Structural insights into the substrate specificity of Streptococcus pneumoniae ?(1,3)-galactosidase BgaC.

ABSTRACT: The surface-exposed ?-galactosidase BgaC from Streptococcus pneumoniae was reported to be a virulence factor because of its specific hydrolysis activity toward the ?(1,3)-linked galactose and N-acetylglucosamine (Gal?(1,3)NAG) moiety of oligosaccharides on the host molecules. Here we report the crystal structure of BgaC at 1.8 ? and its complex with galactose at 1.95 ?. At pH 5.5-8.0, BgaC exists as a stable homodimer, each subunit of which consists of three distinct domains: a catalytic domain of a classic (?/?)(8) TIM barrel, followed by two all-? domains (ABDs) of unknown function. The side walls of the TIM ?-barrel and a loop extended from the first ABD constitute the active site. Superposition of the galactose-complexed structure to the apo-form revealed significant conformational changes of residues Trp-243 and Tyr-455. Simulation of a putative substrate entrance tunnel and modeling of a complex structure with Gal?(1,3)NAG enabled us to assign three key residues to the specific catalysis. Site-directed mutagenesis in combination with activity assays further proved that residues Trp-240 and Tyr-455 contribute to stabilizing the N-acetylglucosamine moiety, whereas Trp-243 is critical for fixing the galactose ring. Moreover, we propose that BgaC and other galactosidases in the GH-35 family share a common domain organization and a conserved substrate-determinant aromatic residue protruding from the second domain.

SUBMITTER: Cheng W 

PROVIDER: S-EPMC3391139 | biostudies-literature | 2012 Jun

REPOSITORIES: biostudies-literature