| Abstract
| - The reduction potentials of beef heart cytochrome c and cytochromes c2 from Rhodopseudomonas palustris, Rhodobacter sphaeroides, and Rhodobacter capsulatus were measured through directelectrochemistry at a surface-modified gold electrode as a function of temperature in nonisothermalexperiments carried out at neutral and alkaline pH values. The thermodynamic parameters for proteinreduction (ΔS°rc and ΔH°rc) were determined for the native and alkaline conformers. Enthalpy and entropyterms underlying species-dependent differences in E° and pH- and temperature-induced E° changes for agiven cytochrome were analyzed. The difference of about +0.1 V in E° between cytochromes c2 and theeukaryotic species can be separated into an enthalpic term (−ΔΔH°rc/F) of +0.130 V and an entropicterm (TΔΔS°rc/F) of −0.040 V. Hence, the higher potential of the bacterial species appears to be determinedentirely by a greater enthalpic stabilization of the reduced state. Analogously, the much lower potentialof the alkaline conformer(s) as compared to the native species is by far enthalpic in origin for both proteinfamilies, and is largely determined by the substitution of Met for Lys in axial heme ligation. Instead, thebiphasic E°/temperature profile for the native cytochromes is due to a difference in reduction entropybetween the conformers at low and high temperatures. Temperature-dependent 1H NMR experimentssuggest that the temperature-induced transition also involves a change in orientation of the axial methionineligand with respect to the heme plane.
|
