Ferric ion (hydr)oxo clusters in the “Venus flytrap” cleft of FbpA : Mössbauer, calorimetric and mass spectrometric studies

Isothermal calorimetric studies of the binding of iron(III) citrate to ferric ion binding protein from Neisseria gonorrhoeae suggested the complexation of a tetranuclear iron(III) cluster as a single step binding event (apparent binding constant K appITC = 6.0(5) × 105 M−1). High-resolution Fourier transform ion cyclotron resonance mass spectrometric data supported the binding of a tetranuclear oxo(hydroxo) iron(III) cluster of formula [Fe4O2(OH)4(H2O)(cit)]+ in the interdomain binding cleft of FbpA. The mutant H9Y-nFbpA showed a twofold increase in the apparent binding constant [K appITC = 1.1(7) × 106 M−1] for the tetranuclear iron(III) cluster compared to the wild-type protein. Mössbauer spectra of Escherichia coli cells overexpressing FbpA and cultured in the presence of added 57Fe citrate were indicative of the presence of dinuclear and polynuclear clusters. FbpA therefore appears to have a strong affinity for iron clusters in iron-rich environments, a property which might endow the protein with new biological functions

Assembly of a Functional Triiron(III) Cluster in L-Ferritin

The L‐subunits of mammalian ferritins are generally accepted to facilitate iron biomineral formation by providing nucleation sites. The formation of a μ3‐oxo trinuclear iron cluster on the inner cage of homopolymeric recombinant human L ferritin and natural horse spleen ferritin has been observed upon diffusion of ferrous ions through metal‐free ferritin crystals. Three glutamate side chains (Glu60, Glu61, and Glu64) act as bridging ligands between iron pairs, thus driving the cluster assembly. In the fully formed cluster, observed after 60′ diffusion in the human L ferritin, the iron ions are also bridged by peroxide anions, which could originate from ferrous iron oxidation by dioxygen. Substitution of Glu60, Glu61, and Glu64 by alanine residues significantly reduces the iron biomineralization rates, thus suggesting that the observed cluster represents the biomineral nucleation site