Ferritin subunits

Figure 6.1 Schematic representation of human isoferritins of different subunit composition. Each ferritin subunit is represented as a sausage and subunits are packed in a symmetrical shell. Twelve of the 24 subunits are visible, with H-chain subunits stippled and L-chain subunits plain. Homopolymers of H-chain and L-chain subunits are at the top and bottom of the figure respectively. The sources of various ferritins are listed in the right hand column. Reprinted from Harrison and Arosio, 1996. Copyright (1996), with permission from Elsevier Science.

Plants contain phytoferritins, which accumulate in non-green plastids1 in conditions of iron loading. They are targeted to the plastids by a putative-transit peptide at their N-terminal extremity, and possess the specific residues for ferroxidase activity and iron nucleation, found in mammalian H-type or L-type ferritin subunits. 

Figure 19.6 The three-dimensional structure of the Listeria innocua ferritin subunit (a) and the 12-mer (dodecameric) structure viewed down a three-fold symmetry axis. (From Lewin et al., 2005. Copyright with permission from The Royal Society of Chemistry, 2005.)...

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 stractures of several members of the ferritin family have been solved, and show a high level of conservation. Ferritin subunits, with a molecular weight of 20kDa, are four-helix bundles with a fifth short helix at the C-terminal... 

White, K. Munro, H. N. Induction of ferritin subunit synthesis by iron is regulated at both the transcriptional and translational levels. J. Biol. Chem. 263 8938-8942 1988. 

Some ferritin subunits, notably in ferritin from bacteria, bind heme in a ratio of less than one heme per two subunits. A possible role of such heme in the oxidation and reduction of iron in the core is being investigated. 

The iron mineral is stable inside ferritin until the addition of reductant and/or chelators [48]. The rate at which the iron is released by reductive chelation (FMN/ NADH -I- bipyridyl) is relatively constant for recombinant H- and L-type ferritins [49] and for natural mixtures of subunits from different tissues, except when post-translationally modified [50, 51]. The site for iron release is unknown, but rates of release are similar in recombinant ferritins composed of all the ferritin subunit types currently known. 

Ferritin subunit type Initial rate (s" ) % Fe released at 5 minutes... 

Ag Binding Peptide (NPSSLFRYLPSD) b CoPt Binding Peptide (KTHEiHSPLLHK) Ferritin Subunit / j Hsp Subunit... 

CE has proven to be useful in characterizing different molecular forms of various metalloproteins like metal-lothionein, transferrin, and conalbumin. Molecular forms arise from differences in the amino acid sequence of proteins (isoforms), differences in the amount or type of metal bound (metalloforms), or from differences in the type and amount of carbohydrate side chains linked to the protein (glycoforms). CZE was used to follow the formation of the oligomeric iron core and its incorporation into ferritin, to detect and quantify ferritin species or ferritin subunit proteins in purified or partially purified states, and to study the interaction of different metal ions with ferritin. 

The above-presented properties of brain ferritin and hemosiderin suggest a different metabolism of iron in the brain and in organs such as the liver. As the H-ferritin subunits are involved more than L subunits in taking up and possibly also in releasing iron from ferritin, the significantly higher H/L ratio in the brain may possibly point to a higher turnover of iron in the brain [22]. 

Westin G, Schaffner W (1988) Heavy metal ions in transcription factors from HeLa cells Spl, but not octamer transcription factor requires zinc for DNA binding and for activator function. Nucleic Acids Res 16 5771-5781 White K, Munro HN (1988) Induction of ferritin subunit synthesis by iron is regulated at both the transcriptional and translational levels. J Biol Chem 263 8938-8942... 

Crystal structures for a number of ferritins and bacterioferritins from different sources have become available, and have recently been reviewed [448], These structures include those for ferritins from horse spleen, bullfrog, the H subunit and the mitochondrial form from human, the L subunits from horse and mouse, as well as for bacterioferritin from E. coli, Rhodobacter capsulatus, and D. desulfuricans ATCC 27774 [433,447,449-462], In addition, the crystal structures of ferritins from the halophile Halobacterium salinarium, from S. solfataricus, from the hyperthermophilic archaeon Archaeoglobus fulgidus, and from Azotobacter vinelandii have recently been solved [463-467], Each ferritin subunit independent of source and type is folded into a characteristic four a-helix bundle (each helix 27 A long). 

Takagi H, Shi DS, Ha Y, Allewell NM, Theil EC. 1998. Localized unfolding at the junction of three ferritin subunits A mechardsm for iron release. J Biol Chem 273 18685-18688. 

Iron stored in ferritin ean be mobilized during iron deficiency for metabolic needs, but the meehanism is incompletely characterized. Two models regarding iron exit and re-utilization have been proposed they are lysosome-mediated protein degradation and iron-releasing model and ferritin pores at the junetions of ferritin subunits as iron channel mediating iron release model. 

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