The electron transfer system of Pseudomonas aeruginosa: a study of the pH-dependent transitions between redox forms of azurin and cytochrome c551

Fast reaction kinetic experiments on the electron transfer reaction between azurin and cytochrome c₅₅₁ isolated from Pseudomonas aeruginosa confirmed the existence of two redox forms of reduced azurin previously reported. The pH dependence of the amplitudes of the relaxation processes observed in temperature jump experiments indicate that these two redox forms are in pH dependent equilibrium. The pH independence of the overall equilibrium constant indicates that redox active and inactive forms of cytochrome c₅₅₁ may also exist. Evidence that reduced cytochrome c₅₅₁ undergoes a pH transition is given by optical spectrophotometry. The nature of the transition is discussed in the context of recent nmr studies and in terms of the Marcus theory of electron transfer. The metabolic consequences of these transitions are also discussed.