Structure of the peptidoglycan of bacterial cell walls. II

Abstract

X-ray diffraction results are presented for cell walls and extracted peptidoglycans isolated from six species of bacteria. The selection of cells includes both Grampositive and Gram-negative organisms and three of the four chemotypes are represented. Orientated diffraction patterns were obtained from layered specimens of peptidoglycans from the three bacteria where the glycan (polysaccharide) chains are loosely cross-linked by peptide. Conversely, no orientation was achieved for the tightly cross-linked peptidoglycans. The orientated features of the peptidoglycan diffraction patterns concerned a single sharp “meridional” reflection and a broad “equatorial” reflection. Both these spacings varied with specimen water content, the sharp reflection (0.98 nm wet to 0.94 nm dry) being identified with the axial advance of a glycan helix and the equatorial reflection (1.9 nm wet to 0.9 nm dry) with the separation by cross-linking peptide chains of adjacent glycan chains. Using the diffraction pattern and the known primary structure of peptidoglycan as constraints in conformational mapping, a range of stereochemically acceptable helical models is deduced for the glycan chains; insufficient information is available to select a single model. A conformation for the glycan chain with attached peptide was selected for study with the glycan in the form of a fourfold right-handed helix. The optical diffraction of this model is compared with the peptidoglycan X-ray diagram. Good agreement between the patterns is achieved if the model helix is subjected to axial distortions within the ranges allowed by the conformation maps. The undistorted helix allows the possibility of two systematic sets of hydrogen bonds. An assessment of the changes in the X-ray patterns, on extracting the teichoic and teichuronic accessory polymers to produce peptidoglycan, leads to the conclusion that these polymers are distributed throughout the volume of the cell wall. The glycan chains run parallel to the surface of the peptidoglycans and, so far as can be defined by X-ray diffraction, the various peptidoglycans appear to have the same general structure.