Multiple evolutionary origins of magnetotaxis in bacteria

Magnetosomes are intracellular, iron-rich, membrane-enclosed magnetic particles that allow magnetotactic bacteria to orient in the earth's geomagnetic field as they swim. The magnetosomes of most magnetotactic bacteria contain iron oxide particles, but some magnetotactic species contain iron sulfide particles instead. Phylogenetic analyses of small subunit ribosomal RNA sequences showed that all known magnetotactic bacteria of the iron oxide type are associated with the a subgroup of the Proteobacteria in the domain Bacteria. In contrast, uncultured magnetotactic bacteria of the iron sulfide type are specifically related to the dissimilatory sulfate-reducing bacteria within the delta subdivision of the Proteobacteria. These findings indicate a polyphyletic origin for magnetotactic bacteria and suggest that magnetotaxis based on iron oxides and iron sulfides evolved independently.

Magnetic polarity fractions in magnetotactic bacterial populations near the geomagnetic equator

The relative numbers of North-seeking and South-seeking polarity types in natural populations of magnetotactic bacteria were determined at sites on the coast of Brazil. These sites were South of the geomagnetic equator and had upward geomagnetic inclinations of 1-12 degrees . For upward inclinations >6 degrees , South-seeking cells predominated over North-seeking cells by more than a factor of 10. For upward inclinations <6 degrees , the fraction of North-seeking cells in the population increased with decreasing geomagnetic inclination, approaching 0.5 at the geomagnetic equator. We present a simple statistical model of a stochastic process that qualitatively accounts for the dynamics of the two polarity types in a magnetotactic bacterial population as a function of the geomagnetic field inclination.

Spatiotemporal distribution of marine magnetotactic bacteria in a seasonally stratified coastal salt pond

The occurrence and distribution of magnetotactic bacteria (MB) were studied as a function of the physical and chemical conditions in meromictic Salt Pond, Falmouth, Mass., throughout summer 2002. Three dominant MB morphotypes were observed to occur within the chemocline. Small microaerophilic magnetite-producing cocci were present at the top of the chemocline, while a greigite-producing packet-forming bacterium occurred at the base of the chemocline. The distributions of these groups displayed sharp changes in abundance over small length scales within the water column as well as strong seasonal fluctuations in population abundance. We identified a novel, greigite-producing rod in the sulfidic hypolimnion that was present in relatively constant abundance over the course of the season. This rod is the first MB that appears to belong to the gamma-Proteobacteria, which may suggest an iron- rather than sulfur-based respiratory metabolism. Its distribution and phylogenetic identity suggest that an alternative model for the ecological and physiological role of magnetotaxis is needed for greigite-producing MB.

Magnetite morphology and life on Mars

Nanocrystals of magnetite (Fe(3)O(4)) in a meteorite from Mars provide the strongest, albeit controversial, evidence for the former presence of extraterrestrial life. The morphological and size resemblance of the crystals from meteorite ALH84001 to crystals formed by certain terrestrial bacteria has been used in support of the biological origin of the extraterrestrial minerals. By using tomographic and holographic methods in a transmission electron microscope, we show that the three-dimensional shapes of such nanocrystals can be defined, that the detailed morphologies of individual crystals from three bacterial strains differ, and that none uniquely match those reported from the Martian meteorite. In contrast to previous accounts, we argue that the existing crystallographic and morphological evidence is inadequate to support the inference of former life on Mars.

Magnetite biomineralization and ancient life on Mars

Certain chemical and mineral features of the Martian meteorite ALH84001 were reported in 1996 to be probable evidence of ancient life on Mars. In spite of new observations and interpretations, the question of ancient life on Mars remains unresolved. Putative biogenic, nanometer magnetite has now become a leading focus in the debate.

Bacterial magnetosomes: microbiology, biomineralization and biotechnological applications

Magnetotactic bacteria orient and migrate along geomagnetic field lines. This ability is based on intracellular magnetic structures, the magnetosomes, which comprise nanometer-sized, membrane-bound crystals of the magnetic iron minerals magnetite (Fe3O4) or greigite (Fe3S4). Magnetosome formation is achieved by a mineralization process with biological control over the accumulation of iron and the deposition of the mineral particle with specific size and orientation within a membrane vesicle at specific locations in the cell. This review focuses on the current knowledge about magnetotactic bacteria and will outline aspects of the physiology and molecular biology of the biomineralization process. Potential biotechnological applications of magnetotactic bacteria and their magnetosomes as well as perspectives for further research are discussed.

