• Prosthetic group features
• Bacterioferritin in motif databases
• Bacterioferritin in alignment databases
• Bacterioferritin in 3­D databases
• References

Haem type	Haem iron coordination	Axial iron ligands		Formal iron oxidation/spin states
Haem b	Hexacoordinate	SMet; SMet		FeII (S=0); FeIII (S=1/2)
Binuclear iron centre		Iron ligands		Formal iron oxidation/spin states
		Fe1	Fe2
		NHis; OGlu	NHis; OGlu	2×FeII (S=2)
		2 × µ­1:1­OGlu
		NHis; OGlu; H2O	NHis; 2 × OGlu	2×FeIII (S=5/2)
		µ­1:1­OGlu; µ­O

Bacterioferritins (BFR; also known as cyt b₁, cyt b₅₅₇, cyt b_557.5, or cyt b₅₅₉), are unusual oligomeric proteins which contain both a binuclear iron centre and haem b. The tertiary and quaternary structure of BFR is very similar to that of ferritins [1]. The physiological functions of BFR, which may be other than just iron uptake, are still unclear [2].

BFR forms a hollow, nearly spherical shell composed of 24 identical protein chains and 12 haem groups [3] (see Figure 1BFR). Iron is stored in its central cavity, of about 80 Å diameter, as a hydrated ferric oxide mineral (essentially hydrated ferric phosphate [1]). The building block for the shell is a protein dimer (subunits A and B) binding the single haem group (see Figure 1BCF). Each subunit consists of four nearly parallel ­helices. The haem is bound symmetrically to subunits A and B by the residues Met_A­52 and Met_B­52 [3]. Each subunit includes a binuclear metal­binding site linking together the four major helices of the subunit, which has been identified as the ferroxidase centre of BFR [4]. The diiron site is situated at the centre of the four­­helix bundle. Each iron atom is ligated by one His and one Glu. In the diferrous form, the two Fe atoms are bridged by two carboxy groups whereas the breakage of one carboxylate bridge (Glu­127) may accompany the iron oxidation [4]. BFR mutants with Met­52 replaced are haem­free, but appear to be correctly assembled and are capable of accumulating iron [5].

BFR from Pseudomonas aeruginosa, unlike other BFRs, is found to contain two subunit types, which differ considerably in their amino acid sequences [6]. A fungal ferritin showing sequence homology to BFR has been purified [7].

Bacterioferritin in motif databases

PRINTS ID	PRINTS AC	PROSITE/BLOCKS ID	PROSITE AC	BLOCKS AC
BACFERRITIN	PR00601	BACTERIOFERRITIN	PS00549	BL00549

Bacterioferritin in alignment databases

Protein Superfamily	Pfam	LPFC 3­D alignment
01705; bacterioferritin	-	-

PDB	MSD	scop	BSM	RELI Base	Header	¹
1bcf	1bcf	1bcf	1bcf	1bcf	Bacterioferritin (cytochrome b1) (Mn2+ substituted for Fe2+) (dimer); Escherichia coli	MS5CD3
1bfr	1bfr	1bfr	1bfr	1bfr	Bacterioferritin (cytochrome b1) (Mn2+ substituted for Fe2+) (24­mer); Escherichia coli	-

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

References

1. Harrison, P.M. and Arosio, P. (1996) The ferritins: molecular properties, iron storage function and cellular regulation. Biochim. Biophys. Acta 1275, 161-203.
2. Moore, G.R. (1991) Bacterial 4­­helical bundle cytochromes. Biochim. Biophys. Acta 1058, 38-41.
3. Frolow, F., Kalb (Gilboa), A.J. and Yariv, J. (1994) Structure of a unique twofold symmetric haem­binding site. Nature Struct. Biol. 1, 453-460.
4. Le Brun, N.E., Andrews, S.C., Guest, J.R., Harrison, P.M., Moore, G.R. and Thomson, A.J. (1995) Identification of the ferroxidase centre of Escherichia coli bacterioferritin. Biochem. J. 312, 385-392.
5. Andrews, S.C., Le Brun, N.E., Barynin, V., Thomson, A.J., Moore, G.R., Guest, J.R. and Harrison, P.M. (1995) Site­directed replacement of the coaxial heme ligands of bacterioferritin generates heme­free variants. J. Biol. Chem. 270, 23268-23274.
6. Moore, G.R., Kadir, F.H.A., al­Massad, F.K., Le Brun, N.E., Thomson, A.J., Greenwood, C., Keen, J.N. and Findlay, J.B.C. (1994) Structural heterogeneity of Pseudomonas aeruginosa bacterioferritin. Biochem. J. 304, 493-497.
7. Carrano, C.J., Bohnke, R. and Matzanke, B.F. (1996) Fungal ferritins: the ferritin from mycelia of Absidia spinosa is a bacterioferritin. FEBS Lett. 390, 261-264.

Bibliography on structural studies of bacterioferritin