Structural characterization of a unique marine animal family 7 cellobiohydrolase suggests a mechanism of cellulase salt tolerance
- aCentre for Novel Agricultural Products, Department of Biology, University of York, York YO10 5DD, United Kingdom;
- bSchool of Biological Sciences, University of Portsmouth, Portsmouth PO1 2DY, United Kingdom;
- cDepartment of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506;
- dBiosciences Center and
- fNational Bioenergy Center, National Renewable Energy Laboratory, Golden, CO 80401; and
- eNovozymes A/S, 2880 Bagsvaerd, Denmark
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Edited by Alexis T. Bell, University of California, Berkeley, CA, and approved May 8, 2013 (received for review January 24, 2013)

Abstract
Nature uses a diversity of glycoside hydrolase (GH) enzymes to convert polysaccharides to sugars. As lignocellulosic biomass deconstruction for biofuel production remains costly, natural GH diversity offers a starting point for developing industrial enzymes, and fungal GH family 7 (GH7) cellobiohydrolases, in particular, provide significant hydrolytic potential in industrial mixtures. Recently, GH7 enzymes have been found in other kingdoms of life besides fungi, including in animals and protists. Here, we describe the in vivo spatial expression distribution, properties, and structure of a unique endogenous GH7 cellulase from an animal, the marine wood borer Limnoria quadripunctata (LqCel7B). RT-quantitative PCR and Western blot studies show that LqCel7B is expressed in the hepatopancreas and secreted into the gut for wood degradation. We produced recombinant LqCel7B, with which we demonstrate that LqCel7B is a cellobiohydrolase and obtained four high-resolution crystal structures. Based on a crystallographic and computational comparison of LqCel7B to the well-characterized Hypocrea jecorina GH7 cellobiohydrolase, LqCel7B exhibits an extended substrate-binding motif at the tunnel entrance, which may aid in substrate acquisition and processivity. Interestingly, LqCel7B exhibits striking surface charges relative to fungal GH7 enzymes, which likely results from evolution in marine environments. We demonstrate that LqCel7B stability and activity remain unchanged, or increase at high salt concentration, and that the L. quadripunctata GH mixture generally contains cellulolytic enzymes with highly acidic surface charge compared with enzymes derived from terrestrial microbes. Overall, this study suggests that marine cellulases offer significant potential for utilization in high-solids industrial biomass conversion processes.
Footnotes
↵1M.K. and J.E.M. contributed equally to this work.
- ↵2To whom correspondence may be addressed. E-mail: simon.mcqueenmason{at}york.ac.uk, gregg.beckham{at}nrel.gov, Simon.Cragg{at}port.ac.uk, or neil.bruce{at}york.ac.uk.
Author contributions: M.K., J.E.M., K.S., G.T.B., N.C.B., and S.J.M.-M. designed research; M.K., J.E.M., S.D.S., R.N.A.M., K.B., L.E., W.E., G.P.M., C.M.P., K.S., and G.T.B. performed research; M.K., J.E.M., S.D.S., C.M.P., M.E.H., G.T.B., S.M.C., N.C.B., and S.J.M.-M. analyzed data; and M.K., J.E.M., C.M.P., G.T.B., S.M.C., N.C.B., and S.J.M.-M. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: The atomic coordinates and structure factors have been deposited in the Protein Data Bank, www.pdb.org (PDB ID codes 4GWA, 4HAP, 4HAQ, and 4IPM) and the sequence reported in this paper has been deposited in the GenBank database (accession no. KC776193).
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1301502110/-/DCSupplemental.