Direct observation of bis-sulfur ligation to the heme of bacterioferritin

X-ray absorption spectroscopy was used to examine the ligation of the heme of Azobacter vinelandii bacterioferritin scrupulously cleaned of non-heme iron. We find that the iron of the protoporphyrin IX of the protein has two axial sulfur ligands at 2.35 A, with four nitrogen ligands at 1.97 A. This result confirms the previous suggestion of Cheesman et al. (Nature 1990, 346, 771-773; Biochem. J. 1992, 786, 361-367.), on the basis of less direct spectroscopic techniques, that the heme of bacterioferritin is ligated by a pair of methionine ligands. To date, this mode of coordination is unknown in any other heme protein