Structural basis for cyclodextrin recognition by Thermoactinomyces vulgaris cyclo/maltodextrin-binding protein.

The crystal structure of a Thermoactinomyces vulgaris cyclo/maltodextrin-binding protein ( TvuCMBP) complexed with γ-cyclodextrin has been determined. Like Escherichia coli maltodextrin-binding protein ( EcoMBP) and other bacterial sugar-binding proteins, TvuCMBP consists of two domains, an N- and a C-domain, both of which are composed of a central β-sheet surrounded by α-helices; the domains are joined by a hinge region containing three segments. γ-Cyclodextrin is located at a cleft formed by the two domains. A common functional conformational change has been reported in this protein family, which involves switching from an open form to a sugar-transporter bindable form, designated a closed form. The TvuCMBP–γ-cyclodextrin complex structurally resembles the closed form of EcoMBP, indicating that TvuCMBP complexed with γ-cyclodextrin adopts the closed form. The fluorescence measurements also showed that the affinities of TvuCMBP for cyclodextrins were almost equal to those for maltooligosaccharides. Despite having similar folds, the sugar-binding site of the N-domain part of TvuCMBP and other bacterial sugar-binding proteins are strikingly different. In TvuCMBP, the side-chain of Leu59 protrudes from the N-domain part into the sugar-binding cleft and orients toward the central cavity of γ-cyclodextrin, thus Leu59 appears to play the key role in binding. The cleft of the sugar-binding site of TvuCMBP is also wider than that of EcoMBP. These findings suggest that the sugar-binding site of the N-domain part and the wide cleft are critical in determining the specificity of TvuCMBP for γ-cyclodextrin. [ABSTRACT FROM AUTHOR]