A big part of the problem with yeasts altered to take up xylose is that they will suck up all the glucose in a mixture before they will touch the xylose, Jin said. A glucose transporter on the surface of the yeast prefers to bind to glucose.
"It's like giving meat and broccoli to my kids," he said. "They usually eat the meat first and the broccoli later."
The yeast's extremely slow metabolism of xylose also adds significantly to the cost of biofuels production.
Jin and his colleagues wanted to induce the yeast to quickly and efficiently consume both types of sugar at once, a process called co-fermentation. The research effort involved researchers from Illinois, the Lawrence Berkeley National Laboratory, the University of California at Berkeley, Seoul National University and BP.
In a painstaking process of adjustments to the original yeast, Jin and his colleagues converted it to one that will consume both types of sugar faster and more efficiently than any strain currently in use in the biofuel industry. In fact, the new yeast strain simultaneously converts cellobiose (a precursor of glucose) and xylose to ethanol just as quickly as it can ferment either sugar alone.
"If you do the fermentation by using only cellobiose or xylose, it takes 48 hours," said postdoctoral researcher and lead author Suk-Jin Ha. "But if you do the co-fermentation with the cellobiose and xylose, double the amount of sugar is consumed in the same amount of time and produces more than double the amount of ethanol. It's a huge synergistic effect of co-fermentation."
The new yeast strain is at least 20 percent more efficient at converting xylose to ethanol than other strains, making it "the best xylose-fermenting strain" reported in any study, Jin said.
The team achieved these outcomes by making several critical changes to the organism. First, they gave the yeast a cellobiose transport
|Contact: Diana Yates|
University of Illinois at Urbana-Champaign