KINETIC-STUDIES OF ELECTRON-TRANSFER IN THE CYT B(5)METHB SYSTEM

The kinetics of methemoglobin reduction by cytochrome b(5) has been st udied by stopped-flow and saturation transfer NMR. A forward rate cons tant k(f) = 2.44 x 10(4) M(-1) s(-1) and a reverse rate constant k(b) = 540 M(-1) have been observed at 10 mM, pH 6.20, 25 degrees C. The ra tio k(f)/k(b) = k(eq) = 43.6 is in good agreement with the equilibrium constant calculated from the electrochemical potential between cyt b( 5) and methemoglobin. A bimolecular collisional mechanism is proposed for the electron transfer from cyt b(5) to methemoglobin based on the kinetic data analysis. The dependence of the rate constants on ionic s trengths supports such collisional mechanism. It is also found that th e reaction rate strongly depends on the conformations of methemoglobin .