Pseudomurein

The origin of 2-amino-2-deoxy-L-taluronic acid,213 a constituent of pseudomurein of Methanobacterium, is difficult to explain at present. 

Nucleoside diphosphate disaccharides and peptides are the anabolic precursors of pseudomurein, the peptidoglycan of archaebacteria [65,66], The central intermediate is a UDP-activated glycopentapeptide consisting of a pentapeptide which is covalently linked to the disaccharide N-acetyltalosaminuronic acid (Pl-3)iV-acetylglucosaminide. In eubacteria a pentapeptide is attached to UDP-activated iV-acetylmuraminic acid (1) (Fig. 5). 

The pseudomurein sacculi could be isolated using the same methods as those usually applied for the isolation of the murein sacculi of gram-positive bacteria [8]. The harvested... 

Fig. 1. Thin section of Methanothermus fervidus (symbols S, S-layer Ps, pseudomurein sacculus ...

Fig. 2. Primary structure of dimers of murein and pseudomurein (compounds in parentheses may be missing in some cases). From ref. [46].

One of the two glutamic acid residues of pseudomurein is replaced by aspartic acid (e.g., Methanobacterium alcaliphilum O. Kandler, F. Fiedler and J. Winter, paper in preparation). 

Fig. 3. Proposed amino acid sequences of cross-linked subunits of pseudomurein and its modifications. Compounds in parentheses indicate alternative sequences observed in some species. Modified from...

Modifications of the amino-acid composition of pseudomurein could be induced by addition of glycine, threonine, ornithine, or aspartic acid in elevated concentrations to the culture medium [21]. The isolated sacculi from several species also contain varying amounts of monosaccharides (Table 2). They could be extracted with hot formamide, indicating that they belong to polysaccharides associated with but not covalently bound to pseudomurein. Teichoic acid or teichuronic acid-like polymers are, however, not present in gram-positive methanogens. 

X-ray diffraction measurements and structural calculations on murein [22-25] and pseudomurein [26-28] have revealed several common structural features in both polymers. Murein and pseudomurein sacculi possess a density of p= 1. 39-1.46g/cm which is characteristic of highly ordered material. A much lower density, in the range of p= 1.24-1.32g/cm is to be expected for amorphous polymers [26]. X-ray diffraction showed diffuse Debye-Scherer rings with Bragg periodicities of about 0.45 nm and 0.94 nm in the planes and of 4.3-4.5nm vertically to the planes of both types of cell walls. These data have been interpreted in two different ways ... 

The first steps of murein biosynthesis take place in the cytoplasm, whereas the two final steps occur at the inner and outer face of the cytoplasmic membrane, respectively [38]. This may also be true for the pseudomurein, according to a tentative scheme of the biosynthesis of pseudomurein (Fig. 4) [39-43], proposed on the basis of the structure of putative precursors isolated from cell extracts. 

Fig. 4. Comparison of the proposed schemes of the biosynthesis of murein and pseudomurein [40].

The participation of a nucleotide-activated disaccharide and UDP-activated peptide intermediates are unique features of pseudomurein biosynthesis. Usually, oligosaccharide precursors of bacterial cell-wall polymers are formed at the lipid stage [38,44] and amino acids carrying a nucleotide residue at the. /V -amino group have not been found in nature so far. The distinct differences between the two biosynthetic routes support the hypothesis that murein and pseudomurein represent independent inventions made after the domains bacteria and archaea had been separated from each other during evolution [40,46]. 

Murein components are known to exhibit a series of biological activities in humans and higher animals [47] so far much less information on the biological activities of pseudomurein components is available. 

Bacterial infections elicit a series of acute-phase responses which include central nervous system effects such as changes in body temperature and increased slow-wave sleep. Dead bacteria [54] and murein preparations [55] as well as other bacterial cell-wall products induce similar responses. Intravenous injections of rabbits with suspensions of pseudomurein from Methanobacterium thermoautotrophicum also alter sleep and brain temperature. The mechanisms responsible for these somnogenic and pyrogenic effects are unknown [56]. It has been demonstrated that in a rat arthritis model, intra-articular injection of high doses of pseudomurein-polysaccharide fragments from Methanobacterium formicicum caused an acute inflammation [57]. 

The cell walls consist of highly sulfated heteropolysaccharide which consists of a complex mixture of neutral and amino sugars, uronic acids, glycine and an aminuronic acid (gulosaminuronic acid) (Table 4) [80-84]. The typical components of minein or pseudomurein are missing. 

Considering the multitude of patterns, it is difficult to think of any common fimction for S-layers. In some cases, the S-layer contributes to the stability of the cell, similar to murein or pseudomurein cell-wall sacculi, for instance in Thermoproteus tenax which possesses an extremely stable S-layer. In many cases, the interspace between the... 

Important distinguishing features between eubacteria and A. are the cell wall structures of A. are heterogeneous (pseudomurein, proteins, glycoproteins, heteropolysaccharides), all A. lack muramic acid, a typical building block of the eubacterial cell wall. Thus, various antibiotics (e. g. penicillin, chloramphenicol) do not exhibit any inhibitory action. The cytoplasm membranes contain glycerol ethers with Cjo- and C4o-iso-prenoids instead of fatty acid glycerol esters. In addition to special biosynthetic pathways and unusual co-... 

Figure 1 Schematic illustration of the supramolecular architecture of the three major classes of prokaryotic cell envelopes containing crystalline bacterial cell surtece layers (S-layers). (a) Cell envelope structure of Gram-negative archaea with S-layers as the only cell wall component external to the cytoplasmic membrane, (b) Cell envelope as observed in Gram-positive archaea and bacteria. In bacteria the rigid wall component is primarily composed of peptidoglycan. In archaea other wall polymers (e.g., pseudomurein or methanochondroitin) are found, (c) Cell envelope profile of Gram-negative bacteria, composed of a thin peptidoglycan layer and an outer membrane. If present the S-layer is closely associated with the lipopolysaccharide of the outer membrane. (Modified after U.B. Sleytr, P. Messner, D. Pum, and M. Sdra. Crystalline Bacterial Cell Surface Proteins. Austin, TX R.G. Landes/Academic Press, 1996. With permission.)...

---