Exploring the worldÕs most abundant material at a molecular level
by Stacie Nishimoto
ItÕs 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. 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 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 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 plantÕs 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 SomervilleÕs 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 observed cellulose synthase molecules moving in linear tracks on either side of the microtubules.
In a separate experiment, researchers were able to influence microtubule alignment by shining blue light on one side of the plant. Due to an effect known as phototropism, the light caused the microtubules to align themselves horizontally rather than vertically. Interestingly, the tagged cellulose synthase molecules continued to move in paths defined by the microtubules despite their new arrangement.
To further establish this relationship, Professor SomervilleÕs 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.
Cellulose as Fuel
Perhaps the most exciting potential application of SomervilleÕs 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 goal, Somerville explained that cellulose, with its highly-bonded 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: ÒStarch is also a polymer of glucose, but itÕs an alpha-linked polymer [as opposed to beta-linked cellulose]. And of course, weÕre extremely efficient at breaking down starch. ThatÕs why weÕre able to live on bread. 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."
Somerville added that some of his students are pursuing questions related to structural manipulation of plant cells in an attempt to solve this problem. ÒInstead 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,Ó he explained. Other research topics being explored in the Somerville lab include analysis of cellulose synthase protein structure to figure out why the enzyme only binds to glucose. ÒIf 2% of the time cellulose synthase added galactose instead of glucose,Ó Somerville suggests, Òthe strands would have irregularities in them and they would be much easier to break down enzymatically.Ó
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. ÒIn some way,Ó added Somerville, ÒI think thatÕs what research at a great university is supposed to be about: solving the fundamental, core problems.Ó Current investigations will provide the basis for future engineering of cellulose applications. Somerville's research on how cellulose is made has immense potential, and weÕve only scratched the surface.
Side Notes:
Comparing Cellulose and Starch:
Cellulose: Glucose monomers are linked together to form cellulose via beta linkage. As a result of the bond angles in the beta linkage, cellulose is mostly a linear chain.
Starch: Glucose monomers are linked together to form cellulose via alpha linkage. As a result of the bond angles in the alpha linkage, starch forms a coiled spring-like spiral.