杏吧原创

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鈥檚 more, if the energy needed to drive the
process came from sources that don鈥檛 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鈥檚 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. 鈥淲hen it solidifies, the enzymes
get trapped,鈥� says Dave. 鈥淭he enzymes can鈥檛 get in or out, but the small
reactants can,鈥� 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. 鈥淭he idea is that you鈥檇
feed in current directly,鈥� 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.

鈥淚鈥檇 like to see the energetics of the reaction, because you don鈥檛 get energy
for nothing,鈥� 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.