The co-ordination of ligands by iron porphyrins: a comparison of ligand binding by myoglobin from sperm whale and the haem undecapeptide from cytochrome c

Abstract

The effect of the protein on the ligand binding properties of a ferric porphyrin was investigated by determining spectrophotometrically the temperature dependence of the binding constants, log K_eq, and hence the thermodynamic parameters ΔH and ΔS, for coordination of F⁻, N₃⁻, SCN⁻, CN⁻ and imidazole by the haemundecapeptide from cytochrome c, known as N-acetyl microperoxidase-11 (NAcMP11), and ferric myoglobin from sperm whale. The axial binding site of the iron porphyrin in NAcMP11 is relatively open to the solvent, but in metMb it is buried in the protein and surrounded by tightly-packed distal amino acid residues. Whereas the N-donor ligands ethanolamine, glycine, pyridine, ammonia and imidazole are readily coordinated by NAcMP11, only the latter is coordinated by metMb, although the affinity of the haemoprotein for imidazole is nearly two orders of magnitude smaller than for NAcMP11 because of a much less favourable reaction enthalpy. Examination of the crystal structures of metMb and its imidazole complex suggests that steric factors are largely responsible. Of all the halides, only F⁻ is coordinated by metMb. In contrast with imidazole, the anionic ligands N₃⁻, SCN⁻, F⁻ and CN⁻ are coordinated more strongly by metMb than by NAcMP11. In the case of the first two, the difference in affinity is enthalpically driven, although there is a compensating entropic effect; this is attributed to the difference in the polarity of the environment of the porphyrin, with the relatively apolar environment of the haemoprotein stabilising the complex due to an electrostatic interaction between the anionic ligand and the residual positive charge at the metal centre. The effect is not generally true for all anionic ligands. Thus, although both F⁻ and CN⁻ are coordinated more strongly by metMb than by NAcMP11, the difference in affinity now stems largely from an entropic effect. Hence, whereas very significant differences in affinity for an exogenous ligand exist between the relatively exposed ferric porphyrin of the haempeptide and the buried ferric porphyrin of the haemoprotein, such differences cannot be attributed to a single factor only.