Abstract
Carbonic anhydrase, a zinc enzyme found in organisms from all kingdoms, catalyses the reversible hydration of carbon dioxide and is used for inorganic carbon acquisition by phytoplankton. In the oceans, where zinc is nearly depleted, diatoms use cadmium as a catalytic metal atom in cadmium carbonic anhydrase (CDCA). Here we report the crystal structures of CDCA in four distinct forms: cadmium-bound, zinc-bound, metal-free and acetate-bound. Despite lack of sequence homology, CDCA is a structural mimic of a functional β-carbonic anhydrase dimer, with striking similarity in the spatial organization of the active site residues. CDCA readily exchanges cadmium and zinc at its active site—an apparently unique adaptation to oceanic life that is explained by a stable opening of the metal coordinating site in the absence of metal. Given the central role of diatoms in exporting carbon to the deep sea, their use of cadmium in an enzyme critical for carbon acquisition establishes a remarkable link between the global cycles of cadmium and carbon.
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Acknowledgements
We thank A. Saxena at the NSLS for assistance and Patrick McGinn for help with CA assays. This work was supported by start-up funds from Princeton University (to Y.S.), the NSF and the NSF-funded Center for Environmental Bioinorganic Chemistry (to F.M.M.M.).
Author Contributions Y.X. performed all the biochemical experiments; L.F. crystallized all forms of CDCA1; P.D.J. solved the structures; and F.M.M.M. and Y.S. supervised the work and wrote the paper. All authors discussed the results and commented on the manuscript.
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The file contains Supplementary Tables 1-2 with the X-ray data statistics and Supplementary Figures 1-5 with Legends. (PDF 3573 kb)
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Xu, Y., Feng, L., Jeffrey, P. et al. Structure and metal exchange in the cadmium carbonic anhydrase of marine diatoms. Nature 452, 56–61 (2008). https://doi.org/10.1038/nature06636
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DOI: https://doi.org/10.1038/nature06636
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