Three-dimensional structure of the bacterial cell wall peptidoglycan

Abstract

The 3D structure of the bacterial peptidoglycan, the major constituent of the cell wall, is one of the most important, yet still unsolved, structural problems in biochemistry. The peptidoglycan comprises alternating N-acetylglucosamine (NAG) and N-acetylmuramic disaccharide (NAM) saccharides, the latter of which has a peptide stem. Adjacent peptide stems are cross-linked by the transpeptidase enzymes of cell wall biosynthesis to provide the cell wall polymer with the structural integrity required by the bacterium. The cell wall and its biosynthetic enzymes are targets of antibiotics. The 3D structure of the cell wall has been elusive because of its complexity and the lack of pure samples. Herein we report the 3D solution structure as determined by NMR of the 2-kDa NAG-NAM(pentapeptide)-NAG-NAM(pentapeptide) synthetic fragment of the cell wall. The glycan backbone of this peptidoglycan forms a right-handed helix with a periodicity of three for the NAG-NAM repeat (per turn of the helix). The first two amino acids of the pentapeptide adopt a limited number of conformations. Based on this structure a model for the bacterial cell wall is proposed. This pentapeptide stem participates in an interglycan cross-linking reaction, thus creating the cell wall polymer. In contrast to the two other ␤-1,4-linked glycan biopolymers, cellulose (repeating glucose) (1-4) and chitin (repeating NAG) (5-7) for which the 3D structure is solved, the structure of the bacterial cell wall has remained elusive because of its complexity and the lack of pure and discrete segments for structural study (8-18). Herein we describe the 3D structure, determined in aqueous solution by NMR, of a 2-kDa synthetic NAG-NAM(pentapeptide)-NAG-NAM(pentapeptide) tetrasaccharide cell wall segment. The defining aspect of this structure is an ordered, right-handed helical saccharide conformation corresponding to three NAG-NAM pairs per turn of the helix. The structure of this peptidoglycan segment is the basis for a proposal for the structure of the bacterial cell wall polymer.Results and Discussion 3D Structure of the Peptidoglycan. Because of the critical significance of the cell wall to bacterial survival, and the exploitation of the cell wall biosynthetic enzymes for the chemotherapeutic intervention of infections, many experimental and theoretical studies have addressed the cell wall structure. Despite diffraction studies carried out Ͼ30 years ago on cell wall extracted from bacteria, which strongly suggested that the peptidoglycan polymer possessed regular order (11), the 3D structure of the cell wall is not known. An excellent account of the historical development of the hypotheses for the cell wall structure is given by Dmitriev, Toukach, and Ehlers in their recent review (18). The major reason for the lack of progress is the absence of a pure fragment of the cell wall, having both the peptide and disaccharide components of the peptidoglycan, for structural investigation. To address this limitation we completed the 37-step synthesis of such a segment (1...

Cell wall peptidoglycan architecture in Bacillus subtilis

“…Peptidoglycan strands have a natural right-handed twist (15,30), nascent (17) and whole material (Fig. 2 C and D) is arranged helically, and cells of B. subtilis can grow helically under some conditions (18).…”

“…The application of high-resolution, atomic force microscopy (AFM) to the study of the structure of the outer surface of S. aureus and Bacillus atrophaeus cell wall peptidoglycan reveals a surface network of thin fibers (possibly of only a few glycan strands) with large empty spaces in between (13,14). This is supported by an NMR structural analysis in which a ''honeycomb'' architecture, based on the scaffold model, is proposed for the peptidoglycan of E. coli (15).…”

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.…”

Structural insights into the bactericidal mechanism of human peptidoglycan recognition proteins

“…This structure, combined with the recent NMR structure of a dimeric PGN fragment (Fig. 1C) (17), enabled us to identify conformational changes in PGN induced by PGRP binding. These interactions suggest a mechanism whereby PGRPs disrupt cell wall maturation in a manner that is related to, yet distinct from, the mechanism used by glycopeptide antibiotics.…”