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  • Bacillus cereus iron uptake protein fishes out an unstable ferric citrate trimer.

Bacillus cereus iron uptake protein fishes out an unstable ferric citrate trimer.

Proceedings of the National Academy of Sciences of the United States of America (2012-10-03)
Tatsuya Fukushima, Allyson K Sia, Benjamin E Allred, Rita Nichiporuk, Zhongrui Zhou, Ulla N Andersen, Kenneth N Raymond
ABSTRACT

Citrate is a common biomolecule that chelates Fe(III). Many bacteria and plants use ferric citrate to fulfill their nutritional requirement for iron. Only the Escherichia coli ferric citrate outer-membrane transport protein FecA has been characterized; little is known about other ferric citrate-binding proteins. Here we report a unique siderophore-binding protein from the gram-positive pathogenic bacterium Bacillus cereus that binds multinuclear ferric citrate complexes. We have demonstrated that B. cereus ATCC 14579 takes up (55)Fe radiolabeled ferric citrate and that a protein, BC_3466 [renamed FctC (ferric citrate-binding protein C)], binds ferric citrate. The dissociation constant (K(d)) of FctC at pH 7.4 with ferric citrate (molar ratio 1:50) is 2.6 nM. This is the tightest binding observed of any B. cereus siderophore-binding protein. Nano electrospray ionization-mass spectrometry (nano ESI-MS) analysis of FctC and ferric citrate complexes or citrate alone show that FctC binds diferric di-citrate, and triferric tricitrate, but does not bind ferric di-citrate, ferric monocitrate, or citrate alone. Significantly, the protein selectively binds triferric tricitrate even though this species is naturally present at very low equilibrium concentrations.

MATERIALS
Product Number
Brand
Product Description

Supelco
Iron(III) citrate tribasic monohydrate, 18-20% Fe basis (T)
Sigma-Aldrich
Ferric citrate, BioReagent, suitable for cell culture
Sigma-Aldrich
Iron(III) citrate, technical grade