Haem and non-haem iron sites in Escherichia coli bacterioferritin: spectroscopic and model building studies.
The bacterioferritin (BFR) of Escherichia coli is an iron-storage protein containing 24 identical subunits and between three and 11 protohaem IX groups per molecule. Titration with additional haem gave a maximum loading of 12-14 haems per molecule. The e.p.r. spectra and magnetic c.d. spectra of the protein-bound haem show it to be low-spin Fe(III), and coordinated by two methionine residues as previously reported for BFRs isolated from Pseudomonas aeruginosa and Azotobacter vinelandii [Cheesman, Thomson, Greenwood, Moore and Kadir, Nature (London) (1990) 346, 771-773]. A recent sequence alignment indicated that BFR may be structurally related to ferritin. The molecular model proposed for E. coli BFR has a four-alpha-helix-bundle subunit conformation and a quaternary structure similar to those of mammalian ferritins. In this model there are two types of hydrophobic pocket within which two methionine residues are correctly disposed to bind haem. The e.p.r. spectra also reveal a monomeric non-haem Fe(III) species with spin, S = 5/2. On the basis of sequence comparisons, a ferroxidase centre has recently been proposed to be present in BFR [Andrews, Smith, Yewdall, Guest and Harrison (1991) FEBS Lett. 293, 164-168] and the possibility that this Fe(III) ion may reside at or near the ferroxidase centre is discussed.
Centre for Metalloprotein Spectroscopy and Biology, School of Chemical Sciences, University of East Anglia, Norwich, U.K.
M R
MR
Cheesman
N E
NE
le Brun
F H
FH
Kadir
A J
AJ
Thomson
G R
GR
Moore
S C
SC
Andrews
J R
JR
Guest
P M
PM
Harrison
J M
JM
Smith
S J
SJ
Yewdall
D000595:Amino Acid Sequence; D001426:Bacterial Proteins; D002570:Ceruloplasmin / Q000378:metabolism; D002942:Circular Dichroism; D003573:Cytochrome b Group / Q000737:chemistry / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D004926:Escherichia coli / Q000378:metabolism*; D005293:Ferritins / Q000737:chemistry / Q000378:metabolism*; D006418:Heme / Q000378:metabolism*; D006801:Humans; D007501:Iron / Q000378:metabolism*; D046911:Macromolecular Substances; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017510:Protein Folding; D016415:Sequence Alignment; D013056:Spectrophotometry, Ultraviolet
D001426:Bacterial Proteins; D003573:Cytochrome b Group; D046911:Macromolecular Substances; D006418:Heme; D005293:Ferritins; C018032:bacterioferritin; D007501:Iron; D002570:Ceruloplasmin