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Carbonic anhydrases catalyze the interconversion of carbon dioxide and carbonic acid. In animals, they are important in respiration. In plants, they are involved in the fixation of carbon dioxide during photosynthesis. These enzymes require a metal for activity, usually zinc. Under conditions where zinc is scarce, however, some marine organisms produce a cadmium-specific carbonic anhydrase, according to studies by Princeton University researcher Todd W. Lane and geochemistry professor François M. M. Morel [Proc. Natl. Acad. Sci. USA, 97, 4627 (2000)]. The work shows cadmium "is not just a toxic metal ion," comments Alison Butler, a professor of chemistry and biochemistry at the University of California, Santa Barbara. And it provides another example of the bioinorganic chemistry arising from the unique transition metal composition of the ocean, she adds. Lane (left) and Morel found biological function for cadmium. [Photo by Pat Denton] Cadmium often is lumped with mercury and lead as one of the really bad metals from a biological point of view. But its nutrientlike behavior in oceanic environments hints that it might have a biological role, Morel says. Nutrients--such as phosphates, nitrates, and silicates, as well as biologically essential trace metals--have a typical distribution in the ocean's water column. The concentrations are almost zero at the surface and increase with depth. This profile reflects uptake by phytoplankton at the surface and decomposition and remineralization farther down. Cadmium has exactly that kind of concentration profile in the ocean, Morel says. "Why is that so, when all that we know of cadmium is that it is toxic and bad for organisms?" he asks. In the search for a biological function for cadmium, work in Morel's lab had established earlier that, under certain conditions, some organisms grow better with cadmium. Now Lane and Morel show that the marine diatom Thalassiosira weissflogii produces a cadmium-specific carbonic anhydrase when it is starved of zinc and cannot make enough of the zinc-requiring carbonic anhydrase called TWCA1. "We presume that cadmium-centered carbonic anhydrases are probably general" in organisms that exist in zinc-limited environments, Morel says. Several lines of evidence indicate that the two enzymes are different and that they do not represent a case of one metal substituting for another in the same protein. First, the two enzymes have different mobilities on protein gels, even when denatured. Second, an antibody to TWCA1 does not react with the cadmium enzyme. And third, the mass of TWCA1 is 27 kilodaltons, whereas that of the cadmium enzyme is 43 kilodaltons. These findings do not directly prove that cadmium is the catalytic center. But it is very likely that cadmium is playing the same role as zinc, given their similar electronic structures. But definitive proof, Morel says, would require showing that the enzyme cannot work without cadmium. To that end, his group is isolating the enzyme and working to overexpress it in Escherichia coli. "We're going to get the enzyme without cadmium," he says. "If it doesn't work without cadmium and then works when we put the metal in, we prove the point." The discovery helps explain how organisms cope with low levels of essential metals in the ocean surface waters. With zinc, for example, concentrations are on the order of only 100 picomolar, says Kenneth Bruland, a marine biogeochemist and professor of ocean sciences at the University of California, Santa Cruz. And more than 90% of that is chelated, leaving free zinc at only 1 to 10 picomolar. The new cadmium enzyme represents one way that "oceanic phytoplankton have evolved to cope with these extremely low concentrations," he says. The findings also have implications for the current use of cadmium as a paleotracer of phosphate in the ocean's water column. According to Morel, in the modern ocean, the concentrations of phosphate and cadmium are very highly correlated. However, unlike cadmium, phosphate amounts in sediments do not reflect past levels of phosphate. "Cadmium at a given time and depth in the ancient ocean is estimated by measuring the cadmium in shells of foraminifera [marine protozoans] conserved in sediments at a depth whose age can be calculated and that were formed at a depth in the water column known on the basis of the ecology of the organism," Morel explains. Using the cadmium/phosphate correlation, the phosphate concentration in the ancient ocean is then estimated. "Now we have to be careful," Morel says. "Because depending on the conditions, that relation may change." |
| UPDATE | 04.00 |
| AUTHOR | Princeton University researcher Todd W. Lane and geochemistry professor François M. M. Morel |
| LITERATURE REF. |
[Proc. Natl. Acad. Sci. USA, 97, 4627 (2000)]. |
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