Reduction of horse heart ferricytochrome c by bovine liver ferrocytochrome b5. Experimental and theoretical analysis.
The reduction of horse heart ferricytochrome c by the tryptic fragment of bovine liver cytochrome b5 and its dimethyl ester heme (DME)-substituted derivative has been studied as a function of ionic strength, pH, and temperature under solution conditions where the reaction is bimolecular. The rate constant for ferricytochrome c reduction by native ferrocytochrome b5 is 1.8 (+/- 0.2) x 10(7) M-1 s-1 (25 degrees C) with delta H++ = 7.5 (+/- 0.2) kcal/mol and delta S++ = -0.3 (+/- 0.6) eu (pH 7.0, I = 0.348 M). Under the same solution conditions, the reduction of ferricytochrome c by DME-ferrocytochrome b5 proceeds with a rate constant of 1.7 (+/- 0.1) x 10(7) M-1 s-1 with delta H++ = 7.9 (+/- 0.4) kcal/mol and delta S++ = 1 (+/- 1) eu. The rate constants for both reactions are strongly dependent on ionic strength. A detailed electrostatic analysis of the proteins has been performed. Two relatively simple Brownian dynamics simulation models predict rate constants for the reaction between the two native proteins that demonstrate a dependence on ionic strength similar to that observed experimentally. In one of these models, the proteins are treated as spheres with reactive surface patches that are defined by a 5 degrees cone generated about the dipole vector calculated for each protein and aligned with the presumed electron-transfer site near the partially exposed heme edge. The second model replaces the reactive patch assumption with an exponential distance dependence for the probability of reaction that permits estimation of a value for the distance-dependence factor alpha. Calculations with this latter model in combination with the aligned dipole assumption provide a reasonable approximation to the observed ionic strength dependence for the reaction and are consistent with a value of alpha = 1.2 A-1.
Eltis
L D
LD
Department of Biochemistry, University of British Columbia, Vancouver, Canada.
Herbert
R G
RG
Barker
P D
PD
Mauk
A G
AG
Northrup
S H
SH
D000818:Animals; D002417:Cattle; D003574:Cytochrome c Group / Q000378:metabolism*; D015786:Cytochromes b5 / Q000378:metabolism*; D004058:Diffusion; D004579:Electron Transport; D006418:Heme / Q000378:metabolism; D006736:Horses; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D008954:Models, Biological; D008958:Models, Molecular; D009206:Myocardium / Q000201:enzymology*; D010084:Oxidation-Reduction; D013816:Thermodynamics
D003574:Cytochrome c Group; D006418:Heme; D015786:Cytochromes b5