The Covalently Linked Sacculus: the Nona-Muropeptide Model.

A bacterial cell wears a stress-bearing suit of armor: the sacculus or exoskeleton. In E. coli it is composed of hundreds of thousands of disaccharide penta-muropeptides polymerized together. There are even more in the Gram-positive B. subtilis. Of course, the penta-muropeptide units are held together by strong covalent bonds, as shown in Figure 6.1, and these individual units are bound together with additional covalent bonds to make the sacculus strong. The bacterial wall can be a two- or three-dimensional covering. There are two styles: Gram-positive and Gram-negative. They both have different strategies to export a small unit and polymerize it into the growing sacculus while maintaining saccular strength. The rationale for these architectural differences comes simply because the basic disaccharide penta-muropeptide units must pass through the cytoplasmic membrane surface in both cases and then must be linked together to cover a unit of surface area in a make-before-break fashion but in different ways (see Chapters 11 and 12). Because the enzymes that create the linkages are also membrane bound, fundamentally only a two-dimensional structure can be created. Three-dimensional walls, as for example in B. subtilis, are composed as an onion is constructed by the successive addition of layers. However, there are probably some bonds linking together adjacent layers. The ability to form a layer from the basic disaccharide penta-muropeptide units with the facilities available from the membrane-bound enzymes is quite limited. The hexoses of one unit can be tied to the hexoses of another by β(1–4) bonds to form the glycan strand. One penta-muropeptide extending from a NAM residue can be tied to another penta-muropeptide by a tail-to-tail peptide (really an amide) bond to make a nona-muropeptide that has linked together two glycan chains. This chapter presents how the organic conformational chemistry of the murein controls formation of new murein in a safe manner. [ABSTRACT FROM AUTHOR]

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