Bis-methionine axial ligation of haem in bacterioferritin from Pseudomonas aeruginosa

Abstract

The iron-containing bacterioferritins contain the protoporphyrin IX haem group. It has been established that Escherichia coli cytochrome b1, cytochrome b557 and bacterioferritin are identical. The optical spectra at room temperature of the haem group show it to be predominantly low-spin in both the ferrous and ferric states. The nature of the axial ligands binding the haem group to the polypeptide has, however, remained unknown. Low-spin, bis-coordinate haem centres in proteins typically have a role in rapid electron transfer as redox changes at the metal ion lead to little structural rearrangement. There are only four amino acids with side-chains that have ligand field strengths sufficient to generate the low-spin state of haem, namely, histidine, lysine, methionine and cysteine. Hence there are, potentially, ten different pairs of these four ligands which could be discovered in electron transfer haemoproteins. To date only three have been established with certainty. They are bis-histidine, as in mammalian cytochrome b5, methionine-histidine, typified by cytochrome c and lysine-histidine, recently recognized by spectroscopic methods in cytochrome f. Here we report the electron paramagnetic resonance and near infrared magnetic circular dichroism spectra of the oxidized state of Ps. aeruginosa bacterioferritin which enable the axial ligands to be identified as the thioether side chains of two methionine residues, a ligation scheme not previously reported for haem in any protein.

Fungal ferritins: The ferritin from mycelia of Absidia spinosa is a bacterioferritin

“…These data in combination with the observation of a heme spectrum in the purified protein leave no doubt that the ferritin isolated from A. spinosa mycelia is a bacterioferritin. Since it is has been proposed that the heine groups in Bfrs have bismethionine ligation [25,26], it is perhaps noteworthy that amongst all the Bfr sequences determined up to now, only one Met is absolutely conserved, i.e. the N-terminal Met-1 [27].…”

Overproduction, purification and characterization of the bacterioferritin of Escherichia coli and a C‐terminally extended variant

“…Watt, personnal communication cited in Moore et al, 1992). The reason for the difference in haem capacity is unknown, although the haem pocket is likely to be equivalent in the bacterioferritins of P. aeruginosa, and E. coli and A. vinelandii, since the proteins appear to be structurally related and the spectral properties of their haem groups are very similar (Cheesman et al, 1990;1993).…”

Kinetic and structural characterization of an intermediate in the biomineralization of bacterioferritin

“…EPR spectroscopy has been used previously [ The EPR spectrum of an aerobic sample of apo-BFR frozen approximately 2 min after the addition of 50 Fe(I1) ions per BFR molecule contains, in addition to the haem group resonances at g = 2.88, 2.31, and 1.46 [22], a high intensity signal at g = 4.28 (Fig. 5a).…”

“…by 2 methionine residues [22], whilst the ferritins, as isolated, do not contain haem. The formation of non-haem iron cores by the ironstorage proteins ferritin [l-8] and bacterioferritin [9-1 I], of magnetite particles by magnetotactic organisms , and of goethite-and lepidocrocite-containing teeth by marine organisms [I 51, is a subject of considerable biological and chemical interest.…”