• Prosthetic group features
• Ferroxidase reaction
• Thermodynamic properties
• Ferritins in motif databases
• Ferritins in alignment databases
• Ferritins in 3­D databases
• References

Protein	Binuclear iron centre	Iron ligands		Formal iron oxidation states
		Fe1	Fe2
Human ferritin (H­chain)		NHis; 1­OGlu; H2O	2 × 1­OGlu	2×FeII; 2×FeIII
		µ­1:1­OGlu
E. coli ferritin		NHis; 2­OGlu	1­OGlu; 2­OGlu	2×FeII; 2×FeIII
		µ­1:1­OGlu

Ferritins, which belong to Class II diiron-carboxylate proteins [1], play a key role in iron metabolism [2, 3]. All ferritins have the capacity to remove Fe²⁺ ions from solution in the presence of dioxygen (O₂), and to deposit iron into the protein interior in a mineral form. The tertiary and quaternary structure of ferritins is highly conserved. All ferritins have 24 protein chains arranged in 432 symmetry to give a hollow, symmetrical shell with outside and inside diameters about 125 and 80 Å, respectively. Iron is stored in its central cavity as a hydrated ferric oxide mineral: mainly ferrihydrite (5Fe₂O₃·9H₂O) in animal ferritins and hydrated ferric phosphate in bacterial ferritins (cf. bacterioferritin). The subunits have overall dimensions about 25 × 25 × 50 Å and are folded as four­­helix bundles, each having a fifth short helix at roughly 60° to the bundle axis and a long extended loop [3]. Subunits pack tightly together except that at 3­fold axes there are narrow channels traversing the shell. It has been found that the 3­fold channels are the major, and probably the only, sites of iron transfer into the cavity of ferritins [4].

Steps in iron storage within ferritin molecules consist of Fe²⁺ oxidation (1), Fe³⁺ migration and the nucleation and growth of the iron core mineral [3].

Fe2+ + O2 + H2O Fe3+ + H2O2

(1)

Ferritin molecules isolated from vertebrates are composed of two types of subunit (H and L), whereas those from plants and bacteria contain only H­type chains (`H­type' is associated with the presence of ferroxidase centres). Mammalian L­ and H­chains show about 54% sequence identity. In mixed subunit 24­mers (heteropolymers) H­ and L­subunits have similar conformations, principally four­­helix bundles. Ferritins isolated from mammalian tissues are composed of variable proportions of H­ and L­subunits. The role of L­chains in ferritin iron incorporation is unclear. As a rule, L­rich ferritins are characteristic of organs storing iron (liver and spleen) and usually have a relatively high average iron content (> 1500 Fe atoms/molecule); H­rich ferritins, typical for heart and brain, have relatively low average iron content (< 1000 Fe atoms/molecule). Human serum ferritin and horse spleen ferritin contain only L­chains. No naturally occuring H­chain 24­mers have been isolated so far [3]. Recombinant H­chain homopolymers form massive protein aggregates, while L­chain homopolymers remain mostly soluble. On the basis of experiments with synthetic ferritin heteropolymers it has been concluded that the ferritins with high L:H­chain ratios are the most efficient in incorporating iron [5].

A metal centre identified by X­ray crystallography of recombinant human H­chain ferritin (HuHF) homopolymers [6] has been studied by site­directed mutagenesis. The substitution of residues at this centre (E27A, Y34F, E62K + H65G, E107A and Q141E) leads to a diminution of ferroxidase activity. The diiron site is situated at the centre of the four­­helix bundle. In Escherichia coli ferritin (EcFTN), X­ray analysis has revealed the presence of three iron­binding sites per subunit [7]. Sites Fe1 and Fe2 (3.8 Å apart) lie within the bundle at positions similar to those found in HuHF, and the third site (Fe3) lies on the inner surface of the protein shell, 7 Å away from the diiron site. HuHF and EcFTN contain only one His ligand (of Fe1). Sites Fe1 and Fe2 of the ferroxidase centres have a common bridging carboxylate residue (Glu­62 in HuHF and Glu­50 in EcFTN) and have a Gln residue (Gln­141 in HuHF and Gln­127 in EcFTN) instead of the second bridging Glu (cf. diiron centres in bacterioferritin and rubrerythrin). The experiments on substitution of Fe1 ligands (E27A­HuHF and E17A­EcFTN) and Fe2 ligands (E107A­HuHF and E94A­EcFTN) show that in wild­type ferritins both iron atoms at the diiron site are required for fast Fe²⁺ oxidation: modification of site Fe1 interferes with Fe²⁺ binding, whereas modification of site Fe2 drastically inhibits oxidation [7].

