Abstract

Cellobiohydrolases (CBHs) are typically major components of natural enzyme cocktails for biomass degradation. Their active sites are enclosed in a tunnel, enabling processive hydrolysis of cellulose chains. Glycoside hydrolase Family 6 (GH6) CBHs act from nonreducing ends by an inverting mechanism and are present in many cellulolytic fungi and bacteria. The bacterial Thermobifida fusca Cel6B (TfuCel6B) exhibits a longer and more enclosed active site tunnel than its fungal counterparts. Here, we determine the structures of two TfuCel6B mutants co-crystallized with cellobiose, D274A (catalytic acid), and the double mutant D226A/S232A, which targets the putative catalytic base and a conserved serine that binds the nucleophilic water. The ligand binding and the structure of the active site are retained when compared with the wild type structure, supporting the hypothesis that these residues are directly involved in catalysis. One structure exhibits crystallographic waters that enable construction of a model of the alpha-anomer product after hydrolysis. Interestingly, the product sites of TfuCel6B are completely enclosed by an "exit loop" not present in fungal GH6 CBHs and by an extended "bottom loop". From the structures, we hypothesize that either of the loops enclosing the product subsites in the TfuCel6B active site tunnel must open substantially for product release. With simulation, we demonstrate that both loops can readily open to allow product release with equal probability in solution or when the enzyme is engaged on cellulose. Overall, this study reveals new structural details of GH6 CBHs likely important for functional differences among enzymes from this important family.

Keywords

Cellulase; Enzyme Structure; Glycoside Hydrolases; Molecular Dynamics; X-ray Crystallography; Thermobifida fusca; Cellobiohydrolase

Published in

Journal of Biological Chemistry
2013, Volume: 288, number: 46, pages: 33107-33117
Publisher: AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC

SLU Authors

• Wu, Miao

  • Department of Molecular Biology, Swedish University of Agricultural Sciences
    

• Sandgren, Mats

  • Department of Molecular Biology, Swedish University of Agricultural Sciences
    

• Ståhlberg, Jerry

  • Department of Molecular Biology, Swedish University of Agricultural Sciences
    

• Hansson, Henrik

  • Department of Molecular Biology, Swedish University of Agricultural Sciences
    

UKÄ Subject classification

Structural Biology
Renewable Bioenergy Research
Biochemistry and Molecular Biology


Publication identifier

DOI: https://doi.org/10.1074/jbc.M113.502765

Permanent link to this page (URI)

https://res.slu.se/id/publ/53161