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
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- 作者：Maja M. Ivkovi<img src="http ; //pubs.acs.org/images/entities/cacute.gif" border="0">-Jensen and and Nenad M. Kosti<img src="http ; //pubs.acs.org/images/entities/cacute.gif" border="0">
- 刊名：Biochemistry
- 出版年：1996
- 出版时间：November 26, 1996
- 年：1996
- 卷：35
- 期：47
- 页码：15095 - 15106
- 全文大小：549K
- 年卷期：v.35,no.47(November 26, 1996)
- ISSN：1520-4995
文摘
This is a study of the effects of temperature (in therange 273.3-307.7 K) and of ionic strength(in the range 2.5-100 mM) on the kinetics of photoinducedelectron-transfer reaction 3Zncyt/pc(II)<IMG SRC="/images/entities/rarr.gif">Zncyt+/pc(I) within the electrostatic complex ofzinc cytochrome c and cupriplastocyanin at pH 7.0.Inorder to separate direct and indirect effects of temperature on therate constants, viscosity of the solutionswas fixed, at different values, by additions of sucrose. Theactivation parameters for the reaction withinthe preformed complex, at the low ionic strength, are
H 
= 13 ± 2 kJ/mol andS = -97 ± 4 J/Kmol. The activation parameters for the reaction within theencounter complex, at the higher ionic strength,are H = 13 ± 1 kJ/mol andS = -96 ± 3 J/K mol.Evidently, the two complexes are the same.The proteins associate similarly in the persistent and thetransient complex, i.e., at different ionic strengths.In both complexes, however, electron transfer is gated by arearrangement, as previous studies from thislaboratory showed. Changes in the solution viscosity modulate thisrearrangement by affecting H ,notS . The activation parameters areanalyzed by empirical methods. The thermodynamicparametersH and S for the formation of the complexZncyt/pc(II) are determined and related to changes inhydrophilic and hydrophobic surfaces upon protein association in threeconfigurations. A differencebetween the values of H for the configuration providingoptimal electronic coupling between the redoxsites and the configuration providing optimal docking equals theexperimental value H = 13kJ/molfor the rearrangement of the latter configuration into the former.Enthalpy of activation may reflect achange in the character of the exposed surface as the diprotein complexrearranges. Entropy of activationmay reflect tightening of the contact between the associatedproteins.
H = 13 ± 2 kJ/mol andS = -97 ± 4 J/Kmol. The activation parameters for the reaction within theencounter complex, at the higher ionic strength,are H = 13 ± 1 kJ/mol andS = -96 ± 3 J/K mol.Evidently, the two complexes are the same.The proteins associate similarly in the persistent and thetransient complex, i.e., at different ionic strengths.In both complexes, however, electron transfer is gated by arearrangement, as previous studies from thislaboratory showed. Changes in the solution viscosity modulate thisrearrangement by affecting H ,notS . The activation parameters areanalyzed by empirical methods. The thermodynamicparametersH and S for the formation of the complexZncyt/pc(II) are determined and related to changes inhydrophilic and hydrophobic surfaces upon protein association in threeconfigurations. A differencebetween the values of H for the configuration providingoptimal electronic coupling between the redoxsites and the configuration providing optimal docking equals theexperimental value H = 13kJ/molfor the rearrangement of the latter configuration into the former.Enthalpy of activation may reflect achange in the character of the exposed surface as the diprotein complexrearranges. Entropy of activationmay reflect tightening of the contact between the associatedproteins.