Ssm Winter 2007 Engtech Cellulose

engineering + technology

Cellulose Research Paves the Way for New Biofuels Exploring the world’s most abundant material at a molecular level by Stacie Nishimoto Photo Credit: ©sxc.hu/John Evans

I

t’s remarkable how little we know about the most abundant organic material on the planet. Cellulose is a major component of the clothing we wear, the paper we use, and the plants in our environment, yet many of its molecular properties remain a mystery. Dr. Christopher Somerville, Professor of Biological Sciences and Director of the Carnegie Institute of Plant Biology at Stanford, is leading a team of researchers to determine the arrangement of cellulose within plant cells. His findings may pave the way to realizing the potential applications of cellulose, including its use as an alternative energy source.

Cellulose Structure The basic structure of cellulose consists of simple glucose molecules that polymerize to form glucans, strings of glucose thousands of molecules long. These glucan fibers fuse through extensive hydrogen bonding to form fibrils that wind around a plant cell, giving the plant its tensile strength.

If cellulose can be broken down into its constituent sugars, it can be converted into ethanol for energy. The enzyme responsible for cellulose construction is cellulose synthase, which catalyzes the formation of polymers from sugar subunits. In studies of other plant enzymes, plant material is ground up and Photo Credit: Professor Somerville the enzyme extracted for investigation. However, this method does not work with cellulose synthase for unknown reasons. Extracts of plants exhibit little or no cellulose synthase enzyme activity. Tracking Cellulose Synthesis Somerville and his research team have been working to unravel the secrets of cellulose

50 stanford scientific

synthase using live cell confocal microscopy. This technique allows his team to view individual cellulose synthase molecules as they move along microtubules. Microtubules, components of a plant’s molecular cytoskeleton, also play a large role in plant wall strength. Forty years ago, Stanford scientist Paul Green suggested that microtubules and cellulose structure were somehow linked, though at the time the details of this association were unclear. In Somerville’s recent investigation, his group tagged cellulose synthase molecules with YFP, a yellow fluorescent marker, and tagged microtubules with CFP, a blue fluorescent marker. By analyzing time lapse videos of the experiment, the team observedsynthase molecules moving in linear tracks on either side of the microtubules. In a separate experiment, Comparing researchers were able to Cellulose and Starch influence microtubule Cellulose: Glucose monomers alignment by shining are linked together to form blue light on one side of cellulose via beta linkage. As the plant. Due to an effect a result of the bond angles in known as phototropism, the beta linkage, cellulose is the light caused the mostly a linear chain. microtubules to align themselves horizontally Starch: Glucose monomers rather than vertically. are linked together to form Interestingly, the tagged cellulose via alpha linkage. cellulose synthase As a result of the bond molecules continued to angles in the alpha linkage, move in paths defined by starch forms a coiled springthe microtubules despite like spiral. their new arrangement.

Cellulose synthase molecules are tagged with green fluorescent marker (left). Microtubules are tagged with red fluorescent marker (middle). Combined images (right), showing significant correlation between cellulose synthase movement and microtubule alignment.

Cellulose as Fuel Perhaps the most exciting potential application of Somerville’s research is the goal of using cellulose as an alternative energy source. If cellulose can be broken down into its constituent sugars, it can be converted into ethanol for energy. When asked about the hurdles to achieving this

“To really be successful at making biofuels, we need to convert cellulose into free sugars with similar efficiency to our ability to convert starch to free sugar.” - Sommerville goal, Somerville explained that cellulose, with its highlybonded fibers of glucans, is nearly crystalline, and therefore resistant to efficient breakdown by all known enzymes. To give a clearer picture of what cellulose-driven biofuels require, Somerville drew an analogy to starch, the most abundant compound in our diets: “Starch is also a polymer of glucose, but it’s an alpha-linked polymer [as opposed to beta-linked cellulose]. And of course, we’re extremely efficient at breaking down starch. That’s why we’re able to live on bread. To really be successful at making biofuels, we need to convert cellulose Photo Credit: Professor Somerville; R. Atalla , unpublished) into free sugars with similar efficiency to our ability to convert starch to free sugar.” Somerville added that some of his students are pursuing questions related to structural manipulation of plant cells in an attempt to solve Thirty-six strands of glucans make up the packing structure of cellulose. this problem. “Instead of having 36 strands in a cellulose fibril, if there were 18 maybe it would be good enough to provide strength for the plant, and certainly a lot easier to break down,” he explained. Other research topics

layout design: Jessica Chia-Rong Lee

engineering + technology

Poto Credit: Professor Somerville

To furthur establish this relationship, Professor Somerville’s team added oryzalin, a chemical known to disrupt microtubule growth, and saw a corresponding disorientation of the cellulose synthase. The enzyme tracks were chaotic, making weaving paths rather than the linear trajectories they assumed when microtubules were present. Unlike conventional biochemical research, where the measurement a scientist takes is an average of millions of individual reactions, this new insight into cellulose has the ability to answer questions about how specific chemicals affect an enzyme’s lifetime or rate of movement. These studies are providing scientists with information on how to manipulate cellulose for useful applications.

No Oryzalin

10uM Oryzalin 45min

Imaging over time, identifying individual cellulose synthase movement in plant cells. Normal microtubule growth (left) with linear cellulose synthase trajectories. When microtubule alignment is affected by oryzalin (right), cellulose synthase molecules exhibit disoriented behavior.

being explored in the Somerville lab include analysis of cellulose synthase protein structure to figure out why the enzyme only binds to glucose. “If 2% of the time cellulose synthase added galactose instead of glucose,” Somerville suggests, “the strands would have irregularities in them and they would be much easier to break down enzymatically.”

Research for the Future Although his team is interested in alternative energy, Somerville emphasizes that they are also seeking answers to important mechanistic questions in molecular biology. “In some way,” added Somerville, “I think that’s what research at a great university is supposed to be about: solving the fundamental, core problems.” Current investigations will provide the basis for future engineering of cellulose applications. Somerville’s research on how cellulose is made has immense potential, and we’ve only scratched the surface. S STACIE NISHIMOTO is a sophomore considering a major in Biological Sciences, with a minor in Physics or English. In addition to science writing, she enjoys belting out songs from musicals in the shower, snorkeling, squishing her toes in the sand, and post-its.

To Learn More Visit the departmental website of Dr. Christopher Somerville, http://www-ciwdpb.stanford.edu/research/research_ csomerville.php Read Dynamic Visualization of Cellulose Synthase Demonstrates Functional Association with Cortical Microtubules. Science, in press [Paredez, A., Somerville, C.R., Ehrhardt, D. (2006)]

Somerville Team; (front and center) Professor Somerville

Photo Credit: Professor Somerville

volume v

51