carbohydrate-binding modules from Bacteroides thetaiotaomicron SusE, SusF and similar proteins.
This group includes five starch-specific CBMs (carbohydrate-binding modules) of SusE and SusF, two cell surface lipoproteins within the Sus (Starch-utilization system) system of the human gut symbiont Bacteroides thetaiotaomicron. These CBMs have no enzymatic activity. The precise mechanistic roles of SusE and SusF in starch metabolism are unclear. Both proteins contain an N-terminal domain which may belong to the immunoglobulin superfamily (IgSF), followed by two or three tandem starch-binding CBMs. SusF has three CBMs (CBM-Fa, -Fb, and -Fc; F denotes SusF, and they are labeled alphabetically from the N- to C- terminus). SusE has two CBMs (CBM-Eb and -Ec, corresponding to CBM-Fb and -Fc). Each starch-binding site contains an arc of aromatic amino acids for hydrophobic stacking with glucose, and hydrogen-bonding acceptors and donors for interacting with the O-2 and O-3 of glucose. These five CBMs show differences in their affinity for various different starch oligosaccharides, and they also contribute differently to binding insoluble starch. CBM-Fa (the CBM unique to SusF), does not bind insoluble starch; CBM-Fb and CBM-Fc both do, deletion of one or the other results in a decrease in the overall affinity of SusF for starch. Both CBM-Eb and CBM-Ec are needed for SusE to bind tightly to starch. CBM-Ec has an additional starch-binding loop that may mediate interactions with partially unwound single helical forms of starch or small starch-breakdown products. Proteins in this group are present in the species of the Gram-negative Bacteroidetes phylum.
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