Cell wall model proposed

The structure of the bacterial cell wall, which is a target for antibacterial drugs, has been difficult to clarify because of its complexity and the difficulty in obtaining individual components. The cell wall is formed from a peptidoglycan scaffold consisting of repeating disaccharide units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), with a pentapeptide attached to each NAM. University of Notre Dame chemistry professor Shahriar Mobashery and coworkers have obtained a 3-D NMR structure of a 2-kilodalton synthetic fragment of this scaffold (Proc. Natl. Acad. Sci. USA, published online March 9, dx.doi.org/10.1073/pnas.0510182103). From this fragment, they predict that the longer native peptidoglycan strand is a right-handed helix with three repeats per turn and threefold symmetry along the helical axis; each strand can cross-link with up to three neighboring strands. They extrapolate to a model of the bacterial cell wall with a layered honeycomb pattern of varying pore sizes (top view shown), depending on the number of cross-links. Waldemar Vollmer of the University of Tübingen in Germany cautions that the model is "highly speculative" and "contradicts a number of experimental data."