From French bread to non-fossil fuel?

Yeast can ferment corn starch into ethanol to be added to gasoline, but that diverts millions of tons of food from hungry people. Researchers are trying to ferment many other plant carbohydrates, especially cellulose, the tough chain-like molecule that stiffens the cell wall so plants can stand by themselves.

Unfortunately, the yeasts used to make ethanol have no taste for cellulose.

In this week’s Science, Jamie Cate, in the department of molecular and cell biology at the University of California at Berkeley, reports a transfer of two genes from a fungus to ethanol-making yeast. Although the fungus was discovered on French bread in the 1840s, the result was not exactly a fine Burgundy, or even a gallon of cheap jug wine, but it was a proof of principle that a single organism could, almost single-yeastedly, convert cellulose into ethanol.

Mon dieu!

The advance may hasten the day when waste wood, crop residues and fast-growing crops such as switchgrass can replace edible crops like corn and sugar cane in producing fuel.

Raise a glass to success!

“It’s a proof of principle using lab strains,” says Cate.

The genetic transfer enabled a single strain of yeast to convert cellulose in plant cell walls into ethanol. After commercial enzymes busted the cellulose into short chains of glucose units, the yeast:

The short chains of glucose that the Neurospora crassa fungus extracts from cellulose do not normally enter the yeast cell, but the transporters ensure that they will enter the transformed yeast, enabling the yeast to make ethanol from normally indigestible compounds.

Taking lessons from fungi

The research began with a basic question. A large portion of plant biomass is cellulose, and “microorganisms in the wild live on plants; they obviously have figured out how to degrade plants as food,” says Cate. “Plants have been figuring out ways to prevent microbes from doing this, so there’s this ongoing battle, and we knew some fungi would be very good at decomposing cellulose.”

Cate focused on N. crassa, a well-studied fungus that lives in burned-over areas, but also has a taste for a stale baguette. The research team moved two genes from the fungus into Saccharomyces cerevisiae, a yeast widely used to ferment sugar into ethanol.

One gene forms structures in the yeast’s cell wall that draw short chains of glucose into the cell. The second gene makes beta-glucosidase, an enzyme that the fungus (and now the yeast) use inside the cell to snip the short chains of glucose into individual glucose molecules, where the yeast converts them into ethanol.

Raise a glass to success!

Although the short chains of glucose that the fungus extracts from cellulose is not digestible to normal yeast, the transformed yeast used these short chains to produce an abundance of ethanol. “It’s a proof of principle using lab strains,” says Cate. “We in the Energy Bioscience Institute [a collaboration of UC-Berkeley, the University of Illinois, Lawrence Berkeley Laboratory and BP] have colleagues who are helping us look at some really robust, industrial yeasts to see how the transporters work in those systems.”

Cate says transporters are key. “Any cell is a fortress, with a membrane or a cell wall that keeps things out to protect its innards. To get a small molecule in or out, there has to be a way, and these are the transporters, which live in the cell membrane, with parts on the outside and parts on the inside.”

The study was “a pretty slick example of how genomic technology can rapidly get you to the gene you care about,” says Steven Slater, associate director of the Great Lakes Bioenergy Research Center. “They used a combination of published literature on genes that are differentially expressed when several fungi are exposed to cellulose, and were able to rapidly go from there down to something that looks like transporter.”

Training a workhorse

The key, Slater says, is that “they took a workhorse organism that is primarily used for the production of ethanol and gave it a new genetic tool that could be used to get things other than glucose inside the cell; that’s important for producing ethanol from cellulosic biomass.”

Indeed, Cate says, many organisms have ways to transport fragments of cellulose: “You can find these all over in nature, including in the black truffle, a fungal delicacy that grows symbiotically on oak trees.”

Cate expects further progress. “We in the Energy Bioscience Institute [a collaboration of UC-Berkeley, the University of Illinois, Lawrence Berkeley Laboratory and BP] are testing some really robust, industrial yeasts.”

This process may not be limited to ethanol, Cate says. “It’s modular, and it may benefit research groups that have been working on yeast to make all sorts of interesting biofuels: alcohols, or things like diesel or jet fuel.”

David J. Tenenbaum

Goldman explains that one compartment inside onion cells contains an enzyme, called allinase, while another compartment holds the enzyme’s substrate: a suite of sulfur compounds known as ACSO for short. Because sulfur is an essential nutrient, the onion stores sulfur from the soil as ACSO for later use.

When onion cells rupture – whether through an insect’s nibble or a knife’s cut – allinase and ACSO mix together and react, producing another set of sulfur compounds called thiosulfinates. In addition to giving onions their familiar taste and odor, thiosulfinates repel pests that attack onion bulbs underground.

They aren’t the chemicals that cause tears to well, however. Before the thiosulfinates are produced, the reaction of allinase and ACSO releases a volatile chemical that wafts into the air and reacts with the water in our eyes. “Basically it produces something like sulfuric acid in your eye,” says Goldman.

The mild acid irritates the eye’s nerve cells, stimulating tears that help wash it away. Chilling onions before chopping can reduce the crying, says Goldman, because “this is an enzyme-mediated process, and low temperatures slow enzymes down.”

Sweet onion varieties, like Vidalia, grown in the low-sulfur soils of Georgia and Texas also show less of this activity and are less pungent, he adds.

Almost immediately, it begins altering polyphenols, a group of health-promoting chemicals with antioxidant activity that naturally occur in apples and other produce.

“As soon as you break the apple cells, you expose (the enzyme) to polyphenols and oxygen, and then it goes after the polyphenols and breaks them down. There’s a whole set of chemicals produced, and one set of the chemicals is colored,” explains Rich Hartel, a professor of food science at UW-Madison.

It is believed that this reaction, called enzymatic browning, is a form of plant defense against pests and pathogens, says Hartel. Some of the chemicals created via this reaction have been shown to help ward off invading organisms.

In addition to apples, enzymatic browning is evident in avocadoes and potatoes. It’s also what gives raisins, prunes and apple cider their deep hues. Interestingly, these brownish-colored chemicals belong to a family of pigments known as melanins, some of which give human skin its color.