Electron transfer between azurin from Alcaligenes faecalis and cytochrome c551 from Pseudomonas aeruginosa

The electron transfer equilibrium and kinetics between azurin from Alcaligenes faecalis and cytochrome c551 from Pseudomonas aeruginosa have been studied. The equilibrium constant K = ([Cyt(III)] . [Az(I)])/([Cyt(II)] . [Az(II))]) = 0.5 at 25 degrees C is about seven times smaller than that observed between the cytochrome c551 and the titrations confirmed a 43-mV difference between the mid-point potentials of +266 mV and +309 mV for the Alcaligenes and Pseudomonas azurins respectively. The kinetics of the reaction between Alcaligenes azurin and Pseudomonas cytochrome c551 were investigated 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 time observed in the reaction between P. aeruginosa azurin and cytochrome c551 is not observed with the Alcaligenes azurin. The simplest mechanism that can therefore be ascribed to the investigated system is: [formula: see text]. This scheme is similar to that proposed earlier for the reaction between P. aeruginosa azurin and cytochrome c551 but does not involve the conformational transition proposed for azurin. The specific rates for the electron transfer are still fast: 1.8 x 10(6) M-1 . s-1 and 3.0 x 10(6) M-1 . s-1 respectively at 25 degrees C.