Ståhlberg, Jerry
- Department of Molecular Biology, Swedish University of Agricultural Sciences
Research article2001Peer reviewedOpen access
Engineering of a glycosidase Family 7 cellobiohydrolase to more alkaline pH optimum: the pH behaviour of Trichoderma reesei CeI7A and its E223S/A224H/L225V/T226A/D262G mutant
Becker, Dieter; Braet, Christophe; Brumer III, Harry; Claeyssens, Marc; Divne, Christina; Fagerström, B. Richard; Harris, Mark; Jones, T. Alwyn; Kleywegt, Gerard J.; Koivula, Anu; Mahdi, Sabah; Piens, Kathleen; Sinnott, Michael L.; Ståhlberg, Jerry; Teeri, Tuula T.; Underwood, Melanie; Wohlfahrt, Gerd
Abstract
The crystal structures of Family 7 glycohydrolases suggest that a histidine residue near the acid/base catalyst could account for the higher pH optimum of the Humicola insolens endoglucanase Cel7B, than the corresponding Trichoderma reesei enzymes. Modelling studies indicated that introduction of histidine at the homologous position in T. reesei Cel7A (Ala(224)) required additional changes to accommodate the bulkier histidine side chain. X-ray crystallography of the catalytic domain of the E223S/A224H/L225V/T226A/D262G mutant reveals that major differences from the wild-type are confined to the mutations themselves, The introduced histidine residue is in plane with its counterpart in H. insolens Cel7B, but is 1.0 Angstrom (= 0.1 nm) closer to the acid/base Glu(217) residue, with a 3.1 Angstrom contact between N-2 and O'(1). The pH variation of k(cat)/K-m for 3,4-dinitrophenyl lactoside hydrolysis was accurately bell-shaped for both wildtype and mutant, with pK(1) shifting from 2.22+/-0.03 in the wild-type to 3.19+/-0.03 in the mutant, and pK(2) shifting from 5.99+/-0.02 to 6.78+/-0.02. With this poor substrate, the ionizations probably represent those of the free enzyme. The relative k(cat) for 2-chloro-4-nitrophenyl lactoside showed similar behaviour. The shift in the mutant pH optimum was associated with lower k(cat)/K-m values for both lactosides and cellobiosides, and a marginally lower stability. However, k(cat) values for cellobiosides are higher for the mutant. This we attribute to reduced nonproductive binding in the +1 and +2 subsites; inhibition by cellobiose is certainly relieved in the mutant. The weaker binding of cellobiose is due to the loss of two water-mediated hydrogen bonds.
Keywords
cellulase; cellulose; endoglucanase; enzyme kinetics; pH-dependence
Published in
Biochemical Journal
2001, Volume: 356, article number: biochemical journal
SLU Authors
UKÄ Subject classification
Structural Biology
Biochemistry and Molecular Biology
Renewable Bioenergy Research
Publication identifier
DOI: https://doi.org/10.1042/0264-6021:3560019
Permanent link to this page (URI)
https://res.slu.se/id/publ/60396