carbohydrate-binding modules Ec and Fc from SusE and SusF, respectively, and similar CBMs
Included in this subgroup are CBM-Ec and CBM-Fc, starch-specific carbohydrate-binding modules of SusE and SusF, 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 have 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 contribute differently to binding insoluble starch. CBM-Fb and CBM-Fc both bind insoluble starch, 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. Proteins in this group are present in the species of the Gram-negative Bacteroidetes phylum.
Feature 1:starch binding site [chemical binding site]
Evidence:
Structure:4FCH: Bacteroides thetaiotaomicron SusE second CBM (CBM-Ec) binds maltoheptaose; contacts at 4A
Comment:CBM-Ec has the most extensive set of protein-ligand interactions of all 5 CBMs contained in SusE and SusF.
Comment:The most N- and C-terminal Trp residues create the aromatic platform for hydrophobic stacking, the Trp residue wedged between these residues provides an additional hydrogen-binding donor.
Jiyao W et al.(2023). "The conserved domain database in 2023.",