EPR Analysis of Imidazole Binding to Methanosarcina acetivorans Protoglobin

Abstract

Protoglobins form an unusual class of globin proteins with surprising structural features, such as a highly ruffled heme. Electron paramagnetic resonance spectra of the frozen solution of the ferric imidazole complex of Methanosarcina acetivorans protoglobin reveal the existence of two low-spin ferric heme complexes. Hyperfine sublevel correlation spectroscopy was used to study the imidazole complex of this protoglobin as well as the imidazole complex of horse heart myoglobin for comparison. The spin Hamiltonian values obtained for the dominant contribution of the imidazole-ligated protoglobin agree with a heme conformation in which the plane of the exogenous imidazole is turned more than 30° versus the imidazole plane of the proximal histidine ligand. In turn, the electron paramagnetic resonance data of the minority complex indicate a second conformer, in which the two ligand planes are aligned almost parallel to each other with a lengthened iron–nitrogen bond for the exogenous imidazole ligand.