Structural basis for the substrate specificity of a Bacillus 1,3-1,4-beta-glucanase.

Depolymerization of polysaccharides is catalyzed by highly specific enzymes that promote hydrolysis of the scissile glycosidic bond by an activated water molecule. 1,3-1,4-beta-Glucanases selectively cleave beta-1,4 glycosidic bonds in 3-O-substituted glucopyranosyl units within polysaccharides with mixed linkage. The reaction follows a double-displacement mechanism by which the configuration of the anomeric C(1)-atom of the glucosyl unit in subsite -I is retained. Here we report the high-resolution crystal structure of the hybrid 1,3-1,4-beta-glucanase H(A16-M)(E105Q/E109Q) in complex with a beta-glucan tetrasaccharide. The structure shows four beta-d-glucosyl moieties bound to the substrate-binding cleft covering subsites -IV to -I, thus corresponding to the reaction product. The ten active-site residues Asn26, Glu63, Arg65, Phe92, Tyr94, Glu105, Asp107, Glu109, Asn182 and Trp184 form a network of hydrogen bonds and hydrophobic stacking interactions with the substrate. These residues were previously identified by mutational analysis as significant for stabilization of the enzyme-carbohydrate complex, with Glu105 and Glu109 being the catalytic residues. Compared to the Michaelis complex model, the tetrasaccharide moiety is slightly shifted toward that part of the cleft binding the non-reducing end of the substrate, but shows previously unanticipated strong stacking interactions with Phe92 in subsite -I. A number of specific hydrogen-bond contacts between the enzyme and the equatorial O(2), O(3) and O(6) hydroxyl groups of the glucosyl residues in subsites -I, -II and -III are the structural basis for the observed substrate specificity of 1,3-1,4-beta-glucanases. Kinetic analysis of enzyme variants with the all beta-1,3 linked polysaccharide laminarin identified key residues mediating substrate specificity in good agreement with the structural data. The comparison with structures of the apo-enzyme H(A16-M) and a covalent enzyme-inhibitor (E.I) complex, together with kinetic and mutagenesis data, yields new insights into the structural requirements for substrate binding and catalysis. A detailed view of enzyme-carbohydrate interactions is presented and mechanistic implications are discussed.

Solution structure, backbone dynamics, and association behavior of the C-terminal BRCT domain from the breast cancer-associated protein BRCA1.

BRCA1 is a tumor suppressor protein associated with breast and ovarian cancer. The C-terminal region of BRCA1 consists of two closely spaced BRCT domains which mediate essential biological functions, including regulation of transcription and control of cell-cycle progression by their interaction with phosphorylated effector proteins. Here we report the NMR structure of the isolated C-terminal BRCT domain (BRCT-c) from human BRCA1. BRCT-c is well-structured in solution, folding independently in the absence of its BRCT-n counterpart. Ultracentrifugation experiments and size exclusion chromatography reveal that BRCT-c exists as a monomer under near-physiological conditions. Dynamics measurements from NMR data show three loops which coincide with the most variable sequence regions in BRCT domains, to be genuinely flexible in solution. The solution structure of BRCT-c shows subtle conformational changes when compared to the crystal structure of BRCT-c in the tandem repeat of BRCA1. These affect sites involved in formation of the BRCT-n-BRCT-c interface and the binding to phosphoserine-containing peptides. The results suggest that the presence of native BRCT-n and a properly aligned BRCT-n-BRCT-c interface are essential if BRCT-c is to adopt a biologically active conformation. Structural consequences of cancer-associated mutations and biological implications of the dynamic behavior are discussed.