Multiple Conformations of Catalytic Serine and Histidine in Acetylxylan Esterase at 0.90 Å*

Acetylxylan esterase (AXEII; 207 amino acids) from Penicillium purpurogenum has substrate specificities toward acetate esters of d-xylopyranose residues in xylan and belongs to a new class of α/β hydrolases. The crystal structure of AXEII has been determined by single isomorphous replacement and anomalous scattering, and refined at 0.90- and 1.10-Å resolutions with data collected at 85 K and 295 K, respectively. The tertiary structure consists of a doubly wound α/β sandwich, having a central six-stranded parallel β-sheet flanked by two parallel α-helices on each side. The catalytic residues Ser90, His187, and Asp175are located at the C-terminal end of the sheet, an exposed region of the molecule. The serine and histidine side chains in the 295 K structure show the frequently observed conformations in which Ser90 is trans and the hydroxyl group is in the plane of the imidazole ring of His187. However, the structure at 85 K displays an additional conformation in which Ser90 side-chain hydroxyl is away from the plane of the imidazole ring of His187. The His187 side chain forms a hydrogen bond with a sulfate ion and adopts an altered conformation. The only other known hydrolase that has a similar tertiary structure is Fusarium solani cutinase. The exposed nature of the catalytic triad suggests that AXEII is a pure esterase,i.e. an α/β hydrolase with specificity for nonlipidic polar substrates.