Forming the phosphate layer in reconstituted horse spleen ferritin and the role of phosphate in promoting core surface redox reactions

Apo horse spleen ferritin (apo HoSF) was reconstituted to various core sizes (100-3500 Fe3+/HoSF) by depositing Fe(OH)3 within the hollow HoSF interior by air oxidation of Fe2+. Fe2+ and phosphate (Pi) were then added anaerobically at a 1:4 ratio, and both Fe2+ and Pi were incorporated into the HoSF cores. The resulting Pi layer consisted of Fe2+ and Pi at about a 1:3 ratio which is strongly attached to the reconstituted ferritin mineral core surface and is stable even after air oxidation of the bound Fe2+. The total amount of Fe2+ and Pi bound to the iron core surface increases as the core volume increases up to a maximum near 2500 iron atoms, above which the size of the Pi layer decreases with increasing core size. Mössbauer spectroscopic measurements of the Pi-reconstituted HoSF cores using 57Fe2+ show that 57Fe3+ is the major species present under anaerobic conditions. This result suggests that the incoming 57Fe2+ undergoes an internal redox reaction to form 57Fe3+ during the formation of the Pi layer. Addition of bipyridine removes the 57Fe3+ bound in the Pi layer as [57Fe(bipy)3]2+, showing that the bound 57Fe2+ has not undergone irreversible oxidation. This result is related to previous studies showing that 57Fe2+ bound to native core is reversibly oxidized under anaerobic conditions in native holo bacterial and HoSF ferritins. Attempts to bury the Pi layer of native or reconstituted HoSF by adding 1000 additional iron atoms were not successful, suggesting that after its formation, the Pi layer "floats" on the developing iron mineral core.

Redox reactivity of animal apoferritins and apoheteropolymers assembled from recombinant heavy and light human chain ferritins

The redox reactivities of air-oxidized apo horse spleen ferritin (HoSF) and apo rat liver ferritin (RaF) were examined by microcoulometry and reductive optical titrations. Microcoulometry on several independent lots of commercial HoSF revealed two distinct types of redox activity: one requiring 3-4 electrons and one requiring 6-7 electrons for full reduction of the protein shell. ApoRaF required 8-9 electrons to fully reduce the oxidized form. Reductive optical titrations confirmed the microcoulometric reduction stoichiometry and, in addition, showed that the spectra of both oxidized and reduced apoHoSF were distinct and possessed absorbances tailing into the visible region. The redox reactivity of both apoRaF and apoHoSF correlated with their H-subunit composition. Identical microcoulometric and optical experiments were conducted with recombinant apo human liver heavy (rHuHF) and light (rHuLF) ferritins, but neither was redox-active. These results suggest that the redox reactivity of native ferritins is due to their heteropolymeric nature. This was confirmed by mixing various proportions of rHuHF and rHuLF, dissociating the 24-mers into individual subunits with guanidine hydrochloride at pH 3.5, and renaturing to form heteropolymeric 24-mers. Microcoulometric measurements of these apoheteropolymers reassembled in vitro showed that they were redox-active like their native apoheteropolymer counterparts. The redox activity of these apoheteropolymers increased with H-subunit composition, reached a maximum near 12 H- and 12 L-subunits, and then declined to zero with increasing L-subunit composition. The decline in redox reactivity at high L-subunit concentrations indicates that both H- and L-subunits are involved in forming the observed redox centers. Apoheteropolymers formed from rHuLF and W93F (an H-chain mutant) were redox-inactive, suggesting that the conserved tryptophan is necessary for redox center formation.

Magnetic microstructure of magnetotactic bacteria by electron holography

Off-axis electron holography in the transmission electron microscope was used to correlate the physical and magnetic microstructure of magnetite nanocrystals in magnetotactic bacteria. The magnetite crystals were all single magnetic domains, and the magnetization directions of small superparamagnetic crystals were constrained by magnetic interactions with larger crystals in the chains. Shape anisotropy was found to dominate magnetocrystalline anisotropy in elongated crystals. A coercive field between 300 and 450 oersted was determined for one chain.

