Bacteria use 'molecular lasso' to cop copper

The bacteria that destroy about one-third of the potent greenhouse gas methane before it can reach the atmosphere use a neat trick to gather a key nutrient for the job. They produce a small organic compound and release it into the surrounding environment, where it "lassos" atoms of copper. The bacteria then reabsorb the compound and use the copper as a weapon against methane, from which they extract energy. The crystal structure of the compound - called methanobactin - will be reported in the Sept. 10 issue of Science. The research was led by Hyung J. Kim, who did much of the work as a graduate student at the University of Kansas and is now a postdoctoral associate at the University of Minnesota College of Biological Sciences.

Methanobactin may have antibacterial properties, and its ability to absorb copper may find application in the semiconductor industry, which needs copper-free water.

The bacteria that make methanobactin are quite common.

"These bacteria are often found in rice paddies and wetlands," said Kim. "Methane is produced in the bottom muck and diffuses into the water, where these bacteria live. The bacteria sequester the methane and turn it into methyl alcohol."

According to estimates made in the 1990s, the amount of methane produced from all sources worldwide is about 120 billion tons per year, said Kim. About 40 percent comes from paddies and wetlands, and the methane-eating bacteria, known as methanotrophs, remove 80 to 90 percent of it. That translates to a methane diet of close to 43 billion tons a year.

Playing a pivotal role in this drama is the methanobactin molecule, a tiny, pyramid-shaped compound with a cleft that holds a single atom of copper in place. The bacteria churn out methanobactin molecules in large numbers and send them into the environment to fetch copper. When the compound returns with its booty, it is thought that the copper is incorporated into molecules of a key enzyme that converts methane to methyl alcohol. A very reactive atom, copper is just the ticket for metabolizing methane, which--chemically speaking--is a hard nut to crack. Their reactivity also makes copper atoms toxic to the bacteria. Thus, methanobactin serves to keep copper under control and protect the bacterial cells from it.

One piece of the story still to be learned is how the methanobactin is retrieved by bacterial cells, Kim said. The cells apparently latch onto copper-bearing methanobactin molecules, but what happens next isn't known. Also, unlike a cowboy's lasso, methanobactin has no tether to its mother cell. Therefore, when bacterial cells release their methanobactin molecules, they probably never see them again; instead, they take delivery of copper from methanobactin released by other cells of the same species. Thus, copper gathering amounts to a bacterial free-trading market.

Methanobactin also seems to keep other bacteria out of the market.

"Synthesized compounds analogous to some parts of the methanobactin molecule have been shown to be antibacterial," said Kim. "Researchers in the laboratory of Alan DiSpirito at Iowa State University are exploring the antibacterial properties of this compound."

Besides Kim, co-authors of the Science paper include DiSpirito and David Graham, who was Kim's adviser at the time the work was done and is an associate professor of civil, architectural and environmental engineering at the University of Kansas. Kim is currently working in the laboratory of Alan Hooper, professor of biochemistry, molecular biology and biophysics at the University of Minnesota. The work was supported by the National Science Foundation, the U.S. Department of Energy and the KU (University of Kansas) Research Development Fund.

09-sep-2004