Cellulolytic Enzymes

Cellulolytic Enzymes

Frances Arnold is designing better enzymes for making biofuels from cellulose.

In December, President Bush signed the Energy Independence and Security Act of 2007, which calls for U.S. production of renewable fuels to reach 36 billion gallons a year--nearly five times current levels--by 2022. Of that total, cellulosic biofuels derived from sources such as agricultural waste, wood chips, and prairie grasses are supposed to account for 16 billion gallons. If the mandates are met, gasoline consumption should decline significantly, reducing both greenhouse-gas emissions and imports of foreign oil.

The ambitious plan faces a significant hurdle, however: no one has yet demonstrated a cost-competitive industrial process for making cellulosic biofuels. Today, nearly all the ethanol produced in the United States is made from the starch in corn kernels, which is easily broken down into the sugars that are fermented to make fuel. Making ethanol from cheaper sources will require an efficient way to free sugar molecules packed together to form crystalline chains of cellulose, the key structural component of plants. That's "the most expensive limiting step right now for the large-scale commercialization of [cellulosic] biofuels," says protein engineer Frances Arnold, a professor of chemical engineering and biochemistry at Caltech.

The key to more efficiently and cheaply breaking down cellulose, Arnold and many others believe, is better enzymes. And Arnold, who has spent the last two decades designing enzymes for use in everything from drugs to stain removers, is confident that she's well on her way to finding them.

Cellulosic biofuels have many advantages over both gasoline and corn ethanol. Burning cellulosic ethanol rather than gasoline, for instance, could cut cars' greenhouse-gas emissions by 87 percent; corn ethanol achieves reductions of just 18 to 28 percent. And cellulose is the most abundant organic material on earth.

But whereas converting...