Structural studies on the bacterial cell wall peptidoglycan pseudomurein. I. Conformational energy calculations on the glycan strands in C1 conformation and comparison with murein.

Conformational energy calculations have been used to explore the conformations which may be realized for the sugar moiety of murein and pseudomurein. For the building blocks of the pseudomurein sugar strands, i.e. for the monosaccharides beta-D-N-acetylglucosamine (NAG) and alpha-L-N-acetyltalosaminuronic acid (NAT), both in C1 ring conformation, as well as for their 1,3 and 1,4 linked disaccharides, the favoured conformations were obtained. The helical parameters of sugar strands of both linkage types, which describe the regular structure of the corresponding polysaccharides, poly-(1,3-NAT-NAG) and poly-(1,4-NAT-NAG), were calculated. Both types of polysaccharides poly-(NAG-NAT) considered in this study favoured extended conformations, which in the case of 1,3 linked polymers showed less gain of length per saccharide unit compared to 1,4 linked poly-(NAG-NAT) residues. For a 1,3 linked sugar moiety of pseudomurein every pair of neighbouring peptides attached to glycan chain pointed in favoured conformations approximately to opposite sides of the strands, whereas in a 1,4 linked poly-(NAG-NAT) the peptides protruded approximately to the same side of the glycan moiety. A comparison between pseudomurein and murein revealed that the sugar moieties of both peptidoglycans have similar features in respect to their overall structure, i.e. both favoured more or less extended structures. In contrast to these data the shapes of the resulting polysaccharide moieties were remarkably different. In poly-(1,3-NAG-NAT) the glycan chains possessed a zig-zag-like arrangement, whereas for glycan chains of the murein type relatively flat structures were preferred. These remaining differences in the conformational arrangement between both peptidoglycans depend strongly on the C1 chair conformation of NAT. It is, therefore, attractive to speculate about an hypothetical pseudomurein sugar chain configuration comprising beta-L-N-acetyltalosaminuronic acid in its 1C conformation.