Redox equilibrium of sperm-whale myoglobin, Aplysia myoglobin, and Chironomus thummi hemoglobin

The reaction of Fe(III) myoglobin fluoride with reducing agents

The reaction of reducing agents with Fe(III) myoglobin fluoride from sperm whale was studied at alkaline pH values. The rate of reduction by dithionite was indistinguishable from the rate of ligand dissociation even when the values of the rate constants for both were only 1.0 X 10(-3)S-1 (at pH 10.6). Reduction by the reduced Methyl Viologen radical ion and reduced Safranine was faster than the rate of dissociation, providing evidence that these reductants can donate electrons to the iron centre via a pathway involving an (undetectable) liganded Fe(II) intermediate.

The reduction by dithionite of Fe(III) myoglobin derivatives with different ligands attached to the iron atom. A study by rapid-wavelength-scanning stopped-flow spectrophotometry

1. The reductions of a number of sperm whale Fe(III) myoglobin-ligand complexes by sodium dithionite in a phosphate buffer pH 6.4, were investigated by using rapid-wavelength-scanning stopped-flow spectrophotometry. The ligands were azide, cyanide, fluoride, imidazole, thiocyanate and water. 2. The reduction of Fe(III) myoglobin cyanide led to the transient formation of Fe(II) myoglobin cyanide but no intermediate species were observable during the reductions of the other derivatives. The final product of the reaction in all cases was unliganded Fe(II)myoglobin. 3. Invesigation of the effect of dithionite concentration on the rate of reduction indicated that the SO2- radical ion was the active species in reducing the azide, cyanide, fluoride and thiocyanate derivatives. 4. Comparison of the observed rates of reduction at different ligand concentrations with those predicted for a pathway of reduction involving prior dissociation of the ligand, allowed us to estimate the rate of reduction with the ligand in position (outer-sphere reduction). There was a large variation in the relative rates of outer-sphere reduction in the order imidazole greater than CN- greater than SCN- greater than N3- greater than F-. The fluoride derivative was so resistant to outer-sphere reduction that the reaction with SO2- proceeded only by a pathway involving dissociation of F- before reduction. It was calculated that any direct reduction of this complex was at least 100 times slower than that of the azide derivative. 5. The results are discussed in terms of the possible rôle of the axial ligands in haem proteins and it is suggested that the pathway of the electron to the Fe(III) centre may be via the ppi orbitals of these ligands.

Heme-linked proton dissociation of carbon monoxide complexes of myoglobin and peroxidase

It was found from spectrophotometric titration and proton balance measurement that the pKa value of a heme-linked protonation group of horseradish ferro-peroxidase C (donor:H2O2 oxidoreductase, EC 1.11.1.7) shifted from 7.25 to 8.25 upon combination with CO. The spectrophotometric titration experiment with myoglobin also revealed the presence of a heme-linked protonation group, the pKa value being 5.57 in myoglobin and 5.67 in the CO-myoglobin complex. It was concluded that the distinct shift of the pKa value in the case of peroxidase was attributable to the presence of a hydrogen bond between the sixth ligand and the distal base. The difference in the strength of such hydrogen bonding between peroxidase and myoglobin was discussed.