Researchers uncover structure of enzyme that makes plant cellulose

September 24, 2014  


Nicholas Carpita

Nicholas Carpita (Purdue Agricultural Communication photo/Tom Campbell
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WEST LAFAYETTE, Ind. - Purdue researchers have discovered the structure of the enzyme that makes cellulose, a finding that could lead to easier ways of breaking down plant materials to make biofuels and other products and materials.

The research also provides the most detailed glimpse to date of the complicated process by which cellulose - the foundation of the plant cell wall and the most abundant organic compound on the planet - is produced.

"Despite the abundance of cellulose, the nitty-gritty of how it is made is still a mystery," said Nicholas Carpita, professor of plant biology. "Now we're getting down to the molecular structure of the individual enzyme proteins that synthesize cellulose."

Cellulose is composed of several dozen strands of glucose sugars linked together in a cablelike structure and condensed into a crystal. The rigidity of cellulose allows plants to stand upright and lends wood its strength.

"Pound for pound, cellulose is stronger than steel," Carpita said.

A large protein complex synthesizes cellulose at the surface of the plant cell. The basic unit of this complex is an enzyme known as cellulose synthase. The protein complex contains up to 36 of these enzymes, each of which has a region known as the catalytic domain, the site where single sugars are added to an ever-lengthening strand of glucose that will be fixed in the plant cell wall as one of the strands in the cellulose "cable."

Carpita and a team of researchers used X-ray scattering to show that cellulose synthase is an elongated molecule with two regions - the catalytic domain and a smaller region that couples with another cellulose synthase enzyme to form a dimer, two molecules that are stuck together. These dimers are the fundamental building blocks of the much larger protein complex that produces cellulose.

"Determining the shape of cellulose synthase and how it fits together into the protein complex represents a significant advance in understanding how these plant enzymes work," Carpita said.

The findings could be used to redesign the structure of cellulose for different material applications, he said. For example, cellulose - the base for many textiles such as cotton and rayon - could be modified to better absorb dyes without chemical treatments. The structure of cellulose could also be altered to break down more easily for the production of cellulosic biofuels.

"For decades, we've been doing our best to replace cellulose and other natural products with compounds made from oil," Carpita said. "Plant biologists are now beginning to do the reverse - combining new knowledge from genetics, genomics and biochemistry to make new kinds of natural products to replace those we now make from oil."

Collaborators on the study include Anna Olek of Purdue's Department of Botany and Plant Pathology; Catherine Rayon of the University of Picardie Jules Verne; Lee Makowski of Northeastern University; and Daisuke Kihara of Purdue's Department of Biological Sciences and the Department of Computer Science.

The paper was published in The Plant Cell and is available at http://www.plantcell.org/content/26/7/2996.full?sid=94a79f59-8b41-4160-ae87-fce1c73d5f86.

This work was supported as part of the Center for the Direct Catalytic Conversion of Biomass to Biofuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award DE-SC0000997. Funding was also provided by the National Science Foundation, the National Institutes of Health and the National Research Foundation of Korea.

Writer:  Natalie van Hoose, 765-496-2050, nvanhoos@purdue.edu

Source: Nicholas Carpita, 765-494-4653, carpita@purdue.edu


ABSTRACT

The structure of the catalytic domain of a plant cellulose synthase and its assembly into dimers

Anna T. Olek 1; Catherine Rayon 1*; Lee Makowski 2, 3; Hyung Rae Kim 4; Peter Ciesielski 5; John Badger 6; Lake N. Paul 7; Subhangi Ghosh 4; Daisuke Kihara 4, 8; Michael Crowley 5; Michael E. Himmel 5; Jeffrey T. Bolin 4; Nicholas C. Carpita 1, 4, 7

1 Department of Botany and Plant Pathology, Purdue University, West Lafayette, Indiana 47907-2054

2 Department of Bioengineering, Northeastern University, Boston, Massachusetts 02115

3 Department of Chemistry and Chemical Biology, Northeastern University, Boston, Massachusetts 02115

4 Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907-1971

5 National Renewable Energy Laboratory, Biomolecular Science Group, Golden, Colorado 80401-3305

6 DeltaG Technologies, San Diego, California 92122

7 Bindley Bioscience Center, Purdue University, West Lafayette, Indiana 47907-2057

8 Department of Computer Science, Purdue University, West Lafayette, Indiana 47907-2107

* Current address: EA 3900-BIOPI, University of Picardie Jules Verne, Amiens, France 80039

E-mail: carpita@purdue.edu 

Cellulose microfibrils are para-crystalline arrays of several dozen linear (1→4)-β-D-glucan chains synthesized at the surface of the cell membrane by large, multimeric complexes of synthase proteins. Recombinant catalytic domains of rice (Oryza sativa) CesA8 cellulose synthase form dimers reversibly as the fundamental scaffold units of architecture in the synthase complex. Specificity of binding to UDP and UDP-Glc indicates a properly folded protein, and binding kinetics indicate that each monomer independently synthesizes single glucan chains of cellulose, i.e., two chains per dimer pair. In contrast to structure modeling predictions, solution X-ray scattering studies demonstrate that the monomer is a two-domain, elongated structure, with the smaller domain coupling two monomers into a dimer. The catalytic core of the monomer is accommodated only near its center, with the plant-specific sequences occupying the small domain and an extension distal to the catalytic domain. This configuration is in stark contrast to the domain organization obtained in predicted structures of plant CesA. The arrangement of the catalytic domain within the CesA monomer and dimer provides a foundation for constructing structural models of the synthase complex and defining the relationship between the rosette structure and the cellulose microfibrils they synthesize. 

 

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