Reaction sequence of iron sulfide minerals in bacteria and their use as biomarkers

Some bacteria form intracellular nanometer-scale crystals of greigite (Fe3S4) that cause the bacteria to be oriented in magnetic fields. Transmission electron microscope observations showed that ferrimagnetic greigite in these bacteria forms from nonmagnetic mackinawite (tetragonal FeS) and possibly from cubic FeS. These precursors apparently transform into greigite by rearrangement of iron atoms over a period of days to weeks. Neither pyrrhotite nor pyrite was found. These results have implications for the interpretation of the presence of pyrrhotite and greigite in the martian meteorite ALH84001.

Magneto-aerotaxis in marine coccoid bacteria

Magnetotactic cocci swim persistently along local magnetic field lines in a preferred direction that corresponds to downward migration along geomagnetic field lines. Recently, high cell concentrations of magnetotactic cocci have been found in the water columns of chemically stratified, marine and brackish habitats, and not always in the sediments, as would be expected for persistent, downward-migrating bacteria. Here we report that cells of a pure culture of a marine magnetotactic coccus, designated strain MC-1, formed microaerophilic bands in capillary tubes and used aerotaxis to migrate to a preferred oxygen concentration in an oxygen gradient. Cells were able to swim in either direction along the local magnetic field and used magnetotaxis in conjunction with aerotaxis, i.e., magnetically assisted aerotaxis, or magneto-aerotaxis, to more efficiently migrate to and maintain position at their preferred oxygen concentration. Cells of strain MC-1 had a novel, aerotactic sensory mechanism that appeared to function as a two-way switch, rather than the temporal sensory mechanism used by other bacteria, including Magnetospirillum megnetotacticum, in aerotaxis. The cells also exhibited a response to short-wavelength light (< or = 500 nm), which caused them to swim persistently parallel to the magnetic field during illumination.

Magnetic properties of tunicate blood cells. II. Ascidia ceratodes

The magnetic properties of intact blood cells of the tunicate Ascidia ceratodes have been measured up to 50 kOe with a SQUID susceptometer. Analysis of total metal contents by plasma emission spectroscopy and V(IV) content by epr indicates that approximately 5% of the accumulated vanadium is +4 vanadyl ion. Measured values of the magnetic moment Mp at different values of the applied magnetic field H over the temperature range T = 2-100 K depend on the magnitude of the field indicating magnetic anisotropy of the ground state. The slope of the Mp vs. H/T curve at high temperature is significantly higher than expected from electron spin S = 1 per vanadium(III) ion. The model that fits these data best is a dimer with one V(III) S = 1 ion ferromagnetically coupled to a second V(III) S = 1 ion, with spin-coupling constant J = 3.5 cm-1, and 5% of the total vanadium content in the form of a V(IV) S = 1/2 ion. Since vanadium in A. ceratodes is known to reside in at least three different types of blood cell, the excellent fit indicates that the metal is stored predominantly as a dimer regardless of blood cell type. Ferromagnetic coupling implies that the two vanadium ions in the dimer are connected by an unprotonated mu-oxo bridge.

Controlled Biomineralization of Magnetite (Fe(inf3)O(inf4)) and Greigite (Fe(inf3)S(inf4)) in a Magnetotactic Bacterium

A slowly moving, rod-shaped magnetotactic bacterium was found in relatively large numbers at and below the oxic-anoxic transition zone of a semianaerobic estuarine basin. Unlike all magnetotactic bacteria described to date, cells of this organism produce single-magnetic-domain particles of an iron oxide, magnetite (Fe(inf3)O(inf4)), and an iron sulfide, greigite (Fe(inf3)S(inf4)), within their magnetosomes. The crystals had different morphologies, being arrowhead or tooth shaped for the magnetite particles and roughly rectangular for the greigite particles, and were coorganized within the same chain(s) in the same cell with their long axes along the chain direction. Because the two crystal types have different crystallochemical characteristics, the findings presented here suggest that the formation of the crystal types is controlled by separate biomineralization processes and that the assembly of the magnetosome chain is controlled by a third ultrastructural process. In addition, our results show that in some magnetotactic bacteria, external environmental conditions such as redox and/or oxygen or hydrogen sulfide concentrations may affect the composition of the nonmetal part of the magnetosome mineral phase.

