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Tuesday, 31 May 2005
The First Known Native Cadmium Enzyme Found In Marine Phytoplankton
Todd W. Lane, Mak A. Saito, Graham N. George, Ingrid J. Pickering, Roger C.
Prince and François M.M. Morel
Cadmium is known to be extremely toxic to mammals, and is generally viewed alongside mercury as an environmental problem and toxic element that is not used by nature in any way. A Brief Communication in the May 5 issue of the journal Nature shows that we need to revise our opinion of cadmium. The paper reports the purification and characterization of a previously unknown metalloenzyme from the marine diatom Thalassiosira weissflogii that specifically uses cadmium to achieve its biological function. This is the first cadmium enzyme that has been discovered. The research team responsible includes former SSRL scientists Graham George and Ingrid Pickering (both now at the University of Saskatchewan) and colleagues from Woods Hole Oceanographic Institute, Sandia Laboratories, ExxonMobil Research and Engineering, and Princeton University. Colleagues on the team isolated the genes responsible for the cadmium enzyme, which also appear to be unique, and confirmed that the enzyme is a carbonic anhydrase. Carbonic anhydrases regulate levels of carbon dioxide within cells, and in plants catalyze the first step in the process of photosynthesis. All the other carbonic anhydrases that are known require zinc, and if the diatom is grown in seawater containing ample zinc then it makes an entirely different carbonic anhydrase that contains this element. But the surface waters of the oceans are extremely low in zinc, and this, together with the observation that adding cadmium allows them to grow, caused the researchers to look for a specific cadmium enzyme. X-ray absorption spectroscopy experiments performed at SSRL's BL7-3 allowed these researchers to gain initial structural understanding of the cadmium site and to relate this to that of analogous zinc-containing enzymes of terrestrial plants. The work reported in Nature indicates that cadmium plays a vital role in the global carbon cycle. Despite their microscopic size, marine phytoplankton are very numerous, and make up a significant fraction of the world's plants. They are thus responsible for a significant fraction of the cycling of atmospheric carbon dioxide through photosynthesis. Cadmium is needed for this (at least in diatoms), so it turns out that cadmium is environmentally essential instead of being just a toxic problem. Because many trace metals are found at low concentrations in the oceans, the researchers speculate that enzymes containing unusual metals may be more common in marine than in terrestrial organisms, and could be important for the cycling of trace metals in sea water.
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