REFINED 1.8-ANGSTROM STRUCTURE REVEALS THE MODE OF BINDING OF BETA-CYCLODEXTRIN TO THE MALTODEXTRIN BINDING-PROTEIN

The maltodextrin binding protein from Escherichia coli serves as the i nitial receptor for both the active transport of and chemotaxis toward a range of linear maltose sugars. The X-ray structures of both the ma ltose-bound and sugar-free forms of the protein have been previously d escribed [Spurlino, J. C., Lu, G.-Y., & Quiocho, F. A. (1991) J. Biol. Chem. 266, 5202-5219; Sharff, A. J., Rodseth, L. E., Spurlino, J. C., & Quiocho, F. A. (1992) Biochemistry 31, 10657-10663]. The X-ray crys tal structure of the maltodextrin binding protein complexed with cyclo maltoheptaose (beta-cyclodextrin) has been determined from a single cr ystal. The structure has been refined to a final R-value of 21% at 1.8 -angstrom resolution. Although not a physiological ligand for the malt odextrin binding protein, beta-cyclodextrin has been shown to bind wit h a K(d) of the same order as those of the linear maltodextrin substra tes. The observed structure shows that the complexed protein remains i n the fully open conformation and is almost identical to the structure of the unliganded protein. The sugar sits in the open cleft with thre e glucosyl units bound to the C-domain at the base of the cleft, in a similar position to maltotriose, the most tightly bound ligand. The to p of the ring is loosely bound to the upper edge of the cleft on the N -domain. The sugar makes a total of 94 productive interactions (of les s than 4.0-angstrom length) with the protein and with bound water mole cules. Of these there are only four (of less than 3.4-angstrom length) direct sugar-protein hydrogen bonds, with another four water-mediated hydrogen bonds. Comparison shows that, other than rotation of the C6 hydroxyl groups and some torsional deformations, the structures of bet a-cyclodextrin and the small molecule crystal structure of beta-cyclod extrin are similar.