SusB, an 84-kDa ␣-glucoside hydrolase involved in the starch utilization system (sus) of Bacteroides thetaiotaomicron, belongs to glycoside hydrolase (GH) family 97. We have determined the enzymatic characteristics and the crystal structures in free and acarbose-bound form at 1.6 Å resolution. SusB hydrolyzes the ␣-glucosidic linkage, with inversion of anomeric configuration liberating the ␤-anomer of glucose as the reaction product. The substrate specificity of SusB, hydrolyzing not only ␣-1,4-glucosidic linkages but also ␣-1,6-, ␣-1,3-, and ␣-1,2-glucosidic linkages, is clearly different from other well known glucoamylases belonging to GH15. The structure of SusB was solved by the single-wavelength anomalous diffraction method with sulfur atoms as anomalous scatterers using an in-house x-ray source. SusB includes three domains as follows: the N-terminal, catalytic, and C-terminal domains. The structure of the SusB-acarbose complex shows a constellation of carboxyl groups at the catalytic center; Glu 532 is positioned to provide protonic assistance to leaving group departure, with Glu 439 and Glu 508 both positioned to provide base-catalyzed assistance for inverting nucleophilic attack by water. A structural comparison with other glycoside hydrolases revealed significant similarity between the catalytic domain of SusB and those of ␣-retaining glycoside hydrolases belonging to GH27, -36, and -31 despite the differences in catalytic mechanism. SusB and the other retaining enzymes appear to have diverged from a common ancestor and individually acquired the functional carboxyl groups during the process of evolution. Furthermore, sequence comparison of the active site based on the structure of SusB indicated that GH97 included both retaining and inverting enzymes.

No abstract

The deformation of a pseudo-elliptic vortex ring with circulation F was studied by a three-dimensional vortex blob method which included the viscous diffusion of vorticity. The circumference of this vortex ring consisted of two parallel line segments of the same length connected by two semicircles of the same radius R, the length of the major axis being denoted by L. The cross section of the vortex ring was represented by multiple vortex blobs with overlapping cores, while the circumference of the vortex ring was divided into a number of such sections. Numerical calculations were performed for the aspect ratios L /2R ~" 2.0-12,0, the initial core radius LT cs = 0.20R, and Reynolds number F /v c~1500, v being the kinematic viscosity. The vurtex rings yielded four fundamental patterns of deformation which included the axis switching and the split, depending on the aspect ratio, A flow-visualization experiment confirmed three of the four fundamental patterns, and was in sufficient agreement with the computation for the period of the axis switching and the time of the split.

Glycoside hydrolases have been implicated in many biological processes. To date, they have been classified into 93 glycoside hydrolase (GH) families based on amino-acid sequence similarity. alpha-Xylosidase from Escherichia coli belongs to GH family 31 and catalyzes the release of alpha-xylose from the non-reducing terminal side of alpha-xyloside. Single crystals of alpha-xylosidase have been grown by vapour diffusion at 293 K from 10%(w/v) PEG 20K, 2%(v/v) 2-propanol, 2%(v/v) glycerol and 0.1 M 2-morpholinoethanesulfonic acid pH 5.5. These crystals belong to space group P2(1)2(1)2(1) and X-ray diffraction data were collected to a resolution of 2.75 A. Crystals of selenomethionyl-substituted alpha-xylosidase were also obtained, which diffracted to at least 3.0 A. Based on the value of VM, the asymmetric unit in these crystals was assumed to contain six molecules.

No abstract