A new proposal for the primary and secondary structure of the glycan moiety of pseudomurein. Conformational energy calculations on the glycan strands with talosaminuronic acid in 1C conformation and comparison with murein.

Using the method of conformational energy calculations, favoured conformations of a pseudomurein sugar strand built up from beta 1,3-linked N-acetyl-D-glucosamine and N-acetyl-L-talosaminuronic acid were obtained. Such a completely beta 1,3-linked polysaccharide primary structure, although contrasting with the originally proposed alternating beta 1,3-alpha 1,3-linked structure [König, H., Kandler, O., Jensen, M. and Rietschel, E. Th. (1983) Hoppe-Seyler's Z. Physiol. Chem. 364, 627-636] would be in agreement with all experimental data hitherto known. Starting from an analysis of favoured conformations of the monosaccharide building blocks and those obtained for disaccharide parts, the favoured helical conformations of the complete polysaccharide chains could be explored. Our completely beta 1,3-linked chain could adopt two types of conformation: extended and hollow; the latter was discarded as unsuitable for cell wall assembly. The extended conformation type was shown to exhibit a remarkable similarity if compared to the secondary structures accessible to murein-type polysaccharide chains. In contrast to the conformations accessible for the beta 1,3-alpha 1,3-linked primary structure, the new hypothetical pseudomurein structure was found to be more extended, possessed a flexible peptide attachment site at every second sugar residue and led to an orientation of consecutive peptide attachment sites analogous to the data known for murein-type chains. From the two possibilities compatible with the experimental data available, the completely beta 1,3-linked sugar strand structure could well be realized in the native pseudomurein network of methanobacteria. In this case the hypothesis for a similar three-dimensional architecture for murein and pseudomurein would be supported.