High Resolution Crystal Structures of Amphibian Red-Cell L Ferritin: Potential Roles for Structural Plasticity and Solvation in Function

Inorganica Chimica Acta

Liu

Tosha

2008

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Synopsis and pictogram: Gated pores in the ferritin family of protein nanocages, illustrated in the pictogram, control transfer of ferrous iron into and out of the cages by regulating contact between hydrated ferric oxide mineral inside the protein cage, and reductants such as FMNH 2 on the outside. The structural and functional homology between the gated ion channel proteins in inaccessible membranes and gated ferritin pores in the stable, water soluble nanoprotein, make studies of ferritin pores models for gated pores in many ion channel proteins.Properties of ferritin gated pores, which control rates of FMNH 2 reduction of ferric iron in hydrated oxide minerals inside the protein nanocage, are discussed in terms of the conserved pore gate residues (arginine 72-apspartate 122 and leucine 110-leucine 134), of pore sensitivity to heat at temperatures 30 °C below that of the nanocage itself, and of pore sensitivity to physiological changes in urea (1-10 mM). Conditions which alter ferritin pore structure/function in solution, coupled with the high evolutionary conservation of the pore gates, suggest the presence of molecular regulators in vivo that recognize the pore gates and hold them either closed or open, depending on biological iron need. The apparent homology between ferrous ion transport through gated pores in the ferritin nanocage and ion transport through gated pores in ion channel proteins embedded in cell membranes, make studies of water soluble ferritin and the pore gating folding/unfolding a useful model for other gated pores.

Section: Identifying Ferritin Nanocage Pore Gatesmentioning

confidence: 99%

Gated pores in the ferritin protein nanocage

Inorganica Chimica Acta

Liu

Tosha

2008

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“…The location of F ox sites in ferritin has been inferred to be in the center of each active subunit for maxiferritins and at junctions of subunit dimers in miniferritins, based on models of protein cocrystals with Fe 2ϩ analogues such as Mg 2ϩ or Ca 2ϩ (7)(8)(9)(10)(11)(12)(13). Such models are reasonable for the ferrous substrate, but not for ferric intermediates such as the diferric peroxo (DFP) complex or products such as diferric oxo͞hydroxo mineral precursors.…”

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confidence: 99%

Ferritin reactions: Direct identification of the site for the diferric peroxide reaction intermediate

Liu

Proc. Natl. Acad. Sci. U.S.A.

2004

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125

Ferritins managing iron-oxygen biochemistry in animals, plants, and microorganisms belong to the diiron carboxylate protein family and concentrate iron as ferric oxide Ϸ10 14 times above the ferric Ks. Ferritin iron (up to 4,500 atoms), used for iron cofactors and heme, or to trap DNA-damaging oxidants in microorganisms, is concentrated in the protein nanocage cavity (5-8 nm) formed during assembly of polypeptide subunits, 24 in maxiferritins and 12 in miniferritins͞DNA protection during starvation proteins. I ron and oxygen are integral to life, but their chemistry requires many proteins with iron cofactors to harness them for respiration, photosynthesis, and DNA synthesis, while minimizing radical side reactions. Ferritins, cage-like nanoproteins, concentrate iron for cofactor synthesis, as a mineral (hydrated ferric oxide), in large, central cavities (5-8 nm). Ferritin nanocavities form during the spontaneous assembly of ellipsoidal, polypeptide subunits (1-3), 24 in maxiferritins (Ϸ480 kDa) and 12 in miniferritins (240 kDa), also called DNA protection during starvation proteins. The biological importance of ferritin is emphasized by the lethality of ferritin gene deletion in mammals (4), defects in the CNS from mutations in humans (5), and oxidant sensitivity after deletions of the multiple ferritins in bacteria (6)(7)(8). Reactions of iron and oxygen catalyzed by maxiferritins, in cells of higher plants, animals including humans, and microorganisms, stabilize iron with oxygen at 10 14 times above the K s to match cellular iron concentrations, whereas miniferritins protect DNA by trapping oxidants with iron in the hydrated ferric oxide mineral.Catalytic ferroxidase (F ox ) sites in ferritin catalyze the first in the series of reactions converting soluble ferrous ions to solid, concentrated ferric biomineral inside ferritin. The location of F ox sites in ferritin has been inferred to be in the center of each active subunit for maxiferritins and at junctions of subunit dimers in miniferritins, based on models of protein cocrystals with Fe 2ϩ analogues such as Mg 2ϩ or Ca 2ϩ (7-13). Such models are reasonable for the ferrous substrate, but not for ferric intermediates such as the diferric peroxo (DFP) complex or products such as diferric oxo͞hydroxo mineral precursors. There are at least four types of metal-protein sites in ferritin (iron entry, exit, nucleation, and F ox substrate). Thus, a number of assumptions are required if a crystal-metal site is assigned to one of the multiple, iron-dependent ferritin functions. Even when two Mg 2ϩ or two Ca 2ϩ ions are relatively near each other in ferritin cocrystals, the metal-metal distances in the models were much longer than the Fe 3ϩ -Fe 3ϩ distance in the functional DFP intermediate, determined by extended x-ray absorption fine structure analysis in solution (11,12,14). F ox site models have also relied on analogies to iron sites of other diiron carboxylate family members that form DFP intermediates (15-17), even when iron is a cofactor and contrasts with ferriti...

“…1). In vertebrates, a subunit (L) with an inactive catalytic center coassembles with the catalytically active subunits (H) with tissue-specific H͞L ratios (3,4) to modulate rapid H subunit iron uptake; this has the cost of peroxide production (2,(5)(6)(7). In addition to concentrating iron, ferritin plays an important role in scavenging intracellular iron to modulate the cellular labile iron pool (8).…”

mentioning

confidence: 99%

“…However, it is clear from the crystal structure of ferritin that the threefold pores are too narrow (3)(4) Å to accommodate [Fe(H 2 O) 6 ] 2ϩ without ligand exchange or a conformational change of the protein to widen the pores (26). Evidence of at least momentary larger pore size is shown by the diffusion of large molecules in and out of the ferritin cavity, exemplified by sugars as large as 13 Å (27), iron chelators (28,29), spin probes (28), and large iron complexes (up to 13 Å) (28,29).…”

mentioning

confidence: 99%

“…Evidence of at least momentary larger pore size is shown by the diffusion of large molecules in and out of the ferritin cavity, exemplified by sugars as large as 13 Å (27), iron chelators (28,29), spin probes (28), and large iron complexes (up to 13 Å) (28,29). Evidence for protein flexibility is the observation of alternate side chain conformations during ferritin crystallization (3) and localized pore unfolding with preservation of global folding and supramolecular assembly (17,18). Microscopy studies indicate that short lived holes in ferritin of 2.0-3.0 nm might occur by dynamic dissociation͞association of subunits (ref.…”

mentioning

confidence: 99%

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Iron uptake in ferritin is blocked by binding of [Cr(TREN)(H ₂ O)(OH)] ²⁺ , a slow dissociating model for [Fe(H ₂ O) ₆ ] ²⁺

Barnes

Proc. Natl. Acad. Sci. U.S.A.

Raymond

2002

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