Bringing Down Walls


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Cellulose fibers in the plant cell wall magnified 50,000 times by an atomic force microscope
Plant biomass can be converted into green fuel using clean, sustainable technologies, but the major challenge involved in producing biofuels from biomass on a large scale is the high cost. Visualizing the minutest details of biomass conversion down to the atomic level can help optimize the process, according to a study performed by researchers from the Weizmann Institute of Science and the National Renewable Energy Laboratory (NREL) in the United States.

The study, reported in Science, has confirmed the basic idea that one of the major obstacles to effective biomass conversion is lignin, the sturdy polymer that gives plants their strength. In fact, to facilitate the conversion of  the  polysaccharides – the complex sugar compounds in the plant cell wall – into the biofuel ethanol, the process usually starts with pretreatment in which the lignin gets mechanically chopped up or chemically destroyed. In the new study, the researchers devised innovative methods that for the first time made it possible to observe lignin destruction at the molecular and atomic resolution, using advanced laser microscopy. The images have revealed that lignin actually interferes with the ability of enzymes to digest polysaccharides. This understanding has enabled the scientists to define the parameters of an ideal pretreatment – one that focuses on removing the lignin without damaging the polysaccharides.
The team further compared two different methods for digesting the polysaccharides. One relied on individual enzymes derived from fungi; the other made use of cellulosomes – natural molecular complexes of several enzymes by means of which bacteria, fungi and other microorganisms degrade plant cellulose. The scientists discovered that the loose enzymes penetrated deeper inside plant cells than the cellulosomes, which acted mainly on the plant cell surface. This new understanding of the mechanisms involved in the deconstruction of cellulosic biomass may help scientists design more effective enzyme systems for bringing down plant cell walls.
Prof. Ed Bayer
Cellulosomes were discovered some three decades ago by Prof. Ed Bayer of the Weizmann Institute’s Biological Chemistry Department, together with his colleague Prof. Raphael Lamed of Tel Aviv University. In the current study, Bayer collaborated with researchers from the NREL, including Dr. Shi-You Ding, who had conducted postdoctoral research in Bayer’s laboratory in the late 1990s. Dr. Ding and Dr. Yu-San Liu of the NREL devised methods for visualizing the effects of different chemicals on the plant cell wall at an enormous range of resolutions, spanning a million-fold difference: from a millimeter to a nanometer (one millionth of a millimeter). Other members of the NREL team included Drs. Michael Himmel, Yining Zeng and John Baker.
The study’s findings can help researchers optimize biomass pretreatment and the activity of the enzymes used to degrade the biomass. Such improvements, in turn, could increase ethanol yields, lowering the cost of making biofuels.
A historic aside: The cellulosome in the study was derived from the microorganism Clostridium thermocellum, which belongs to the genus of bacteria historically linked to the Weizmann Institute and the State of Israel. Dr. Chaim Weizmann, the Institute’s founder and first President of Israel, had used a bacterium of the same genus, Clostridium acetobutylicum, now often called the Weizmann organism, to produce acetone during World War I. Following in Dr. Weizmann’s footsteps, Weizmann Institute researchers have recently established yet another tie between past and present science: Genetic studies have demonstrated that Weizmann’s bacterium, somewhat surprisingly, produces a cellulosome of its own.
Prof. Ed Bayer’s research is supported by the Leona M. and Harry B. Helmsley Charitable Trust; and the Brazilian Friends of the Weizmann Institute of Science. Prof. Bayer is the incumbent of the Maynard I. and Elaine Wishner Professorial Chair of Bio-Organic Chemistry.
Bright-field light microscopy images of plant cell walls, stripped of lignin, degraded by cellulosomes