The iron storage protein, ferritin, plays a key role in iron metabolism. Its ability to sequester the element gives ferritin the dual functions of iron detoxification and iron reserve. The importance of these functions is emphasised by ferritin's ubiquitous distribution among living species. Ferritin's three-dimensional structure is highly conserved. All ferritins have 24 protein subunits arranged in 432 symmetry to give a hollow shell with an 80 A diameter cavity capable of storing up to 4500 Fe(III) atoms as an inorganic complex. Subunits are folded as 4-helix bundles each having a fifth short helix at roughly 60 degrees to the bundle axis. Structural features of ferritins from humans, horse, bullfrog and bacteria are described: all have essentially the same architecture in spite of large variations in primary structure (amino acid sequence identities can be as low as 14%) and the presence in some bacterial ferritins of haem groups. Ferritin molecules isolated from vertebrates are composed of two types of subunit (H and L), whereas those from plants and bacteria contain only H-type chains, where 'H-type' is associated with the presence of centres catalysing the oxidation of two Fe(II) atoms. The similarity between the dinuclear iron centres of ferritin H-chains and those of ribonucleotide reductase and other proteins suggests a possible wider evolutionary linkage. A great deal of research effort is now concentrated on two aspects of ferritin: its functional mechanisms and its regulation. These form the major part of the review. Steps in iron storage within ferritin molecules consist of Fe(II) oxidation, Fe(III) migration and the nucleation and growth of the iron core mineral. H-chains are important for Fe(II) oxidation and L-chains assist in core formation. Iron mobilisation, relevant to ferritin's role as iron reserve, is also discussed. Translational regulation of mammalian ferritin synthesis in response to iron and the apparent links between iron and citrate metabolism through a single molecule with dual function are described. The molecule, when binding a [4Fe-4S] cluster, is a functioning (cytoplasmic) aconitase. When cellular iron is low, loss of the [4Fe-4S] cluster allows the molecule to bind to the 5'-untranslated region (5'-UTR) of the ferritin m-RNA and thus to repress translation. In this form it is known as the iron regulatory protein (IRP) and the stem-loop RNA structure to which it binds is the iron regulatory element (IRE). IREs are found in the 3'-UTR of the transferrin receptor and in the 5'-UTR of erythroid aminolaevulinic acid synthase, enabling tight co-ordination between cellular iron uptake and the synthesis of ferritin and haem. Degradation of ferritin could potentially lead to an increase in toxicity due to uncontrolled release of iron. Degradation within membrane-encapsulated "secondary lysosomes' may avoid this problem and this seems to be the origin of another form of storage iron known as haemosiderin. However, in certain pathological states, massive deposits of "haemosiderin' are f...

Ferritin is important in iron homeostasis. Its twenty-four chains of two types, H and L, assemble as a hollow shell providing an iron-storage cavity. Ferritin molecules in cells containing high levels of iron tend to be rich in L chains, and may have a long-term storage function, whereas H-rich ferritins are more active in iron metabolism. The molecular basis for the greater activity of H-rich ferritins has until now been obscure, largely because the structure of H-chain ferritin has remained unknown owing to the difficulties in obtaining crystals ordered enough for X-ray crystallographic analysis. Here we report the three-dimensional structure of a human ferritin H-chain homopolymer. By genetically engineering a change in the sequence of the intermolecular contact region, we obtained crystals isomorphous with the homologous rat L ferritin and of high enough quality for X-ray diffraction analysis. The X-ray structure of human H ferritin shows a novel metal site embedded within each of its four-helix bundles and we suggest that ferroxidase activity associated with this site accounts for its rapid uptake of iron.

