title: Contribution of protein conformation to heme stereochemistry and reactivity. Low-temperature magnetic circular dichroism data.
abstract: Visible and near infrared magnetic circular dichroism (MCD) spectra of heme proteins and enzymes as well as those of a protein-free heme bound to 2-methylimidazole were recorded and compared at 4.2 K in unrelaxed metastable and relaxed equilibrium heme stereochemistry. The relaxed and unrelaxed stereochemistries of a 5-coordinate ferrous heme were generated by chemical reduction of iron at room temperature before freezing the sample and by photolysis of CO or O2 complexes at 4.2 K, respectively. The results are discussed in terms of a protein contribution into energies of the Fe-N epsilon(His) and Fe-N(pyrrols) bonds and their change on a ligand binding. We observed and analyzed cases of weak (myoglobin, hemoglobin) and strong (leghemoglobin, peroxidases) constraints imposed by the protein conformation on the proximal heme stereochemistry by comparing the bond energies in proteins with those in the protoheme-(2-methylimidazole) model compound. The role of a protein moiety in modulating the ligand binding properties of leghemoglobin and the heme reactivity of horseradish peroxidase is discussed.
authors.affiliation: Institute of Molecular Biology, USSR Academy of Sciences, Moscow.
authors.lastname: Sharonov YuA
authors.forename: V F
authors.initials: VF
authors.lastname: Pismensky
authors.forename: E G
authors.initials: EG
authors.lastname: Yarmola
mesh_terms: D002942:Circular Dichroism; D005916:Globulins; D006418:Heme; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D010544:Peroxidases; D011487:Protein Conformation; D013237:Stereoisomerism; D013696:Temperature
chemical_list: D005916:Globulins; D006418:Heme; D010544:Peroxidases