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Science  07 Apr 2006:
Vol. 312, Issue 5770, pp. 22
DOI: 10.1126/science.312.5770.22a

Any mention of biological polymers often serves as shorthand for proteins or nucleic acids. Sugar-based macromolecules come to mind less readily even though they constitute some of the most abundant and visible manifestations: cellulose in trees and chitin in invertebrate exoskeletons. A third and equally important member of this group is the disaccharide building block (NAG-NAM) of the bacterial cell wall, whose essential contribution to survival is amply illustrated by the use of lysozyme in the laboratory and penicillin (and its descendants) in the clinic.

Meroueh et al. describe the NMR structure of a synthesized fragment incorporating two of these disaacharides and their two pendant peptides, which cross-link the glycan strands in vivo. Modeling this fragment into an average-length glycan (10 saccharide units) yields a helix with the pentapeptides emerging at 120° to each other. Factoring in the critical assumption that these strands run perpendicularly to the membrane surface makes it feasible to situate these helices within a honeycomb structure with pores of diameter 70 Å, which snugly accommodate the TolC efflux channel that bridges the periplasm and outer membrane. — GJC

Proc. Natl. Acad. Sci. U.S.A. 103, 4404 (2006).

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