Iron core formation in horse spleen ferritin: magnetic susceptibility, pH, and compositional studies

Horse spleen ferritin (HoSF) reconstituted with small iron cores ranging in size from 8 to 500 iron atoms was studied by magnetic susceptibility and pH measurements to determine when the added Fe3+ begins to aggregate and form antiferromagnetically coupled clusters and also to determine the hydrolytic state of the iron at low iron loading. The Evans NMR magnetic susceptibility measurements showed that at iron loadings as low as 8 Fe3+/HoSF, at least half of the added iron atoms were involved in antiferromagnetic exchange interactions and the other half were present as isolated iron atoms with S = 5/2. As the core size increased to about 24 iron atoms, the antiferromagnetic exchange interactions among the iron atoms increased until reaching the limiting value of 3.8 Bohr magnetons per iron atom, the value present in holo HoSF. HoSF containing eight or more Fe3+ to which eight Fe2+ were added showed that the Fe2+ ions were at sites remote from the Fe3+ and that the resulting HoSF consisted of individual, noninteracting Fe2+ and the partially aggregated Fe3+. pH measurements for core reduction showed that Fe(OH)3 was initially present at all iron loadings but that in the absence of iron chelators the reduced iron core is partially hydrolyzed. Proton induced x-ray emission spectroscopy showed that Cl- is transported into the iron core during reduction, forming a stable chlorohydroxy Fe(II) mineral phase.

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Magnetoferritin: characterization of a novel superparamagnetic MR contrast agent

A protein-encaged superparamagnetic iron oxide has been developed and characterized by using horse spleen apoferritin as a novel bioreactive environment. The roughly spherical magnetoferritin molecules, 120 A in diameter, are composed of a monocrystalline maghemite or magnetite core 73 A +/- 14 in diameter. Except for the additional presence of iron-rich molecules of higher molecular weight, the appearance and molecular weight (450 kd) of magnetoferritin are identical to that of natural ferritin; the molecules are externally indistinguishable from their precursor, with a pI (isoelectric point) in the range 4.3-4.6. The measured magnetic moment of the superparamagnetic cores is 13,200 Bohr magnetons per molecule, with T1 and T2 relaxivities (r1 and r2) of 8 and 175 L.mmol-1 (Fe).sec-1, respectively, at body temperature and clinical field strengths. The unusually high r2/r1 ratio of 22 is thought to arise from ideal core composition, with no evidence of crystalline paramagnetic inclusions. T2 relaxation enhancement can be well correlated to the field-dependent molecular magnetization, as given by the Langevin magnetization function, raised to a power in the range 1.4-1.6. With its nanodimensional biomimetic protein cage as a rigid, convenient matrix for complexing a plethora of bioactive substances, magnetoferritin may provide a novel template for specific targeting of selected cellular sites.

Electron microscopic studies of magnetosomes in magnetotactic bacteria

Electron microscopic studies on magnetosomes in magnetotactic bacteria have revealed much information on their composition, structure, and even the formation of their mineral phase. The mineral phases of the magnetosomes are of two general types: iron oxides and iron sulfides. Iron oxide-type magnetosomes contain particles of the ferrimagnetic mineral magnetite (Fe3O4) while the iron sulfide-type contain ferrimagnetic greigite (Fe3S4), greigite and non-magnetic pyrite (FeS2), or possibly ferrimagnetic pyrrhotite (Fe7S8). Regardless of their composition, the crystalline particles in magnetosomes have a narrow size range: approximately 35 to 120 nm. Magnetite crystals in this size range are single-magnetic-domains and confer a permanent magnetic dipole moment to the cell. The single-domain size range for greigite is not known but is probably similar to that for magnetite. The morphology of the particles in the bacterial magnetosomes appears to be species-specific. Morphologies of magnetite crystals in different species of magnetotactic bacteria include cubo-octahedra, parallelepipedal (truncated hexahedral or octahedral prisms), and tooth- or bullet-shaped (anisotropic). Morphologies of greigite particles include cubo-octahedra and rectangular prismatic. The greigite-pyrite particles are generally pleomorphic with no consistent crystalline morphology. A membrane has been shown to surround the particles in some organisms and may be involved in the formation of the crystalline phase while also providing physical constraints on the size and the shape of the crystal. These results clearly indicate that the biomineralization process involved in the bacterial magnetosome, a good example of a self-assembled structure on a nanometer scale, is highly controlled by the organism.

