First Crystallographic Structure of a Xylanase from Glycoside Hydrolase Family 5: Implications for Catalysis,

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Abstract
The room-temperature structure of xylanase (EC 3.2.1.8) from the bacterial plant pathogen Erwinia chrysanthemi expressed in Escherichia coli, a 45 kDa, 413-amino acid protein belonging to glycoside hydrolase family 5, has been determined by multiple isomorphous replacement and refined to a resolution of 1.42 Å. This represents the first structure of a xylanase not belonging to either glycoside hydrolase family 10 or family 11. The enzyme is composed of two domains similar to most family 10 xylanases and the α-amylases. The catalytic domain (residues 46−315) has a (β/α)8-barrel motif with a binding cleft along the C-terminal side of the β-barrel. The catalytic residues, Glu165 and Glu253, determined by correspondence to other family 5 and family 10 glycoside hydrolases, lie inside this cleft on the C-terminal ends of β-strands 4 and 7, respectively, with an Oε2···Oε1 distance of 4.22 Å. The smaller domain (residues 31−43 and 323−413) has a β9-barrel motif with five of the strands interfacing with α-helices 7 and 8 of the catalytic domain. The first 13 N-terminal residues form one β-strand of this domain. Residues 44, 45, and 316−322 form the linkers between this domain and the catalytic domain.

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