A breakthrough in the production of biofuel isobutanol in yeast cells has boosted yields by 260 per cent.
In the search for renewable alternatives to gasoline, heavy alcohols such as isobutanol are promising candidates, as not only do they contain more energy than ethanol, but they are also more compatible with existing gasoline-based infrastructure.
MIT chemical engineers and biologists have now devised a way to dramatically boost isobutanol production in yeast, which naturally make it in small amounts, by engineering yeast so that isobutanol synthesis takes place entirely within mitochondria, cell structures that generate energy and also host many biosynthetic pathways.
Though still short of the scale needed for industrial production, the advance suggests that this is a promising approach to engineering not only isobutanol but other useful chemicals as well, says Gregory Stephanopoulos, an MIT professor of chemical engineering and one of the senior authors of a paper describing the work in online journal Nature Biotechnology.
"It's not specific to isobutanol," Stephanopoulos says. "It's opening up the opportunity to make a lot of biochemicals inside an organelle that may be much better suited for this purpose compared to the cytosol of the yeast cells."
Stephanopoulos collaborated with Gerald Fink, an MIT professor of biology and member of the Whitehead Institute, on this research. The lead author of the paper is Jose Avalos, a postdoc at the Whitehead Institute and MIT.
Historically, researchers have tried to decrease isobutanol production in yeast, because it can ruin the flavour of wine and beer.
“Now there's been a push to try to make it for fuel and other chemical purposes," said Dr Avalos.
Normally production of isobutanol happens in two stages, with precursors created in the mitochondria before being transported out to the cell’s cytoplasm, where they are converted by a set of enzymes into isobutanol.
Other researchers have tried to express all the enzymes needed for isobutanol biosynthesis in the cytoplasm, but it is difficult to get some of those enzymes to function in the cytoplasm as well as they do in the mitochondria.
The MIT researchers took the opposite approach and moved the second phase, which naturally occurs in the cytoplasm, into the mitochondria and this enzyme relocation boosted the production of isobutanol by 260 per cent, and yields of two related alcohols, isopentanol and 2-methyl-1-butanol, went up even more — 370 and 500 per cent, respectively.
The researchers are now trying to further boost isobutanol yields and reduce production of ethanol, which is still the major product of sugar breakdown in yeast.
"Knocking out the ethanol pathway is an important step in making this yeast suitable for production of isobutanol," Stephanopoulos says. "Then we need to introduce isobutanol synthesis, replacing one with the other, to maintain everything balanced within the cell."