Spectroscopic identification of the haem axial ligands of haemoferritin and location of possible haem-binding sites in ferritin by molecular modelling.
Horse spleen ferritin will bind up to 16 protoporphyrin IX haem groups per 24 subunits in vitro [Kadir & Moore (1990) FEBS Lett. 276, 81-84] at a site that causes the haem to be low spin for both ferric and ferrous states. E.p.r. spectra at 10 K of the oxidized form of the resulting haemoferritin gives g values of 2.93, 2.26 and 1.55, characteristic of low-spin haem. The near-i.r. magnetic circular dichroism spectrum shows a porphyrin-to-ferric charge-transfer band at 1590 nm. The spectroscopic parameters indicate that the haem group is probably bound by two histidine ligands. Molecular modelling studies reveal one type of potential haem-binding site in horse L-chain ferritin with bis-histidine co-ordination. This is an intersubunit site which lies in a pocket within the ferritin protein shell in the region of the 3-fold channel. The ligands are His-114 and His-124 in horse L-chain. A second possible set of sites in human H-chain ferritin involves His-60 residues in the pockets between pairs of subunits. These are considered less likely sites of haem occupancy. There are three of the intersubunit sites in horse L-chain ferritin at each of the eight 3-fold channels. We propose that conformational crowding between haem-binding sites at a given channel prevents more than two haems per channel being bound.
Moore
G R
GR
Centre for Metalloprotein Spectroscopy and Biology, School of Chemical Sciences, University of East Anglia, Norwich, U.K.
Cheesman
M R
MR
Kadir
F H
FH
Thomson
A J
AJ
Yewdall
S J
SJ
Harrison
P M
PM
D000818:Animals; D001665:Binding Sites; D002942:Circular Dichroism; D004578:Electron Spin Resonance Spectroscopy; D005293:Ferritins / Q000737:chemistry / Q000378:metabolism*; D006418:Heme / Q000737:chemistry / Q000378:metabolism*; D006420:Hemeproteins / Q000737:chemistry / Q000378:metabolism*; D006736:Horses; D006801:Humans; D007501:Iron / Q000737:chemistry; D008958:Models, Molecular; D017433:Protein Structure, Secondary; D011524:Protoporphyrins / Q000737:chemistry / Q000378:metabolism; D013057:Spectrum Analysis
D006420:Hemeproteins; D011524:Protoporphyrins; C073694:haemoferritin; D006418:Heme; D005293:Ferritins; C028025:protoporphyrin IX; D007501:Iron