Identification of the acid/base catalyst of a glycoside hydrolase family 3 (GH3) beta-glucosidase from Aspergillus niger ASKU28
2013 (English)In: Biochimica et Biophysica Acta - General Subjects, ISSN 0304-4165, E-ISSN 1872-8006, Vol. 1830, no 3, 2739-2749 p.Article in journal (Refereed) Published
Background: The commercially important glycoside hydrolase family 3 (GH3) beta-glucosidases from Aspergillus niger are anomeric-configuration-retaining enzymes that operate through the canonical double-displacement glycosidase mechanism. Whereas the catalytic nucleophile is readily identified across all GH3 members by sequence alignments, the acid/base catalyst in this family is phylogenetically variable and less readily divined. Methods: In this report, we employed three-dimensional structure homology modeling and detailed kinetic analysis of site-directed mutants to identify the catalytic acid/base of a GH3 beta-glucosidase from A. niger ASKU28. Results: In comparison to the wild-type enzyme and other mutants, the E490A variant exhibited greatly reduced k(cat) and k(cat)/K-m values toward the natural substrate cellobiose (67,000- and 61,000-fold, respectively). Correspondingly smaller kinetic effects were observed for artificial chromogenic substrates p-nitrophenyl beta-D-glucoside and 2,4-dinitrophenyl beta-D-glucoside, the aglycone leaving groups of which are less dependent on add catalysis, although changes in the rate-determining catalytic step were revealed for both, pH-rate profile analyses also implicated E490 as the general acid/base catalyst. Addition of azide as an exogenous nucleophile partially rescued the activity of the E490A variant with the aryl beta-glucosides and yielded beta-glucosyl azide as a product. Conclusions and general significance: These results strongly support the assignment of E490 as the acid/base catalyst in a beta-glucosidase from A. niger ASKU28, and provide crucial experimental support for the bioinformatic identification of the homologous residue in a range of related GH3 subfamily members.
Place, publisher, year, edition, pages
2013. Vol. 1830, no 3, 2739-2749 p.
beta-Glucosidase, Acid/base catalyst, Aspergillus niger, Glycoside hydrolase family 3, Azide rescue
Biochemistry and Molecular Biology
IdentifiersURN: urn:nbn:se:kth:diva-119724DOI: 10.1016/j.bbagen.2012.11.014ISI: 000315307900026ScopusID: 2-s2.0-84873577162OAI: oai:DiVA.org:kth-119724DiVA: diva2:612778
FunderFormasKnut and Alice Wallenberg Foundation
QC 201303252013-03-252013-03-212013-03-25Bibliographically approved