On the Secondary and Tertiary Structure of Murein

Abstract

X-ray diffraction, density measurements, and stereochemical data were used in order to disclose the architecture of murein, the rigid component of almost all bacterial cell walls. Dry densities of 1.38 -1.39 g/cm3 were observed for Micrococcus luteus and Staphylococcus aureus. The X-ray data for gram-positive (S. uureiis, M . luteus) and gram-negative (Escherichia cofi) strains were almost identical, showing, in addition to some diffuse scattering with broad maxima corresponding to Bragg values of 0.22 and 0.45 nm, two sharp peaks indicating periodicities of about 0.7 and 0.94 nm. These latter periodicities were not found in whole bacteria but emerge immediately after breakage of the whole cells. In gram-positive species a weak oriented reflex at 4.2 nm appeared, while all other reflexes remain ring-shaped. Diffraction patterns obtained under the influence of humidity and certain metal ions showed strong variations in the position of the 0.45-nm ha1 0.These data, together with stereochemical considerations, invalidate the hitherto advanced models of a chitin-like structure of the glycan chains in murein. Instead, the following model proposed and discussed here is consistent with the experimental data. The structure is made up of layers, which in S. aureus and M . luteus are separated by about 4.2 nm. The layers are more or less randomly rotated against each other. The sugar chains run parallel to these layers and, hence, to the surface of the cell wall. They do not possess a twofold screw axis as in chitin or cellulose. Thus, the peptide strands protude from the glycan chain axis in different directions. The peptide strands seem to assume fairly rigid conformations and to be mainly responsible for the regular layer-like arrangement of murein.The main skeletal component of most bacterial cell walls is a unique polymer called peptidoglycan (murein). The murein of gram-positive bacteria has a multilayered arrangement and makes up about 50 % of the cell wall weight, whereas the murein of gramnegative bacteria is probably monolayered, representing only about 5-10% of the cell wall weight [l].The glycan chains are composed of alternating sequences of N-acetylglucosamine and N-acetylmuramic acid. The peptide part is built up by alternating Dand L-amino acids, including a y-bond for D-glutamic acid (Fig. 1) [2-41.The subunits are linked together by p(1-4)-glycosidic bonds in the glycan part and can be crossParts of this paper were presented at the IV. International Conference on Small-Angle Scattering of X-Rays and Neutrons in Gatlinburg/U.S. A., October 1977. linked by peptide bonds between the free amino group of the diamino acid and the carboxyl group of the D-alanine of another peptide strand which, in this case, loses its second D-alanine. The cross-linkage COCH..

Processivity of peptidoglycan synthesis provides a built-in mechanism for the robustness of straight-rod cell morphology

“…3E). The radius of a C. crescentus cell is r ¼ 0.25 μm and the length of a disaccharide glycan subunit is near 1.03 nm [estimated using crystal structure of α-chitin and NMR measurements of glycan fragments (35)(36)(37)(38)]. Therefore, the processivity required to explain the observed straightening coefficient based solely on this mechanism is 287 AE 18 nm, which is equivalent to 279 AE 17 subunits.…”

Tertiary Structure of Staphylococcus aureus Cell Wall Murein

“…In the course of murein biosynthesis, disaccharide repeating units are attached to each other to produce glycan helices of different lengths, while the arms of the peptide side chains interact with each other to generate long oligopeptide bridges, endowing murein with a high degree of cross-linking. When performing the conformational analysis of oligomuropeptides, we made the central conceptual assumptions that within the murein tertiary structure, glycan strands have a helical conformation, which was determined by previous X-ray diffraction studies (6,21), and that the geometry of the oligopeptide chains is undistorted and resembles that of free oligomers released from murein by muramidase digestion.…”

“…Beneath each plane and at right angles to the glycan chains, there was a plane of peptide chains that were crosslinked at considerable distances; the whole structure made a covalently closed sphere joined by glycan chains in one direction and by oligopeptide chains at 90 o to the glycan chains (29,32,36,37,41). This model was discounted by X-ray diffraction data (6,21), which proved that glycan strands do not possess a straight chitin-like conformation but represent right-handed helices with 4 disaccharides units per turn. Although there are no conditions under which helices can be introduced into the proposed type of architecture, the "chitin" model is surprisingly present even in today's microbiology textbooks.…”

“…While the first model (37, 41) accounted for a high degree of cross-linking, it was discounted because it is incompatible with the known helical conformation of glycan strands. The second model ascribed to murein an unusual "plywood"-type architecture with consecutive turns of each glycan layer by 60° (21,22). This type * Corresponding author.…”

Reduced Vancomycin Susceptibility inStaphylococcus aureus, Including Vancomycin-Intermediate and Heterogeneous Vancomycin-Intermediate Strains: Resistance Mechanisms, Laboratory Detection, and Clinical Implications

“…The actual orientation of the glycan chains within the cell wall is uncertain. Early models suggested that the glycan strands were arranged in shell-like, parallel structures around the cell (72,170). A recent model suggests that the glycan and oligopeptide chains are in fact perpendicular to the plasma membrane, with oligopeptide chains adopting a zigzag conformation to connect adjacent glycan strands (72).…”