Bacteria for biofuel
A new study by Prof. Ed Bayer offers promise for alternative energy
Decomposition and growth are like yin and yang, forming two halves of the closed-loop cycle of natural ecosystems. Within the plant kingdom, decomposition pays off, by freeing energy needed for future plant growth.
But according to Prof. Ed Bayer, plant decomposition has the potential to deliver big dividends to humans as well.
“The materials contained in plant cell wall comprise the most abundant source of renewable energy on Earth,” says Prof. Bayer, of the Weizmann Institute’s Department of Biomolecular Sciences. “The challenge is to free this energy efficiently, and harness it for human use.”
The most important structural material in plant tissues is called cellulose, a sugar-based polymer that makes up 40 percent of the cell wall. Long targeted by scientists seeking a way to unlock sequestered sugars so they can be fermented into biofuel - the agricultural equivalent of spinning straw into gold - cellulose is notoriously tough to break down.
“If you imagine the cell wall of plants as poured concrete, cellulose would be the steel rods holding the structure together,” Prof. Bayer says, adding that, because of cellulose’s native resilience, industrial degradation protocols require high-energy processes that are both polluting and prohibitively expensive. ”Luckily, Mother Nature provided us with a bacterium–called Clostridium thermocellum–that breaks down cellulose efficiently. Back in the early 80’s, when I was still working on my postdoc, I decided to examine how this bacteria-based degradation proceeds on the molecular level.”
This investigation–conducted with Raphael Lamed, who later became a professor at Tel Aviv University–resulted in the discovery of the cellulosome, an intricate multi-layer enzyme-driven “machine” located on the bacterial surface that degrades cellulose. By the mid-90's, the scientists had revealed what Prof. Bayer refers to as “nature at play”: the cellulosome’s LEGO-like structure, in which protein complexes anchor to the plant surface, enabling delivery of degradation-triggering enzymes. But while these early studies produced an unprecedentedly detailed picture of how bacteria mediate the “liberation” of sequestered sugars, Prof. Bayer says that, in a world seeking more and better biofuel, characterizing the status quo was simply not enough.
The goal was to turn this natural process into something that could generate sufficient “raw material” for biofuel. “The cellulosome-mediated process we discovered releases sugar, but not enough to make a difference to any biofuel-based energy supply,” he says. “Our ultimate aim is to create engineered cellulosomes capable of releasing industrial quantities of sugar for biofuel, as part of a cost-effective procedure that does not pollute the environment.”
Striding toward biofuel
In a recent publication in Proceedings of the National Academy of Sciences, Prof. Bayer and his team reported on a key step in this direction. They engineered a "designer" cellulosome doctored with an enzyme capable of breaking down lignin–a material in the cell wall that helps organic material put up a stiff fight against degradation and decay.
“We took a lignin-degrading enzyme–isolated from a different class of bacteria–and inserted it into our experimental system,” he says. “Not only did this result in the first-ever cellulosome capable of degrading lignin, the introduction of this foreign enzyme had another, unexpected result: it made the breakdown of cellulose and other cell-surface polysaccharides significantly more efficient. In effect, we engineered a ‘super-charged’ molecular machine on the bacterial surface, capable of freeing a larger-than-normal amount of useful sugar from biomass. This is significant because it may eventually allow us to break through the bottleneck that prevents cost-effective energy production based on biofuel.”
Prof. Bayer will continue to investigate novel enzymes and organisms, in hopes of achieving even more efficient degradation results. But while he hopes to see biofuel take over a greater percentage of the world’s overall energy supplies, he admits there is still a long way to go.
“Advances in biofuel science don’t threaten the petroleum lobby just yet,” he says. “But since our oil and gas supplies won’t last forever, we have to think ahead."
Prof. Ed Bayer is supported by the Dana and Yossie Hollander Center for Structural Proteomics; the Leona M. and Harry B. Helmsley Charitable Trust; and the Jewish Community Endowment Fund. Prof. Bayer is the incumbent of the Maynard I.and Elaine Wishner Professorial Chair of Bio-Organic Chemistry.
Prof. Ed Bayer