Functional Evaluation of Iron Oxypyriporphyrin in Protein Heme Pocket

The iron complex of oxypyriporphyrin, a porphyrinoid containing a keto-substituted pyridine, was coupled with apomyoglobin. The reconstituted ferric myoglobin was found to be five-coordinate without iron-bound water molecules. The anionic ligands such as CN− and N3 − bound the myoglobin with high affinities, while neutral imidazole did not. The IR observation indicated that the azide complex was pure high-spin, although the corresponding native protein was in the spin-state equilibrium. The reduced myoglobin was five-coordinate but exhibited no measurable affinity for O2. The affinity for CO was lowered down to 1/2400 as compared with native myoglobin. These anomalies were ascribed to the deformation in the iron coordination core after the replacement of one of the four pyrroles with a larger pyridine ring. The ligand binding analyses for the ferric and ferrous myoglobin suggest that the proximal histidine pulls the iron atom from the deformed core to reduce the interaction between the iron and exogenous ligands. Similarity of the reconstituted myoglobin with guanylate cyclase, a NO-responsive signaling hemoprotein, was pointed out.