No abstract

In this study, a comparative analysis of metal-related neuronal vulnerability was performed in two brainstem nuclei, the locus coeruleus (LC) and substantia nigra (SN), known targets of the etiological noxae in Parkinson's disease and related disorders. LC and SN pars compacta neurons both degenerate in Parkinson's disease and other Parkinsonisms; however, LC neurons are comparatively less affected and with a variable degree of involvement. In this study, iron, copper, and their major molecular forms like ferritins, ceruloplasmin, neuromelanin (NM), manganese-superoxide dismutase (SOD), and copper͞zinc-SOD were measured in LC and SN of normal subjects at different ages. Iron content in LC was much lower than that in SN, and the ratio heavy-chain ferritin͞iron in LC was higher than in the SN. The NM concentration was similar in LC and SN, but the iron content in NM of LC was much lower than SN. In both regions, heavy-and light-chain ferritins were present only in glia and were not detectable in neurons. These data suggest that in LC neurons, the iron mobilization and toxicity is lower than that in SN and is efficiently buffered by NM. The bigger damage occurring in SN could be related to the higher content of iron. Ferritins accomplish the same function of buffering iron in glial cells. Ceruloplasmin levels were similar in LC and SN, but copper was higher in LC. However, the copper content in NM of LC was higher than that of SN, indicating a higher copper mobilization in LC neurons. Manganese-SOD and copper͞zinc-SOD had similar age trend in LC and SN. These results may explain at least one of the reasons underlying lower vulnerability of LC compared to SN in Parkinsonian syndromes. L ocus coeruleus (LC) is the main brain region containing norepinephrine neurons. The rostral projections from these neurons seem to be involved in the modulation of neuronal activity, metabolism, and memory (1, 2), whereas the spinal cord projections are known to modulate spinal motoneuron function (3, 4).Neuronal loss in LC occurs in conditions such as Parkinson's disease (PD) and Alzheimer's disease and Down's syndrome (5, 6) with different cellular loss in rostral and caudal parts of LC. In Alzheimer's disease and Down's syndrome, it is not clear whether neuronal loss in LC is a primary event or a consequence of retrograde degeneration of cortically projecting cells due to the loss of cortical synapses. In idiopathic PD, most studies have documented a higher degree of neuronal loss in substantia nigra (SN) compared to LC (5-8). However, a recent study reported extensive impairment of LC neurons in PD (9). SN and LC share anatomical and biochemical similarities, being both pigmented because of neuromelanin (NM), and both composed of catecholaminergic neurons. Yet, in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridineintoxicated subjects, LC neurons are spared, whereas a large neuronal depletion occurs in SN (10, 11). Also, in other types of Parkinsonian syndromes caused by exposure to toxins, the LC neurons seem less damaged than those of...

Ferritin is a ubiquitous protein that plays a critical role in regulating intracellular iron homoeostasis by storing iron inside its multimeric shell. It also plays an important role in detoxifying potentially harmful free ferrous iron to the less soluble ferric iron by virtue of the ferroxidase activity of the H subunit. Although excess iron is stored primarily in cytoplasm, most of the metabolically active iron in cells is processed in mitochondria. Little is yet known of how these organelles regulate iron homeostasis and toxicity. Here we report an unusual intronless gene on chromosome 5q23.1 that encodes a 242-amino acid precursor of a ferritin H-like protein. This 30-kDa protein is targeted to mitochondria and processed to a 22-kDa subunit that assembles into typical ferritin shells and has ferroxidase activity. Immunohistochemical analysis showed that it accumulates in high amounts in ironloaded mitochondria of erythroblasts of subjects with impaired heme synthesis. This new ferritin may play an important role in the regulation of mitochondrial iron homeostasis and heme synthesis.

UV-visible spectroscopy, electrode oximetry, and pH stat were used to study Fe(II) oxidation and hydrolysis in horse spleen ferritin (HoSF) and recombinant human H-chain and L-chain ferritins (HuHF and HuLF). Appropriate test reactions and electrode responses were measured, establishing the reliability of oxygen electrode/pH stat for kinetics studies of iron uptake by ferritin. Stoichiometric ratios, Fe(II)/O2 and H+/Fe(II), and rates of oxygen uptake and proton production were simultaneously measured as a function of iron loading of the protein. The data show a clear distinction between the diiron ferroxidase site and mineral surface catalyzed oxidation of Fe(II). The oxidation/hydrolysis reaction attributed to the ferroxidase site has been determined for the first time and is given by 2Fe2+ + O2 + 3H2O --> [Fe2O(OH)2]2+ + H2O2 + 2H+ where [Fe2O(OH)2]2+ represents the hydrolyzed dinuclear iron(III) center postulated to be a mu-oxo-bridged species from UV spectrometric titration data and absorption band maxima. The transfer of iron from the ferroxidase site to the mineral core has been now established to be [Fe2O(OH)2]2+ + H2O --> 2FeOOH(core) + 2H+. Regeneration of protein ferroxidase activity with time is observed for both HoSF and HuHF, consistent with their having enzymatic properties, and is facilitated by higher pH (7.0) and temperature (37 degreesC) and by the presence of L-subunit and is complete within 10 min. In accord with previous studies, the mineral surface reaction is given by 4Fe2+ + O2 + 6H2O --> 4FeOOH(core) + 8H+. As the protein progressively acquires iron, oxidation/hydrolysis increasingly shifts from a ferroxidase site to a mineral surface based mechanism, decreasing the production of H2O2.

Human ferritins sequester and store iron as a stable FeOOH (s) mineral core within a protein shell assembled from 24 subunits of two types, H and L. Core mineralization in recombinant H-and L-subunit homopolymer and heteropolymer ferritins and several site-directed H-subunit variants was investigated to determine the iron oxidation/hydrolysis chemistry as a function of iron flux into the protein.Stopped-flow absorption spectrometry, UV spectrometry, and electrode oximetry revealed that the mineral core forms by at least three pathways, not two as previously thought. They correspond to the ferroxidase, mineral surface, and the Fe(II) + H 2 O 2 detoxification reactions, respectively:

No abstract