Kinetics of electron transfer between cytochrome c and laccase

Rate constants have been determined for the electron-transfer reactions between reduced horse heart cytochrome c and resting Rhus vernicifera laccase as a function of pH, ionic strength, and temperature. The second-order rate constant for the oxidation of reduced cytochrome c was determined to be k = 125 M-1 s-1 at 25 degrees C in 0.2 M phosphate buffer at pH 6.0 with the activation parameters delta H++ = 16.2 kJ mol-1 and delta S++ = 28.9 J mol-1 K-1. The rate constants increased with decreasing buffer concentration, indicating that electron transfer from cytochrome c to laccase is favored by the local electrostatic interaction (ZAZB = -0.9 at pH 6 and -1.3 at pH 4.8) between the basic proteins with positive net charges. From the increase of the rate of electron transfer with decreasing pH, one of the driving forces of the reaction was suggested to be the difference in the redox potentials between the type 1 copper in laccase and the central iron in cytochrome c. Further, on addition of one hexametaphosphate anion per cytochrome c molecule, the rate of the electron transfer was increased, probably because the association of both proteins became more favorable.