Research looks at how cellulose is produced

September 25, 2014, West Lafayette, Ind. – Researchers at
Purdue University 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 www.plantcell.org/content/26/7/2996.full?sid=94a79f59-8b41-4160-ae87-fce1c73d5f86.

Funding for the research was provided by the Center for the
Direct Catalytic Conversion of Biomass to Biofuels, an Energy Frontier Research
Center based at Purdue's Discovery Park; the National Science Foundation; the
National Institutes of Health; and the National Research Foundation of Korea.