Role of phosphate in Fe2+ binding to horse spleen holoferritin

In order to identify the function and location of phosphate associated with the iron core of horse spleen ferritin (HoSF), the phosphate content of native HoSF was altered by two procedures. Adjustment of pH from 7.0 to 10.0 irreversibly released 53% of the phosphate and 10% of the iron, while lowering the pH to 5.0 reversibly released 43% of the phosphate and 35% of the iron. Reversible release of 85% of the initial phosphate (but little iron release) also occurs upon reduction with methyl viologen (MV) or dithionite. Most of the phosphate is released in the early stages of reduction of the iron core, suggesting that the phosphate resides primarily on the mineral core surface. Reduction followed by chelation altered both the iron and phosphate content of the HoSF mineral cores. HoSF iron cores first reconstituted in the absence of phosphate and then incubated with added phosphate did not bind phosphate. However, when HoSF was first reconstituted in the absence of phosphate and then equilibrated anaerobically with both Fe2+ and phosphate, then phosphate was incorporated in amounts similar to native HoSF. Fe2+ binding to native, phosphate altered, and reconstituted HoSF in the presence and absence of phosphate clearly showed that Fe2+ binding to the mineral core depends on the presence of core-bound phosphate. Fe2+ binding to phosphate-depleted mineral cores or to cores reconstituted with 621, 2158, and 3013 Fe/HoSF core in the absence of phosphate bound only eight Fe2+ per entire ferritin molecule, clearly showing that Fe2+ has no measurable affinity for the phosphate-free, reconstituted mineral core.(ABSTRACT TRUNCATED AT 250 WORDS)

Redox reactions of apo mammalian ferritin

Apo horse spleen ferritin undergoes a 6.3 +/- 0.5 electron redox reaction at -310 mV at pH 6.0-8.5 and 25 degrees C to form reduced apoferritin (apoMFred). Reconstituted ferritin containing up to 50 ferric ions undergoes reduction at the same potential, taking up one electron per ferric ion and six additional electrons by the protein. We propose that apo mammalian ferritin (apoMF) contains six redox centers that can be fully oxidized forming oxidized apoferritin (apoMFox) or fully reduced forming apoMFred. ApoMFred can be prepared conveniently by dithionite or methyl viologen reduction. ApoMFred is slowly oxidized by molecular oxygen but more rapidly by Fe(CN)6(3-) to apoMFox. Fe(III)-cytochrome c readily oxidizes apoMFred to apoMFox with a stoichiometry of 6 Fe(III)-cytochrome c per apoMFred, demonstrating a rapid interprotein electron-transfer reaction. Both redox states of apoMF react with added Fe3+ and Fe2+. Addition of eight Fe2+ to apoMFox under anaerobic conditions produced apoMFred and Fe3+, as evidenced by the presence of a strong g = 4.3 EPR signal. Subsequent addition of bipyridyl produced at least six Fe(bipyd)3(2+) per MF, establishing the reversibility of this internal electron-transfer process between the redox centers of apoMF and bound iron. Incubation of apoMFred with the Fe(3+)-ATP complex under anaerobic conditions resulted in the formation and binding of two Fe2+ and four Fe3+ by the protein. The various redox states formed by the binding of Fe2+ and Fe3+ to apoMFox and apoMFred are proposed and discussed. The yellow color of apoMF appears to be an integral characteristic of the apoMF and is possibly associated with its redox activity.

Fe2+ and phosphate interactions in bacterial ferritin from Azotobacter vinelandii

Fe2+ binding to both apo- and holo- bacterial ferritin from Azotobacter vinelandii (AVBF) was measured as a function of pH under carefully controlled anaerobic conditions. Fe2+ binding to apo-AVBF is strongly pH dependent with 25 Fe2+ ions/apo-AVBF binding tightly at pH 5.5 and over 150 Fe2+/apo-AVBF at pH 9.0. Holo-AVBF gave a similar pH-dependent binding profile with over 400 Fe2+/AVBF binding at pH of 9.0. Proton release per Fe2+ bound to either AVBF protein increases with increasing pH until a total of about two protons are released at pH 9.0. These binding results are both qualitatively and quantitatively different from corresponding measurements (Jacobs et al., 1989) on apo- and holo- mammalian ferritin (MF) where less Fe2+ binds in both cases. The high level of Fe2+ binding to holo-AVBF relative to that of mammalian ferritin is a consequence of the higher phosphate content in the core of AVBF. Reduction of AVBF by either dithionite or methyl viologen in the absence of chelating agents demonstrated that phosphate, but not Fe2+, is released from the AVBF core in amounts commensurate with the degree of iron reduction, although even at 100% reduction considerable phosphate remains associated with the reduced mineral core. Fe2+ binding to holo-AVBF made deficient in phosphate was lower than that of native AVBF, while the addition of phosphate to native holo-AVBF increased the Fe2+ binding capacity. These results clearly support the role of phosphate as the site of interaction of Fe2+ with the AVBF mineral core.(ABSTRACT TRUNCATED AT 250 WORDS)

