Structural basis for oligosaccharide recognition by Pyrococcus furiosus maltodextrin-binding protein

maltodextrin-binding protein from Pyococcus furiosus (PfuMBP) has been over produced in Escherichia coli, purified, and crystallized. The crystal struc ture of the protein bound to an oligosaccharide ligand was determined to 1. 85 Angstrom resolution. The fold of I;PfuMBP is very similar to that of the orthologous MBP from E. coli (EcoMBP), despite the moderate level of seque nce identity between the two proteins (27% identity, 46% similarity). PfuMB P is extremely resistant to heat and chemical denaturation, which may be at tributed to a number of factors, such as a tightly packed hydrophobic core, clusters of isoleucine residues, salt-bridges, and the presence of proline residues in key positions. Surprisingly, an attempt to crystallize the com plex of PfuMBP with maltose resulted in a structure that contained maltotri ose in the ligand-binding site. The structure of the complex suggests that there is a considerable energy gain upon binding of maltotriose in comparis on to maltose. Moreover, isothermal titration calorimetry experiments demon strated that the binding of maltotriose to the protein is exothermic and ti ght, whereas no thermal effect was observed upon addition of maltose at thr ee temperatures. Therefore, PfuMBP evidently is designed to bind oligosacch arides composed of three or more glucopyranose units.