An enzyme that breaks down alcohol in the liver could be used to recycle carbon, rather than pumping ever more into the atmosphere, say chemists in Illinois. They have developed a more efficient way of turning the greenhouse gas carbon dioxide into methanol using the enzyme.
Invented by Bakul Dave and Robyn Obert of the Southern Illinois University in Carbondale, the process effectively reverses the chemistry of burning. It promises to be a highly efficient way to produce methanol, a clean-burning fuel that can be used to power cars. What's more, if the energy needed to drive the process came from sources that don't generate CO2, this fuel could be produced and used without adding to the greenhouse effect.
To make methanol, the liver enzyme and two bacterial enzymes are embedded in a sponge-like, glassy material, which is placed in water. When CO2 is bubbled through the water, one of the bacterial enzymes, formate dehydrogenase, converts CO2 into formic acid. Then another, formaldehyde dehydrogenase, transforms the formic acid into formaldehyde. Finally, alcohol dehydrogenase, which normally helps our livers to detoxify alcohol, completes the reaction by turning the formaldehyde into methanol.
Each of the enzyme reactions is reversible, so to drive the process in the right direction, the Illinois team adds a fourth, electron-donating ingredient called nicotinamide adenine dinucleotide (NADH).
The spongy glass, a substance called a silica sol-gel, is the key to the reaction's success. It contains millions of microscopic pores that act as mini-reactors. By mixing the enzymes with the liquid gel, Dave and Obert successfully locked them into the structure. "When it solidifies, the enzymes get trapped," says Dave. "The enzymes can't get in or out, but the small reactants can," he says. So CO2 and NADH can get in, and methanol can diffuse out.
To make the process practical, the NADH will have to be recycled by constantly replenishing the electrons it feeds to the enzymes. Dave and Obert say this might be possible if the sol-gel is made from materials which conduct electricity and feed electrons back into the system. "The idea is that you'd feed in current directly," says Dave.
Ole Kirk, a senior researcher at the Danish enzyme manufacturer, Novo Nordisk, says that regenerating NADH in this way is feasible. NADH is extremely expensive, but if you could recycle it, he says, this would not be a problem.
If Dave and Obert can solve the remaining problems, it might be possible to recycle the CO2 from, say, power stations. But this would be pointless if as much CO2 were produced in generating the electricity needed to drive the reaction.
"I'd like to see the energetics of the reaction, because you don't get energy for nothing," says John Houghton, co-chair of the Intergovernmental Panel on Climate Change, which coordinates research on global warming. He says that the search for cheap, renewable sources of energy is more important, but has lost momentum in many OECD countries.
Author: Andy Coghlan
New Scientist issue 29th January 2000
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Source: Journal of the American Chemical Society (vol 121, p 12 192)