Manganese (III) phthalocyanine-substituted cytochrome c

Manganese phthalocyanine-substituted cytochrome c has been prepared by the reaction of Mn(III) tetrasulfonated phthalocyanine with apocytochrome c in acetate buffer, pH 5.8. Its structure and properties have been investigated by difference spectroscopy, circular dichroism (cd), electron paramagnetic resonance (epr), electrophoresis, molecular weight estimation, and potentiometric measurements. The epr and spectroscopic data show that the manganese phthalocyanine-substituted cytochrome c represents the low spin, six-coordinated. Mn(Ill) complex with the metal ion in the plane of the phthalocyanine ring. The sixth ligand, which is coordinated axially to the metal ion, is probably the methionine-80. Electrophoresis and molecular weight studies show this complex to be a monomer. As is shown by cd experiments, Mn(III)L-apocyt has a more ordered structure than that of apocytochrome c. Its conformation is, however, significantly altered compared to native cytochrome c. The manganese(III)-phthalocyanine complex is able to combine with cyanide. The cyanide derivative gives a stable reduced form upon dithionite reduction. If, however, Mn(IlI)Lapocyt is reduced with dithionite before addition of cyanide, it loses its ability to coordinate with cyanide. Nitric oxide reacts with the manganese(III) complex to form, in all probability, the nitrosyl derivative. The half-reduction potential of Mn(IlI)L-apocyt is about +400 mV, and the complex is reduced by cytochrome c. Spectroscopic data suggest that the mechanism of this process is complicated.