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Synthesis

Bacteria Confiscate Unstable Iron-Citrate Complex

Unique protein may give Bacillus cereus a leg up in sequestering the life-essential element iron

by Jyllian Kemsley
October 8, 2012 | APPEARED IN VOLUME 90, ISSUE 41

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Credit: Proc. Natl. Acad. Sci. USA
A model for Fe3(citrate)3 shows iron in orange, citrate oxygen in red, citrate carbon in white, and protein ligands or water oxygen in green.
09041-scicon-ironcitrate.jpg
Credit: Proc. Natl. Acad. Sci. USA
A model for Fe3(citrate)3 shows iron in orange, citrate oxygen in red, citrate carbon in white, and protein ligands or water oxygen in green.

A unique bacterial protein selectively binds an unstable triferric citrate complex to import iron into Bacillus cereus cells, reports a team from the University of California, Berkeley (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1210131109). Iron is an essential element that bacteria commonly sequester by sending ligands, called siderophores, to chelate insoluble Fe3+ in the environment. Selective binding and transport proteins then convey the complexes back into the cells. Common siderophores include citrate ([COH(COO)(CH2COO)2]3–) and citrate-based ligands. Citrate’s carboxyl and hydroxyl groups coordinate Fe3+. Previously, however, citrate-binding proteins were observed harboring only Fe(citrate)2 and Fe2(citrate)2. Kenneth N. Raymond and colleagues have now identified a B. cereus protein, christened ferric citrate-binding protein C, or FctC, that selectively binds Fe3(citrate)3 even when other iron-citrate species are present in solution. Only a few closely related species have genes for similar proteins, so FctC may give the B. cereus group an advantage by enabling import of iron complexes that other bacteria cannot sequester.

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