Designer Yeast

September 12, 2010

Probably the oldest chemical synthesis known to humans is the production of ethanol (ethyl alcohol) by fermentation.  Yeast acts in a solution of simple sugars, typically glucose, and uses the sugar as food, producing ethanol as a by-product.  The traditional process uses an agricultural product, such as grape juice or malted barley, to provide the sugars, and produces wine or beer as its end product.   More recently, there has been interest in using ethanol as an energy source, to replace, at least in part, fossil fuels.

A problem with the traditional methods of production is that they are not very efficient, in total energy terms.  Because the yeast can only work with simple sugars, the most straightforward approach uses food crops as a source: not necessarily grapes, but things like corn or sugar cane.  There is considerable interest in using other sources of plant material, such as corn husks and stalks, or wood chips.  These materials, and many others, contain a great deal of glucose.  Unfortunately, it is primarily locked up in the form of cellulose, a polysaccharide (sugar polymer) consisting of long, straight chains of glucose molecules, ranging in size from a few hundred to about 10,000 sugar units.  There is plenty of it around — it’s probably the most common organic compound on Earth — but, unfortunately, yeast cannot get at the glucose when it’s locked up in the cellulose structure.  Animals, like us, generally have the same problem.  We can’t really digest cellulose (it’s what your grandma called “roughage”); some ruminants (such as cattle) can, because they have symbiotic bacteria in their digestive systems that produce an enzyme (cellulase) that can break the cellulose down.

Currently, ethanol can be made from cellulose by the addition of a cocktail of enzymes collected from other organisms.  According to reports at Technology Review and Ars Technica, a group of scientists, led by Jamie Cate, a biologist at the University of California, Berkeley, has managed to transfer genes from a common fungus into yeast.  (Their paper was published in Science; the abstract is here.)   The fungus, Neospora crassa, which grows naturally on dead plant material, produces an enzyme that helps break down cellulose, and two proteins that help transport it into cells for digestion.  A preliminary step is still needed, using other enzymes to break the cellulose molecules into shorter chains, called cellodextrins, but the genetically modified yeast can be added to complete the ethanol production process.

This is, at present, still preliminary work.  There is a good deal to be done to translate into even a pilot production facility for demonstration purposes.  Nonetheless, it does show promise; if it can be made to work on a large scale, it could make a new energy source from what is, right now, a big pile of agricultural rubbish.

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