Effects of Temperature on the Kinetics of the Gated Electron-Transfer Reaction between Zinc Cytochrome c and Plastocyanin. Analysis of Configurational Fluctuation of the Diprotein Complex

• Published in: Biochemistry (Easton) Vol. 35; no. 47; pp. 15095 - 15106
• Main Authors: Ivković-Jensen, Maja M, Kostić, Nenad M
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 26.11.1996
• Subjects: Cytochrome c Group - chemistry; Cytochromes c; Electron Transport; Enzyme Activation; Hydrogen-Ion Concentration; Ions; Kinetics; Photochemistry; Plastocyanin - chemistry; Temperature; Viscosity
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi961608g

This is a study of the effects of temperature (in the range 273.3−307.7 K) and of ionic strength (in the range 2.5−100 mM) on the kinetics of photoinduced electron-transfer reaction 3Zncyt/pc(II) → Zncyt+/pc(I) within the electrostatic complex of zinc cytochrome c and cupriplastocyanin at pH 7.0. In order to separate direct and indirect effects of temperature on the rate constants, viscosity of the solutions was fixed, at different values, by additions of sucrose. The activation parameters for the reaction within the preformed complex, at the low ionic strength, are ΔH  ⧧ = 13 ± 2 kJ/mol and ΔS  ⧧ = −97 ± 4 J/K mol. The activation parameters for the reaction within the encounter complex, at the higher ionic strength, are ΔH  ⧧ = 13 ± 1 kJ/mol and ΔS  ⧧ = −96 ± 3 J/K mol. Evidently, the two complexes are the same. The proteins associate similarly in the persistent and the transient complex, i.e., at different ionic strengths. In both complexes, however, electron transfer is gated by a rearrangement, as previous studies from this laboratory showed. Changes in the solution viscosity modulate this rearrangement by affecting ΔH  ⧧, not ΔS  ⧧. The activation parameters are analyzed by empirical methods. The thermodynamic parameters ΔH and ΔS for the formation of the complex Zncyt/pc(II) are determined and related to changes in hydrophilic and hydrophobic surfaces upon protein association in three configurations. A difference between the values of ΔH for the configuration providing optimal electronic coupling between the redox sites and the configuration providing optimal docking equals the experimental value ΔH  ⧧ = 13 kJ/mol for the rearrangement of the latter configuration into the former. Enthalpy of activation may reflect a change in the character of the exposed surface as the diprotein complex rearranges. Entropy of activation may reflect tightening of the contact between the associated proteins.