The cell wall and plasma membrane glycoproteins and glycolipids of plant cells are carbohydrate-based outer layers that serve structural and defensive purposes. It can be challenging to move through this thicket of carbohydrates, either for microbes in the act of feeding on cell wall constituents or for viruses seeking to effect entry into the host cell. Polysaccharide-degrading enzymes often incorporate a carbohydrate-binding module (CBM) alongside the hydrolytic domain. Boraston et al. use the crystal structure of one such domain, called CBM27, and thermodynamic measurements to describe its interaction with an oligosaccharide of the mannan type. The individual binding sites not only manage to discriminate among various combinations of axial and equatorial hydroxyls, but succeed in accommodating side branches so as to afford virtually complete coverage of the heterogeneous polymer of glucose, galactose, and mannose residues. Thobhani et al. show that the multivalency conferred by linked sialidase and CBM introduces a remarkable gain in catalytic efficiency, up to 50-fold. This tethering enables the low-affinity sialidase to clip off terminal sialic acid residues. The difficulty in targeting an enzyme that displays low substrate affinity might thus be overcome by developing an inhibitor that makes use of the adjoining CBM.—GJC
Structure 11, 665 (2003); J. Am. Chem. Soc. 10.1021/ja029759w (2003).