Multiple conformations of catalytic serine and histidine in acetylxylan esterase at 0.90 angstrom

Acetylxylan esterase (AXEII; 207 amino acids) from Penicillium purpurogenum has substrate specificities toward acetate esters of D-xylopyranose residu es in xylan and belongs to a new class of alpha/beta hydrolases. The crysta l structure of AXEII has been determined by single isomorphous replacement and anomalous scattering, and refined at 0.90- and 1.10-Angstrom resolution s with data collected at 85 K and 295 K, respectively. The tertiary structu re consists of a doubly wound alpha/beta sandwich, having a central six-str anded parallel beta -sheet flanked by two parallel ol-helices on each side. The catalytic residues Ser(90), His(187), and Ap(175) are located at the C -terminal end of the sheet, an exposed region of the molecule. The serine a nd histidine side chains in the 295 K structure show the frequently observe d conformations in which Ser(90) is trans and the hydroxyl group is in the plane of the imidazole ring of His(187), However, the structure at 85 K dis plays an additional conformation in which Ser(90) side-chain hydroxyl is aw ay from the plane of the imidazole ring of His(187). The His(187) side chai n forms a hydrogen bond with a sulfate ion and adopts an altered conformati on. The only other known hydrolase that has a similar tertiary structure is Fusarium solani cutinase, The exposed nature of the catalytic triad sugges ts that AXEII is a pure esterase, i.e. an alpha/beta hydrolase with specifi city for nonlipidic polar substrates.