Three-dimensional structure of the catalytic core of acetylxylan esterase from Trichoderma reesei: Insights into the deacetylation mechanism

Acetylxylan esterase from Trichoderma reesei removes acetyl side groups fro m xylan. The crystal structure of the catalytic core of the enzyme was solv ed at 1.9 Angstrom resolution. The core has an alpha/beta/alpha sandwich fo ld, similar to that of homologous acetylxylan esterase from Penicillium pur purogenum and cutinase from Fusarium solani. All three enzymes belong to fa mily 5 of the carbohydrate esterases and the superfamily of the alpha/beta hydrolase fold. Evidently, the enzymes have diverged from a common ancestor and they share the same catalytic mechanism. The catalytic machinery of ac etylxylan esterase from T. reesei was studied by comparison with cutinase, the catalytic site of which is well known. Acetylxylan esterase is a pure s erine esterase having a catalytic triad (Ser90, His187, and Asp175) and an oxyanion hole (Thr13 N, and Thr13 O gamma). Although the catalytic triad of acetylxylan esterase has been reported previously, there has been no menti on of the oxyanion hole. A model for the binding of substrates is presented oil the basis of the docking of xylose. Acetylxylan esterase from T. reese i is able to deacetylate both mono- and double-acetylated residues, but it is not able to remove acetyl groups located close to large side groups such as 4-O-methylglucuronic acid. If the xylopyranoside residue is double-acet ylated, both acetyl groups are removed by the catalytic triad: first one ac etyl group is removed and then the residue is reorientated so that the nucl eophilic oxygen of serine can attack the second acetyl group. (C) 2000 Acad emic Press.