Contacts:
Larry Que, University of Minnesota Chemistry Department, (612) 625-0389 Deane Morrison, News Service, (612) 624-2346, [email protected]
Scientists Find 'Diamond' Drill to Demolish Methane
The secret weapon used by microbes to perform the difficult but environmentally crucial--not to mention commercially desirable--conversion of methane (natural gas) to methyl alcohol has been identified by researchers at the University of Minnesota and Carnegie-Mellon University. The weapon is a diamond-shaped configuration of iron and oxygen atoms at the active site of the enzyme that performs the conversion.
Understanding how it works could help in developing new processes to make plastics and other large chemicals that now are made from crude oil, as well as in making methyl alcohol, a potential alternative to gasoline. It could also lead to cheaper energy and the design of antitumor drugs. The work is described in the Jan. 24 issue of Science.
Methane is a tricky substance to convert to anything except carbon dioxide and water (which is cinchy--just light a match). How to open up the molecule's structure just a little bit to insert a single oxygen atom--thereby forming methyl alcohol--has perplexed chemists for decades.
"Methane is the smallest hydrocarbon, but it's the toughest nut to crack," said Minnesota's Larry Que, principal investigator for the study. "It's the toughest to break down without completely burning it up. So far, we've designed iron-based catalysts that can attack other hydrocarbons, but not methane."
The bacteria that perform this sophisticated chemistry live in lakes and oceans around the world, using methane as their sole source of carbon and energy. Methane is produced by bacteria living in the bottom sediment where no oxygen penetrates. The gas bubbles up, and when it has ascended to a certain level it encounters oxygen that has penetrated from the surface. That's also where "methanotrophic" bacteria live. These bacteria put oxygen and methane together, then live off the resulting metabolic products.
Without these bacteria, methane would build up in the atmosphere and contribute much more heavily to global warming, said Minnesota biochemist John Lipscomb, another author of the paper. With an atmospheric half-life of 20 years--compared to carbon dioxide's one year--methane could eventually compete with CO2 as the major greenhouse gas if it were not removed. About a billion tons of methane are produced by sediment-dwelling bacteria every year, and the methanotrophic bacteria absorb upwards of 95 percent of the gas before it can escape into the air. The bacterial enzyme, called methane monooxygenase, contains iron but is quite different from hemoglobin, the iron-based protein in blood that transports oxygen. When the enzyme is working, it forms a transient diamond-shaped structure every time it grabs a molecule of methane.
Que and his colleagues found the structure by taking a "snapshot" of the enzyme in action with gamma rays and x-rays. Que said he suspects many other enzymes of harboring a similar structure. One is ribonucleotide reductase, the enzyme that catalyzes the first step in DNA synthesis. If this enzyme were knocked out, the cells that contained it couldn't divide--a desirable state of affairs if the cells are cancerous. "We have proposed that this enzyme also uses this diamond core," said Que. "If so, knowing how it works may someday be useful in designing inhibitors that work against this structure and nothing else. These inhibitors would thus act as antitumor agents."
Harnessing the diamond structure to produce methyl alcohol has huge economic implications, Que said. First is the potential of methyl alcohol (methanol) as an alternative to gasoline. Second, methane will outlast supplies of crude oil, so why not use methane to make chemicals now made from crude? Also, the cost of transporting methane from natural gas fields is a huge part of the total expense in using gas for heat and cooking. But transforming it into a liquid such as methyl alcohol would make transport more economical. Que is now using his knowledge of bacterial methane monooxygenase to design synthetic catalysts to do what methanotrophic bacteria have done, without any fanfare, for eons.
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