A Copper Economy

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Science  04 Apr 2003:
Vol. 300, Issue 5616, pp. 21
DOI: 10.1126/science.300.5616.21c

Copper is an essential element, but free copper is toxic, so bacteria have developed regulatory systems, such as the pco system in Escherichia coli and the homologous cop system in Pseudomonas syringae. Four proteins, Pco/CopA, B, C, and D, are expressed under the control of a copper-inducible promoter. Recently, Peariso et al. used x-ray absorption spectroscopy (XAS) to show that the small periplasmic protein PcoC binds both Cu(I) and Cu(II), using different sets of ligands.

Arnesano et al. have determined the solution structure of CopC bound to Cu(I), and show that the binding sites for Cu(I) and Cu(II) in CopC are 30 angstroms apart. Cu(II) is bound by two histidines, an aspartic acid, and a glutamic acid, whereas Cu(I) is bound by a histidine and three methionines. A shift in redox state causes the copper ion to migrate between the sites.

How does CopC contribute to copper homeostasis? CopA is a periplasmic copper oxidase, and CopB and CopD are outer- and inner-membrane proteins, respectively. CopA and CopB contain methionine-rich repeats that, on the basis of the CopC structure, are likely to bind Cu(I). CopC may act as a molecular switch that facilitates either Cu(II) import to the cytoplasm via CopD, or Cu(I) export via CopB. Conversely, CopA may oxidize Cu(I) bound to CopC, or Cu(I) delivered from CopC might catalyze the oxidase activity of CopA.—VV

J. Am. Chem. Soc.125, 342 (2003); Proc. Natl. Acad. Sci. U.S.A.100, 3814 (2003).

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