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The majority of the glycoside hydrolases from this family hydrolyze the glycosidic bond between L-arabinofuranosides side chains of hemicelluloses such as arabinoxylan, arabinogalactan, and L-arabinan. A few enzymes of the family exhibit β-1,4-endoglucanase activity towards carboxymethyl cellulose and xylan [1].
Kinetics and Mechanism
GH51 L-arabinfuranosidases are retaining enzymes and follow a classical Koshland retaining mechanism. Owing to the fast mutarotation and tautomerization rates of arabinose, the stereochemical course of the reaction was monitored in presence of methanol and followed by NMR spectroscopy [2, 3, 4]. Enzymes that have been well studied kinetically include the Geobacillus stearothermophilus T-6 and Thermobacillus xylanilyticus α-L-arabinofuranosidases, for which a detailed kinetic study was performed including kinetics with aryl-α-L-arabinofuranosides bearing various leaving groups, Brønsted plots for the E175A acid-base catalytic residue and azide-rescue for the E294A nucleophilc mutant [3, 4, 5].
Catalytic Residues
The general acid/base was first identified in Thermobacillus xylanilyticus (Glu176) [3] and in Geobacillus stearothermophilus T-6 (Glu175) α-arabinofuranosidases [4] using kinetic analysis, pH dependence profiles, and azide rescue of the catalytic mutant. The catalytic nucleophile was first identified in Geobacillus stearothermophilus α-arabinofuranosidase through detailed kinetic studies for the catalytic mutant including azide rescue.
Three-dimensional structures
Three-dimensional structures for GH51 arabinofuranosidases are available for Geobacillus stearothermophilus [6] Clostridium thermocellum [7] and Thermobacillus xylanilyticus [8]. The enzyme in solution is a hexamer (can be described as a trimer of dimmers) and each monomer is organized into two domains: a ‘clan GH-A’ catalytic (β/α)8 domain and a 12-stranded β sandwich with a jelly-roll topology.
Family Firsts
First sterochemistry determination
Aspergillus niger and Aspergillus aculeatus α-L-arabinfuranosidases carried out in the presence of 2.5 M methanol and followed by 1H-NMR spectroscopy [2].
Thermobacillus xylanilyticus and Geobacillus stearothermophilus T-6 α-L-arabinofuranosidases via detailed kinetic studies for the catalytic mutant including azide rescue [3, 4].
