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The glycoside hydrolases of this family are endo-β-1,4-xylanases. No other activities have been observed. As a historical note, GH11 was one of the first glycoside hydrolase families classified by sequence analysis, and was previously known as "Cellulase Family G" prior to extensive enzymological characterization [1].
Kinetics and Mechanism
Figure. Xylanase reaction.
Family GH11 xylanases are retaining enzymes, as first shown by NMR [2] and follow a classical Koshland double-displacement mechanism. No detailed analyses involving both steady state and pre-steady state kinetic studies have been reported. However, recent studies of the contributions of each substrate hydroxyl to transition state stabilization, and thus to catalysis, have been reported [3].
Catalytic Residues
The catalytic nucleophile was first identified in the Bacillus circulansendo-xylanase as Glu78 in the sequence ITELD through trapping of the 2-deoxy-2-fluoroxylobiosyl-enzyme intermediate and subsequent peptide mapping via LC-MS/MS technologies [4]. The importance of the precise positioning of the nucleophile was probed by detailed kinetic analysis of mutants modified at that position [5]. The general acid/base residue was first identified as Glu127 in this same enzyme through detailed mechanistic analysis of mutants at that position, which included azide rescue experiments [6]. Indeed detailed analyses of the role of the general acid/base catalyst have been described. NMR measurements of changes in pKa values of enzymic residues during the catalytic cycle reveal pKa cycling [7], while the changes that occur upon repositioning or removing the general acid/base residue reveal its contributions [8].
Three-dimensional structures
Three-dimensional structures are available for several Family GH11 enzymes, the first solved being that of the Bacillus circulans xylanase [9]. As members of Clan GH-C they have a jellyroll fold. NMR assignment has allowed insights into both dynamics and electrostatics within the protein [9, 10, 11, 12].
