Multiple conformations of catalytic serine and histidine in acetylxylan esterase at 0.90 A.
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 alpha/beta hydrolases. The crystal structure of AXEII has been determined by single isomorphous replacement and anomalous scattering, and refined at 0.90- and 1.10-A resolutions with data collected at 85 K and 295 K, respectively. The tertiary structure consists of a doubly wound alpha/beta sandwich, having a central six-stranded parallel beta-sheet flanked by two parallel alpha-helices on each side. The catalytic residues Ser(90), His(187), and Asp(175) are 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 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 displays an additional conformation in which Ser(90) side-chain hydroxyl is away from the plane of the imidazole ring of His(187). The His(187) 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 alpha/beta hydrolase with specificity for nonlipidic polar substrates.
Ghosh
D
D
Hauptman-Woodward Medical Research Institute, Buffalo, New York 14203, USA. ghosh@hwi.buffalo.edu
Sawicki
M
M
Lala
P
P
Erman
M
M
Pangborn
W
W
Eyzaguirre
J
J
Gutierrez
R
R
Jornvall
H
H
Thiel
D J
DJ
D000115:Acetylesterase / Q000737:chemistry* / Q000378:metabolism; D002384:Catalysis; D005656:Fungal Proteins / Q000737:chemistry*; D006639:Histidine; D010407:Penicillium / Q000737:chemistry* / Q000201:enzymology; D011487:Protein Conformation; D012694:Serine; D013329:Structure-Activity Relationship
D005656:Fungal Proteins; D012694:Serine; D006639:Histidine; D000115:Acetylesterase; C081851:acetylxylan esterase