ELECTRON-TRANSFER SELF-EXCHANGE KINETICS OF CYTOCHROME B5

The electron-transfer self-exchange rate constant of trypsin-solubilized bovine liver microsomal cytochrome b5 has been measured as a function of temperature and ionic strength. Calculations based on 1H NMR spectra and using the inversion-recovery method determined this rate constant to be 2.6 X 103 M-1 s-1 [pH 7.0, μ = 0.1 M (sodium phosphate), 25 °C] with ΔH* = 5.5 kcal mol-1 and ΔS* = -23 eu (μ = 0.1-0.3 M). This rate constant increases with ionic strength, reaching a value of 4.5 X 104 M-1 s-1 at μ = 1.5 M. Analysis of the data in terms of Marcus theory gives a reorganization energy, λ, for self-exchange in the range 0.9-1.3 eV mol-1. The components of the dipole moments through the exposed heme edge of the reduced and oxidized protein are estimated to be -280 and -250 D, respectively. © 1990, American Chemical Society. All rights reserved.