Changes in the spin state and reactivity of cytochrome c induced by photochemically generated singlet oxygen and free radicals

Abstract

This work compares the effect of photogenerated singlet oxygen (O 2(1Δg)) (type II mechanism) and free radicals (type I mechanism) on cytochrome c structure and reactivity. Both reactive species were obtained by photoexcitation of methylene blue (MB +) in the monomer and dimer forms, respectively. The monomer form is predominant at low dye concentrations (up to 8 φM) or in the presence of an excess of SDS micelles, while dimers are predominant at 0.7 mM SDS. Over a pH range in which cytochrome c is in the native form, O2( 1σg) and free radicals induced a Soret band blue shift (from 409 to 405 nm), predominantly. EPR measurements revealed that the blue shift of the Soret band was compatible with conversion of the heme iron from its native low spin state to a high spin state with axial symmetry (g ∼ 6.0). Soret band bleaching, due to direct attack on the heme group, was only detected under conditions that favored free radical production (MB+ dimer in SDS micelles) or in the presence of a less structured form of the protein (above pH 9.3). Matrix-assisted laser desorption ionization time-of-flight mass spectrometry of the heme group and the polypeptide chain of cytochrome c with Soret band at 405 nm (cytc405) revealed no alterations in the mass of the cytc405 heme group but oxidative modifications on methionine (Met65 and Met80) and tyrosine (Tyr74) residues. Damage of cytc405 tyrosine residue impaired its reduction by diphenylacetaldehyde, but not by β-mercaptoethanol, which was able to reduce cytc405, generating cytochrome c Fe(II) in the high spin state (spin 2).