| STRUCTURE |
When Massachusetts Institute of Technology postdoc Tzanko I. Doukov brought his hard-won three-dimensional crystal structure of a complex bacterial enzyme to show his boss, assistant professor Catherine L. Drennan, her first reaction was to send him back to the lab to try again. The enzyme in question, carbon monoxide dehydrogenase/acetyl-coenzyme A synthase (CODH/ACS), is produced by Moorella thermoacetica, a microbe found in animals' digestive tracts. This 310-kilodalton enzyme uses two different kinds of metalloclusters to convert carbon dioxide into the universal biosynthetic building block acetyl-CoA. The metallocluster found in the CODH subunit reduces CO2 to CO, and that in the ACS subunit uses CO and a methyl group to assemble acetyl-CoA. Doukov's 2.2-Å structure showed that the cluster that assembles acetyl-CoA--predicted to be a cube of four irons and four sulfur atoms linked to a nickel center--contains a copper atom, too [Science, 298, 567 (2002)]. Coordinated by three thiolate ligands, the copper is nestled between a Fe4S4 cluster and a square planar nickel center. Drennan PHOTO BY RUDY BAUM "I didn't believe it at first," Drennan tells C&EN. But careful data analysis confirmed the presence of copper. Drennan's collaborator, biochemist Stephen W. Ragsdale of the University of Nebraska, was also caught by surprise. "We've studied this enzyme for 20 years and never thought it might contain copper," Ragsdale admits. The researchers have used elemental analysis to confirm that CODH/ACS does indeed contain copper. And copper is absolutely necessary for the enzyme to assemble acetyl-CoA, Ragsdale says. Paul A. Lindahl, a biochemist at Texas A&M University who has spent years studying the enzyme, was also "shocked" by the structure. In light of previous spectroscopic data that seem to rule out the presence of copper, he remains cautious about whether the metal will turn up in the native enzyme. Ragsdale, however, claims to have preliminary spectroscopic data confirming the presence of copper in native CODH/ACS. The novel cluster raises fresh questions about how nature combines two one-carbon compounds to make a useful two-carbon metabolic building block. Previous biochemical and spectroscopic experiments have suggested that the methyl group coordinates to nickel and the CO binds to another metal in the cluster. If that metal were the copper, it would ideally position the two reactants to form a copper-bound acetyl group--which then could be plucked off by coenzyme A to give acetyl-CoA. Drennan points out that the structure might just be catching the copper in the act: In addition to three thiolates, a small molecule is ligated to the copper. Although its identity is still unclear, the molecule could be the copper-acetyl intermediate. "We hope to capture a series of structures along the reaction pathway to see how CODH/ACS works," she says. The discovery has elicited a flurry of activity among inorganic chemists trying to model the enzyme's active site and activity, including Charles G. Riordan of the University of Delaware. After he heard about the structure, "I was up much of the night thinking about new synthetic models and new mechanisms," Riordan tells C&EN. Inorganic chemist Michelle Millar of SUNY Stony Brook knows exactly how Riordan feels. The structure is "just spectacular," she says. "Now the race is on to model it." To those who wonder how such a well-studied enzyme can still hold such surprises, Drennan points out, "You often find only what you are looking for. And no one was looking for copper." |
| UPDATE | 10.02 |
| AUTHOR | MIT's Catherine L. Drennan |
| LITERATURE REF. | [Science, 298, 567 (2002)]. |
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