Protein-induced changes of the electronic structure of the heme

Abstract

For an understanding of the various functions of heme proteins it is important to comprehend how a particular protein structure controls the geometric and electronic properties of the heme (and vice versa). The flexible structure of the heme group allows it to exist in various structurally distinct forms depending on the electronic structure of the central Fe ion and on the particular contacts and constraints exercised on the heme by the protein. In studying proteinporphyriniron interactions it is particularly useful to direct one's attention to the symmetry properties of the heme and their changes caused by these interactions. As an example we shall compare heme in solution with heme in myoglobin (Mb) for the 3d 5 configuration of Fe. Information about the symmetry of the heme is provided by several spectroscopic methods; we shall discuss here mainly 1H NMR measurements. The hyperfine shifted magnetic resonances of protons belonging to the heme reflect the electronic properties of the central Fe ion and of the porphyrin [1, 2]. Data are available particularly on the 4 methyl resonances [3, 4], which appear as pairs and originate from methyls related by a twofold rotation axis and by a reflection plane respectively. The range of their chemical shifts, which are centered near −16 ppm (from TMS) in the ferric low spin and near −64 ppm in the ferric high spin states increases to the same extent as proteinheme interactions lower the heme symmetry. The actual symmetry of the heme iron may be approximated by C 4/gu symmetry with a triclinic perturbation. The range of the spread of the methyl resonances relative to the value of the magnetic susceptibility is larger for low spin than for high spin states. This indicates that the triclinic perturbation has a smaller influence on the electron distribution of the ferric high spin Fe than on that of the ferric low spin Fe. The electronic structures of the Fe ion in hemes and heme proteins as inferred from Mössbauer, ESR and magnetic susceptibility measurements [5] serve as a basis for interpreting the NMR results: Ferric high spin Fe in Mb(H 2O) has for C 4υ symmetry a spherically symmetric 6A 1 ground state and low lying 2E, 4A 2 and 2B 2 levels at about 1100, 1400 and 1700 cm −1. A small triclinic perturbation splits the 2E doublet into E + and E − levels separated by about 300 cm −1 in a first approximation. The electron distribution of these two levels is rhombic C 2υ. Only in a second approximation a triclinic perturbation mixes the 2B 2 level with the 2E ± levels which yields a triclinic electron distribution. The 2E ± levels admix to the 6A 1 ground state by spin-orbit coupling. The electron distribution is therefore predominantly spherically symmetric with a small rhombic component C 2υ. Ferric low spin Fe in Mb(CN) has for C 4υ symmetry a 2E ground state and low lying 2E 2 and 4A 2 levels at about 300 and 800 cm −1. A triclinic perturbation splits the 2E doublet into 2E + and the new ground state 2E +. The electron distribution is therefore always of rhombic symmetry C 2υ. It is intended to discuss also the symmetry properties of hemes and heme proteins for the 3d 6 configuration of Fe.