacterial growth. Cells then were washed and placed in methionine-free medium with spectinomycin, gentamicin, the eukaryotic protein synthesis inhibitors [cycloheximide (50 tg/mL) and anisomycin (30 tg/ml),] and 25 dVI calpain inhibitor I. After 30 min, [ S]methionine was added, and the cells were pulse-labeled for periods of 20 to 60 min. Cells... 

The main component of bacterial cell membranes is a mixed polymer known as murein or peptidoglycan. Peptidoglycan is a long polysaccharide chain that is cross-linked with short peptides. 

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