Biochemistry

Curvy Carbohydrates

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Science  25 Jul 2008:
Vol. 321, Issue 5888, pp. 466
DOI: 10.1126/science.321.5888.466c

The global abundance of carbohydrate-based biopolymers and the prevalence of them in our diets support within the human gut a fascinating microbial ecosystem. Its role, from our point of view, is to degrade dietary polysaccharides; the flux through these pathways contributes as much as 10% of our daily calorie intake. One of the inhabitants of the human intestine is Bacteroides, and its starch utilization system (Sus) contains a number of regulatory and metabolic genes.

Koropatkin et al. have determined the structure of the outer membrane protein SusD on its own and with linear and cyclic oligosaccharides bound. They find that the side chains of aromatic residues of SusD align to offer a curved surface that complements the helical conformation of maltoheptaose as well as the curvatures of α- and β-cyclodextrins (composed of six and seven glucose units, respectively). The multivalent, low-affinity interaction may facilitate hydrolysis of longer polysaccharides by the neighboring amylase SusG or the loading of oligosaccharides into the outer membrane importer SusC. — GJC

Structure 16, 1105 (2008).

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