Electron Transfer between Azurin from Alcaligenes faecalis and Cytochrome c 551 from Pseudomonas aeruginosa

The electon transfer equilibrium and kinetics between azurin grom Alcaaligenes faecalis and cytochrome c 551 from Pseudomonas aeruginosa have been studied. The equilibrium constant K {[Cyt(III)] · [Az(I)]}/{[Cyt(II)] · [Az(II)]} = 0.5 at 25°C is about seven times smaller than that observed between the cytochrome c 551 and the azurin both from P. aeruginosa [Rosen, P. and Pecht, I. (1976) Biochemistry,15, 775–786]. Potentiometric titrations confirmed a 43‐mV difference between the mid‐point potentials of +266 mV and +309m V for the Alcaligenes and Pseudomonas azurins respectively. The kinetics of the reaction between Alcaligenes azurin and Pseudomonas cytochrome c 551 were investigataed by the temperature‐jump chemical relaxation method. Only a single relaxation mode was observed throughout the range of concentrations and temperatures examined. Thus, the slow relaxation mode was observed throughout the range of concentrations and temperatures examined. Thus, the slow relaxation time observed in the reaction between. P. aeruginosa azurin and cytochrome c 551 is not observed with the Alcaligenes azurin. The simplest mechanism that can therefore be ascribed to the investigated system is:This scheme is similar to that proposed earlier for the reaction between P. aeruginosa azurin and cytochrome c 551 but does not involve the conformational transition proposed for azurin. The specific rates for the electron transfer are still fast: 1.8×10 6 M −1 · s −1 and 3.0×10 6 M −1 · s −1 respectively at 25°C.