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

Abstract

THE iron-containing bacterioferritins^1–6 contain the protoporphyrin IX haem group⁵. It has been established that Escherichia coli cytochrome b₁, cytochrome b₅₅₇ and bacterioferritin are identical⁷. The optical spectra at room temperature of the haem group show it to be predominantly low-spin in both the ferrous and ferric states⁸. 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 rearrangement⁹. 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 transferhaemoproteins. To date only three have been established with certainty. They are bis-histidine, as in mammalian cytochrome b₅ (ref. 10), methionine-histidine, typified by cytochrome c (ref. 11) and lysine-histidine, recently recognized by spectroscopic methods in cytochrome f (ref. 12). 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.