Fe2+ binding to apo and holo mammalian ferritin

The binding of Fe2+ to both apo and holo mammalian ferritin has been investigated under anaerobic conditions as a function of pH. In the pH range 6.0-7.5, 8.0 +/- 0.5 Fe2+ ions bind to each apoferritin molecule, but above pH 7.5, a pH-dependent Fe2+ binding profile is observed with up to 80 Fe2+ ions binding at pH 10.0. This Fe2+ binding is reversible and is accompanied by up to two H+ being released per Fe2+ bound at pH 10.0. The Fe2+ binding to apoferritin probably occurs in the 3-fold channels. A much larger and more complex pH-dependent Fe2+ binding stoichiometry was observed for holoferritin with up to 300 Fe2+ ions binding at pH 10.0. This pH-dependent Fe2+ binding was interpreted as Fe2+ interaction at the FeOOH mineral surface with displacement of H+ from -OH or phosphate surface groups by the incoming Fe2+ ions. Mossbauer spectroscopic measurements using 57Fe-labeled Fe2+ under anaerobic conditions showed that 57Fe2+ binding to holoferritin was accompanied by electron transfer to the core, yielding 57Fe3+, presumably bound to the mineral surface. Removal of added iron by Fe2+-specific chelating agents yielded 57Fe2+, demonstrating the reversibility of this electron-transfer process. The Fe2+ bound to apo- and holoferritin is readily converted to Fe3+ by exposure to O2 and strongly retained by the respective ferritin species.

Redox reactions associated with iron release from mammalian ferritin

The early redox events involved in iron reduction and mobilization in mammalian ferritin have been investigated by several techniques. Sedimentation velocity measurements of ferritin samples with altered core sizes, prepared by partial reduction and Fe2+ chelation, suggest two different events occur during iron loss from the ferritin core. Reductive optical titrations confirm this biphasic behavior by showing that the first 20-30% of core reduction has different optical properties than the latter 70-80%. Proton uptake during initial core reduction is near zero, but as the percent core reduction increases, the proton uptake (H+/e) values increase to 2 H+/e (2 H+/Fe3+ reduced) as core reduction approaches 1 e/Fe3+. Coulometric reduction of ferritin by mediators of different redox potential and different cross-sectional areas show a two-phase sigmoidal reaction pattern in which initial core reduction occurs at a slower rate than later core reduction. The above experiments were all conducted in the absence of iron chelators so that the observed results were all attributed to core reduction rather than the combined effects of core reduction accompanied by Fe2+ chelation. The coulometric reduction of ferritin by various mediators shows a correlation more with reduction potential than with molecular cross-sectional area. The role of the ferritin channels in core reduction is considered in terms of the reported results.

Magnetite and magnetotaxis in microorganisms

Magnetotactic bacteria from freshwater and marine sediments orient and navigate along geomagnetic field lines. Their magnetotactic response is based on intracellular, single magnetic domains of ferrimagnetic magnetite, which impart a permanent magnetic dipole moment to the cell.

Redox reactivity of bacterial and mammalian ferritin: is reductant entry into the ferritin interior a necessary step for iron release?

Both mammalian and bacterial ferritin undergo rapid reaction with small-molecule reductants, in the absence of Fe2+ chelators, to form ferritins with reduced (Fe2+) mineral cores. Large, low-potential reductants (flavoproteins and ferredoxins) similarly react anaerobically with both ferritin types to quantitatively produce Fe2+ in the ferritin cores. The oxidation of Fe2+ ferritin by large protein oxidants [cytochrome c and Cu(II) proteins] also occurs readily, yielding reduced heme and Cu(I) proteins and ferritins with Fe3+ in their cores. These latter oxidants also convert enthetically added Fe2+, bound in mammalian or bacterial apo- or holoferritin, to the corresponding Fe3+ state in the core of each ferritin type. Because the protein reductants and oxidants are much larger than the channels leading into the mineral core attached to the ferritin interior, we conclude that redox reactions involving the Fe2+/Fe3+ components of the ferritin core can occur without direct interaction of the redox reagent at the mineral core surface. Our results also suggest that the oxo, hydroxy species of the core, composed essentially of Fe(O)OH, arise exclusively from solvent deprotonation. The long-distance ferritin-protein electron transfer observed in this study may occur by electron tunneling.