Thermodynamic properties of ferritin

Protein	pH	ProTherm entry	Mutation	Method
Ferritin L chain; human (recombinant)	2.0	3456	wild type	thermal
	2.2	3457	wild type	thermal
	2.4	3458	wild type	thermal
	2.8	3459	wild type	thermal
Ferritin H chain; human (recombinant)	2.0	3460	wild type	thermal
	2.4	3461	wild type	thermal
	2.8	3462	wild type	thermal

Ferritins in motif databases

PRINTS ID	PRINTS AC	PROSITE/BLOCKS ID	PROSITE AC	BLOCKS AC
-	-	FERRITIN_1 FERRITIN_2	PS00540 PS00204	BL0540

Ferritins in alignment databases

Protein Superfamily	Pfam	LPFC 3­D alignment
01704; ferritin	PF00210; ferritin	-

PDB	scop	BSM	RELI Base	Header	¹
1aew	1aew	1aew	1aew	Apoferritin (L­chain) (complex with Cd2+); horse (Equus caballus)	-
1dat	1dat	1dat	1dat	Apoferritin (L­chain) (complex with Cd2+) (cubic form); horse (Equus caballus)	-
1fha	1fha	1fha	1fha	Apoferritin (H­chain) (K86Q mutant) (complex with Ca2+); human (recombinant form expressed in Escherichia coli)	MMS92061
1hrs	1hrs	1hrs	1hrs	Apoferritin (complex with protoporphyrin IX and Cd2+); horse (Equus caballus)	-
1ier	1ier	1ier	1ier	Apoferritin (L­chain) (complex with Cd2+) (cubic form); horse (Equus caballus)	-
1ies	1ies	1ies	1ies	Apoferritin (L­chain) (complex with Cd2+) (tetragonal form); horse (Equus caballus)	-
1rcc	1rcc	1rcc	1rcc	Apoferritin (L­chain) (E57A, E58A, E59A, E61A mutant) (complex with betaine) (pH 5.5); bullfrog (Rana catesbeiana)	MS5LB24
1rcd	1rcd	1rcd	1rcd	Apoferritin (L­chain) (complex with betaine) (pH 5.5); bullfrog (Rana catesbeiana)	MS5LB24
1rce	1rce	1rce	1rce	Apoferritin (L­chain) (E57A, E58A, E59A, E61A mutant) (complex with betaine) (pH 6.3); bullfrog (Rana catesbeiana)	MS5LB24
1rcg	1rcg	1rcg	1rcg	Apoferritin (L­chain) (complex with betaine) (pH 6.3); bullfrog (Rana catesbeiana)	MS5LB24
1rci	1rci	1rci	1rci	Apoferritin (L­chain) (H25Y mutant) (complex with betaine) (pH 5.5); bullfrog (Rana catesbeiana)	MS5LB24
2fha	2fha	2fha	2fha	Apoferritin (H­chain) (K86Q mutant) (complex with Ca2+); human (recombinant form expressed in Escherichia coli)	-

¹ Macromolecular Structures abstract. Full text is available to BioMedNet Members

References

1. Nordlund, P. and Eklund, H. (1995) Di­iron-carboxylate proteins. Curr. Opin. Struct. Biol. 5, 758-766.
2. Andrews, S.C., Arosio, P., Bottke, W., Briat, J.F., von Darl, M., Harrison, P.M., Laulhere, J.P., Levi, S., Lobreaux, S. and Yewdall, S.J. (1992) Structure, function, and evolution of ferritins. J. Inorg. Biochem. 47, 161-174.
3. Harrison, P.M. and Arosio, P. (1996) The ferritins: molecular properties, iron storage function and cellular regulation. Biochim. Biophys. Acta 1275, 161-203.
4. Levi, S., Santambrogio, P., Corsi, B., Cozzi, A. and Arosio, P. (1996) Evidence that residues exposed on the three­fold channels have active roles in the mechanism of ferritin iron incorporation. Biochem. J. 317, 467-473.
5. Levi, S., Santambrogio, P., Cozzi, A., Rovida, E., Corsi, B., Tamborini, E., Spada, S., Albertini, A. and Arosio, P. (1994) The role of the L­chain in ferritin iron incorporation. Studies of homo and heteropolymers. J. Mol. Biol. 238, 649-654.
6. Lawson, D.M., Artymiuk, P.J., Yewdall, S.J., Smith, J.M.A., Livingstone, J.C., Treffry, A., Luzzago, A., Levi, S., Arosio, P., Cesareni, G., Thomas, C.D., Shaw, W.V. and Harrison, P.M. (1991) Solving the structure of human H ferritin by genetically engineering intermolecular crystal contacts. Nature 349, 541-544.
7. Hempstead, P.D., Hudson, A.J., Artymiuk, P.J., Andrews, S.C., Banfield, M.J., Guest, J.R. and Harrison, P.M. (1994) Direct observation of the iron binding sites in a ferritin. FEBS Lett. 350, 258-262.
8. Treffry, A., Zhao, Z., Quail, M.A., Guest, J.R. and Harrison, P.M. (1997) Dinuclear center of ferritin: studies of iron binding and oxidation show differences in the two iron sites. Biochemistry 36, 432-441.

Bibliography on structural studies of ferritins

Reviews on ferritins