Iron incorporation into ferritins: evidence for the transfer of monomeric Fe(III) between ferritin molecules and for the formation of an unusual mineral in the ferritin of Escherichia coli

Abstract

Iron that has been oxidized by H-chain ferritin can be transferred into other ferritin molecules before it is incorporated into mature ferrihydrite iron cores. Iron(III) dimers are formed at the ferroxidase centres of ferritin H chains at an early stage of Fe(II) oxidation. Mössbauer spectroscopic data now show that the iron is transferred as monomeric species arising from dimer dissociation and that it binds to the iron core of the acceptor ferritin. Human H-chain ferritin variants containing altered threefold channels can act as acceptors, as can the ferritin of Escherichia coli (Ec-FTN). A human H-chain ferritin variant with a substituted tyrosine (rHuHF-Y34F) can act as a donor of Fe(III). Since an Fe(III)-tyrosinate (first identified in bullfrog H-chain ferritin) is absent from variant rHuHF-Y34F, the Fe(III) transferred is not derived from this tyrosinate complex. Mössbauer parameters of the small iron cores formed within Ec-FTN are significantly different from those of mammalian ferritins. Analysis of the spectra suggests that they are derived from both ferrihydrite and non-ferrihydrite components. This provides further evidence that the ferritin protein shell can influence the structure of its iron core.

Direct observation of the iron binding sites in a ferritin

“…A different nucleation site with ligands not present in Ec-FTN, has been postulated for HuHF [8]. The possibility of a unique nucleation centre is intriguing and could explain another finding of Miissbauer spectral analysis [21]. Subspectra attributed to iron clusters at early stages of iron-core formation are more complex than those of HuHF or of horse spleen ferritin (HoHF) [25].…”

“…However, this could be due to diffraction effects and the relatively low resolution of the analysis. p-oxo-bridged Fe(II1) dimers have been observed by Mijssbauer spectroscopy in HuHF [4,5] and also recently in EC-FTN [21] as an early product of Fe(I1) oxidation. The region of EC-FTN shown in Fig.…”

Comparison of the three-dimensional structures of recombinant human H and horse L ferritins at high resolution 1 1Edited by R. Huber

“…Both subunit types contain a central hydrophilic region within the subunit fold, but whereas that of the H subunit has been shown to bind metals and to be associated with ferroxidase activity, in the L subunit, which lacks ferroxidase activity, the metal binding site is replaced by a salt bridge Bauminger et al, 1991Bauminger et al, , 1994. L chains are more stable to denaturants than H chains , but the functional importance of L chains is thought to reside primarily in their ability to promote ferrihydrite nucleation Santambrogio et al, 1996;Bauminger et al, 1991Bauminger et al, , 1994. Regions of structure responsible for these functional differences are compared at high resolution.…”

Comparison of the kinetics of iron release from a marine (Trichodesmium erythraeum) Dps protein and mammalian ferritin in the presence and absence of ligands

“…For example, Bauminger et al showed that monomeric Fe(III) can be exchanged among ferritin minerals in different ferritin molecules [35]. It has also been reported that transfer of Fe(III) to heterologous ferritins occurs suggesting that this process does not requires specific inter-molecular contact [36]. In addition, Bakker and Boyer demonstrated that in the presence of buffer ions that can facilitate Fe transfer Fe(II) oxidized by the ferroxidase center of ferritin could be transferred to transferrin in excess rather than incorporated in the ferritin core [37].…”