[Effect of porphyrin ring ligands on the affinity of heme iron to axial ligands].

To clarify the influence of protein surrounding on the heme reactivity in heme proteins the effect of interaction between a porphyrin ring and pi-acceptor molecule, 1,2,4-trimethyl-pyridinium (TMP), on the affinity of deuteroheme to axial ligands (imidazole and cyanide) has been studied as a model system. It is shown that TMP induces the fourfold decrease in equilibrium constant of imidazole to deuteroheme. From the analysis of the two stages for cyanide binding it is concluded that TMP decreases the binding constant of the first cyanide by 40 times and does not apparently influence the second ligand binding. The effect of TMP on the reactivity of deuteroheme to axial ligands is interpreted as a result of a decrease in the electron density on the iron orbitals which is due to the altered pi-eleectron density in the porphyrin pi-system through the donor-acceptor interaction with TMP molecules. The possible significance of the contacts between the porphyrin and neighboring amino acid residues in determining heme affinity to axial ligands is discussed.