Crystal structure of Streptomyces olivaceoviridis E-86 beta-xylanase containing xylan-binding domain

Xylanases hydrolyse the beta-1,4-glycosidic bonds within the xylan backbone and belong to either family 10 or 11 of the glycoside hydrolases, on the b asis of the amino acid sequence similarities of their catalytic domains. Ge nerally, xylanases have a core catalytic domain, an N and/or C-terminal sub strate-binding domain and a linker region. Until now, X-ray structural anal yses of family 10 xylanases have been reported only for their catalytic dom ains and do not contain substrate-binding domains. We have determined the c rystal structure of a family 10 xylanase containing the xylan-binding domai n (XBD) from Streptomyces olivaceoviridis E-86 at 1.9 Angstrom resolution. The catalytic domain comprises a (beta/alpha)(8)-barrel topologically ident ical to other family 10 xylanases. XBD has three similar subdomains, as sug gested from a triple-repeat sequence, which are assembled against one anoth er around a pseudo-3-fold axis, forming a galactose-binding lectin fold sim ilar to ricin B-chain. The Gly/Pro-rich linker region connecting the cataly tic domain and XBD is not visible in the electron density map, probably bec ause of its flexibility. The interface of the two domains in the crystal is hydrophilic, where five direct hydrogen bonds and water-mediated hydrogen bonds exist. The sugar-binding residues seen in ricin/lactose complex are s patially conserved among the three subdomains in XBD, suggesting that all o f the subdomains in XBD have the capacity to bind sugars. The flexible link er region enables the two domains to move independently and may provide a t riple chance of substrate capturing and catalysis. The structure reported h ere represents an example where the metabolic enzyme uses a ricin-type lect in motif for capturing the insoluble substrate and promoting catalysis. (C) 2000 Academic Press.