Protein coat of ferritin

The Protein Coat. Twenty-four polypeptides assemble into a hollow sphere, of ca. 100-120 X in outer diameter, to form the protein coat of ferritin. The diameter of the interior, which becomes filled with hydrous ferric oxide, is ca. 70-80 A. Subunit assembly appears to be spontaneous the coat remains assembled even without the iron core. Subunit biosynthesis is actually controlled by the amount of iron to be stored by a cell the subunit templates (mRNAs) are stored in the cytoplasm of a cell in a repressed form and are recruited for biosynthesis when the concentration of iron increases (3). 

Many of the current ideas about the shape of the ferritin molecule are derived from the high resolution x-ray crystallographic studies of Harrison and coworkers (5,6) (Figure 1) on the protein coat of ferritin from the spleen of horses, in which essentially all (> 90%) of the polypeptide subunits are identical. However, protein coats of ferritin from other animals, and indeed from different cells and tissues in the same animal, can be composed of assemblages of similar, but distinct, subunits (3). 

Figure 1. The protein coat of ferritin from horse spleen. N refers to the N-terminus and E refers to the location of the E-helix, a short helix with an axis perpendicular to the long axis of the subunit and which lines the channels formed at the fourfold axes. (Reproduced with permission from Ref. 5. Copyright 1983 Elsevier.)...

The Iron/Proteln Interface. Interactions of Iron with the protein coat of ferritin are most easily characterized In the early stages of core formation when most. If not all, of the Iron present Is In contact with the protein coat. In the complete core, bulk Iron Is Inorganic. To date, the protein coat has been little examined early In Iron core formation except In terms of effects on the Iron environment. Studies of the Iron early In core formation will be discussed later. 

Ferritin forms by addition of iron to assembled protein coats, which are stable even with no iron. The protein coats of ferritin can bind a variety of metal ions, e.g. Fe, Cd, Mn, V, Tb, and Zn, but to date, only Fe has been observed to form a core. 

For a more detailed discussion of ferritin biosynthesis and the possible physiological roles of isoferritins of different subunit compositions, the reader is referred to the recent literature (12, 13, 16, 18-22). In this paper the structures of the iron cores and protein coats of ferritins and the hemoferritins of bacteria are compared and the current state of knowledge concerning mineralization processes in these molecules is discussed in relation to this structural information. 

The microenvironment inside the protein coat of ferritin has recently been modeled by encapsulating ferrous ion inside phosphatidylcholine vesicles and studying the oxidation of iron as the pH is raised. The efficacy of such a model is indicated by the observation of relatively stable mixtures of Fe(II)/Fe(III) inside the vesicles, as have also been observed in ferritin reconstituted experimentally from protein coats and ferrous ion. - " ... 

Iron is stored in the protein ferritin. The protein coat of ferritin is a hollow sphere of 24 polypeptide chains through which Fe passes, is oxidized, and mineralizes inside in various forms of hydrated Fe203. Control of the formation and dissolution of the mineral core by the protein and control of protein synthesis by Fe are subjects of current study. 

The stabilization of Fe in aqueous solution by the protein coat of ferritin. In the absence of protein, at neutral pH, in air, flocculent precipitates of ferric hydrous oxide form. The equivalent concentration of Fe(III) in the solution of ferritin is about 10 times greater than in the inorganic solution. Left a solution of Fe(II)S04, pH 7, in air after 15 min. Right, the same solution in the presence of apoferritin, the protein coat of ferritin (reprinted from Reference 6). 

Ferritin, found in plants, animals, and some bacteria, serves as a reserve of iron for iron-proteins, such as those of respiration, photosynthesis, and DNA synthesis, as well as providing a safe site for detoxification of excess iron. The structure of ferritin, unique among proteins, is a protein coat of multiple, highly conserved polypeptides around a core of hydrous ferric oxide with variable amounts of phosphate. 

The protein that stores iron in the body is called ferritin. A ferritin molecule consists of a protein coat and an iron-containing core. The outer coat is made up of 24 pol3q5eptide chains, each with about 175 amino acids. As Figure 20-27 shows, the pol q5eptides pack together to form a sphere. The sphere is hollow, and channels through the protein coat allow movement of iron in and out of the molecule. The core of the protein contains hydrated iron(HI) oxide, FC2 O3 H2 O. The protein retains its shape whether or not iron is stored on the inside. When filled to capacity, one ferritin molecule holds as many as 4500 iron atoms, but the core is only partially filled under normal conditions. In this way, the protein has the capacity to provide iron as needed for hemoglobin s mthesis or to store iron if an excess is absorbed by the body. 

