Iron metabolism in aerobes: managing ferric iron hydrolysis and ferrous iron autoxidation

Abstract

Aerobes and anaerobes alike express a plethora of essential iron enzymes; in the resting state, the iron atom(s) in these proteins are in the ferrous state. For aerobes, ferric iron is the predominant environmental valence form which, given ferric iron's aqueous chemistry, occurs as 'rust', insoluble, bio-inert polymeric ferric oxide that results from the hydrolysis of [Fe(H(2)O)(6)](3+). Mobilizing this iron requires bio-ferrireduction which in turn requires managing the rapid autoxidation of the resulting Fe(II) which occurs at pH > 6. This review examines the aqueous redox chemistry of iron and the mechanisms evolved in aerobes to suppress the 'rusting out' of Fe(III) and the ROS-generating autoxidation of Fe(II) so as to make this metal ion available as the most ubiquitous prosthetic group in metallobiology.

Fig. 1

Iron enzymes common to aerobes. Nitrogenase is included although not common in aerobes. Adapted from unpublished image created by J. Imlay (with permission).

Fig. 2

Oxygenation of the biosphere, the environmental redox potential [8], and iron’s Pourbaix diagram [9].

Fig. 3

Aqueous chemistry of Fe^II and Fe^III. (A) Solubility product constants and H₂O exchange rates. (B) Hydrolysis reaction of [Fe(H₂O)₆]³⁺ . Data from [10].

Fig. 4

Autoxidation of Fe^II. Fe^II→Fe^III conversion is quantified by consumption of O₂ using a Clark electrode; the electron stoichiometry was 3.8:1 at all pH values. Overlapping buffers used were 100 mM acetate, MES and MOPS. Chen and Kosman, unpublished.

Fig. 5

‘Catalysis’ of Fe^II autoxidation by chelation. (A) Autoxidation of FeII in the presence of oxygenous chelators of increasing ligand stability. (B) Rate equilibrium free energy relationship between relative stability of Fe^III complex and autoxidation rate. The buffer was 50 mM MES with [chelator] = 1 mM. In addition to citrate and EDTA, the chelators were: IDA, iminodiacetic acid; EDDP, ethylenediamine-N,N’-dipropionic acid; HIDA, N-(2-hydroxyethyl)iminodiacetic acid; NTA, nitrilotriacetic acid; and HEDTA, N-(2-hydroxyethyl)ethylenediamine-N,N’,N’-triacetic acid. Chen and Kosman, unpublished.

Fig. 6

Dispersion of iron redox potentials in biologically-relevant coordination complexes. Symbols R and O denote lower and upper limits of cellular redox couples involved in iron trafficking. Consequently, to be a substrate for these metabolic pathways, the E°’ of biologic iron must fall within these limits. Diagram based on [15].

Fig. 7

Fe^III coordination in human transferrin (A, 1A8E) and Neisseria gonorrhoeae FpbA (B, 1D9Y). Unresolved solute ligands have been inserted, CO₃^2- in hTf, PO₃^2- and H₂O in FbpA. Note that in both proteins, solute ligation is trans to homologous His and Tyr ligands. In hTf, H-bond network between Nδ1 in H249 and E93 modulates Fe^III binding, possibly by tuning Im→Fe CT.

Fig. 8

Modulation of holo-FbpA stability and Fe^III reduction potential by solute ligand. Citrate effect is specifically noted. Adapted from ref. 37.

Fig. 9

Ferri-reductase, ferri-oxidase Fe-uptake pathway in fungi. The ligand, L, can be anything from citrate to Tf. This redox cycling ‘solves’ the recalcitrant aqueous and redox chemistry of iron at neutral pH as summarized in the text.

Fig. 10

Citrate mobilization of Fe from Tr for Fe-uptake by yeast. Citrate is synergistic in Tf-Fe^III reduction (A) and in ⁵⁹Fe-uptake from from Tf (B). Citrate makes Tf-Fe^III a better oxidant by 135 mV (C); with E°’ = −285 mV, Tf-Fe-Cit is substrate for ferri-reduction and Fe-uptake. In both (A) and (B) yeast cells are incubated with 10 μM FeCl3 for 30 min in absence or presence of citrate as noted. In (A), ferrireduction by Fre1 was quantified by trapping the Fe^II produced with 4,4′-(3-(2-pyridinyl)-1,2,4-triazine-5,6-diyl)bisbenzenesulfonic acid (ferrozine). In (B) Fe uptake via the Fet3, Ftr1 pathway was quantified using the radio-tracer, ⁵⁹Fe. The data in (C) were obtained spectroelectrochemically following the absorbance of Tf•FeIII using a BASi Epsilon workstation. Ziegler and Kosman, unpublished.

Fig. 11

Fe^II ET site in Fet3 (1ZPU). E185 and D409 provide oxygenous ligand set that tunes down E° of bound Fe^II while supporting favorable Marcus SK_a and H_DA terms (Eq. 1).

Fig. 12

Monomeric ferrous citrate [62].