Fe(III) Clusters on Ferritin Protein Coats and Other Aspects of Iron Core Formation... 

Variations in ferritin protein coats coincide with variations in iron metabolism and gene expression, suggesting an Interdependence. Iron core formation from protein coats requires Fe(Il), at least experimentally, which follows a complex path of oxidation and hydrolytic polymerization the roles of the protein and the electron acceptor are only partly understood. It is known that mononuclear and small polynuclear Fe clusters bind to the protein early in core formation. However, variability in the stoichiometry of Fe/oxidant and the apparent sequestration and stabilization of Fe(II) in the protein for long periods of time indicate a complex microenvironment maintained by the protein coats. Full understanding of the relation of the protein to core formation, particularly at intermediate stages, requires a systematic analysis using defined or engineered protein coats. 

Ferritin Structure. Ferritin is a large complex protein composed of a protein coat which surrounds a core of polynuclear hydrous ferric oxide (Fe0 0H or Fe203 nH20). Points of contact which have been observed (4) between the inner surface of the protein coat and the iron core may reflect sites of cluster formation and core nucleatlon. 

Dimeric, trlmeric, and tetramerlc Interactions of ferritin subunits each have distinctive features which may relate to the function of the protein coat. Among vertebrates, at least, the amino acid sequence In such regions Is highly conserved, and forms structures which may have functional roles. 

The function of all ferritin molecules is to store iron. However, the mechanisms by which iron enters the core or is released from the core 1ji vivo is poorly understood. Experimentally, Fe(II), but not Fe(III), mixed with ferritin protein coats forms normal iron cores. Moreover, reductants such as thloglycollate or reduced flavins can reverse the process of core formation and release Fe(II) from the core. Since such reductants occur in vivo, reduction of ferritin cores may also occur vivo. 

Low Fe/Proteln (Fe <10-12/Molecule). Ferritin protein coats have multiple (8-12) binding sites for a variety of metals. Including Fe(II), Fe(IIl), V(IV), Mn(II), Tb(III), Cd(II), Zn(II), and Cu(II) (e.g., 5,34-36, and reviewed In Ref. 37). At least some of the metals bind at the three-fold channels. The location of the nucleatlon sites Is presently unknown. However, If the three-fold channels are the nucleatlon sites for core formation, core growth could block the channels, thus Inhibiting further accretion INSIDE the protein coat and could lead to the addition of Fe OUTSIDE the protein coat. Such an effect would obviate the sequestering function of the protein. Three forms of Fe have been observed bound to ferritin protein coats (apoferrltln) mononuclear, dlnuclear, and multlnuclear clusters. 

Within tissues of animals, plants, and fungi much of the iron is packaged into the red-brown water-soluble protein ferritin, which stores Fe(III) in a soluble, nontoxic, and readily available form.61 64 Although bacteria store very little iron,65 some of them also contain a type of ferritin.66-67 On the other hand, the yeast S. cerevisieae stores iron in polyphosphate-rich granules, even though a ferritin is also present.65 Ferritin contains 17-23% iron as a dense core of hydrated ferric oxide 7 nm in diameter surrounded by a protein coat made up of twenty-four subunits of mo-... 

In vitro a crystalline iron core can be laid down in apoferritin by the addition of an oxidant, such as O2, to an aqueous solution of a ferrous salt and apoferritin (32, 132, 140). The reconstituted core of horse ferritin prepared in the absence of phosphate and with O2 as oxidant is very similar to the native core in terms of its size and Mossbauer properties (85). Electron microscopy, however, reveals that it is less well ordered. Reconstitution in the presence of phosphate leads to smaller cores. Reconstituted A. vinelandii cores in the absence of phosphate were more ordered than were the native cores, and clearly contained ferrihydrite particles and, in some cases, crystal domains (85). Thus the nature of the core is not determined solely by the protein coat the conditions of core formation are also important. This is also indicated by Mossbauer spectroscopy studies of P. aeruginosa cells grown under conditions different than those employed for the large-scale pu-... 

Jacobs et al. (68) that FMN cannot cross the protein coat, and the most probable explanation of the NMR data of Khodr et al. (76), concerning the interaction of ferritin with small amines, is that passage through the coat is severely limited, even for small molecules. 

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