Comparing the Rates and the Activation Parameters for the Forward Reaction between the Triplet State of Zinc Cytochrome c and Cupriplastocyanin and the Back Reaction between the Zinc Cytochrome
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- 作者：Maja M. Ivkovi<img src="http ; //pubs.acs.org/images/entities/cacute.gif" border="0">-Jensen ; G. Matthias Ullmann ; Milan M. Crnogorac ; Mikael Ejdebä ; ck ; Simon Young ; Ö ; rjan Hansson ; and Nenad M.
- 刊名：Biochemistry
- 出版年：1999
- 出版时间：February 2, 1999
- 年：1999
- 卷：38
- 期：5
- 页码：1589 - 1597
- 全文大小：111K
- 年卷期：v.38,no.5(February 2, 1999)
- ISSN：1520-4995
文摘
This is a comparative study of the photoinduced (so-called forward) electron-transfer reaction3Zncyt/pc(II) <IMG SRC="/images/entities/rarr.gif"> Zncyt+/pc(I), between the triplet state of zinc cytochrome c (3Zncyt) and cupriplastocyanin[pc(II)], and the thermal (so-called back) electron-transfer reaction Zncyt+/pc(I) 
Zncyt/pc(II), betweenthe cation (radical) of zinc cytochrome c (Zncyt+) and cuproplastocyanin [pc(I)], which follows it. Bothreactions occur between associated (docked) reactants, and the respective unimolecular rate constants arekF and kB. Our previous studies showed that the forward reaction is gated by a rearrangement of thediprotein complex. Now we examine the back reaction and complare the two. We study the effects oftemperature (in the range 273.3-302.9 K) and viscosity (in the range 1.00-17.4 cP) on the rate constantsand determine enthalpies (
H
), entropies (S), and free energies (G) of activation. We comparewild-type spinach plastocyanin, the single mutants Tyr83Leu and Glu59Lys, and the double mutantGlu59Lys/Glu60Gln. The rate constant kB for wild-type spinach plastocyanin and its mutants markedlydepends on viscosity, an indication that the back reaction is also gated. The activation parameters Hand S show that the forward and back reactions have similar mechanisms, involving a rearrangementof the diprotein complex from the initial binding configuration to the reactive configuration. Therearrangements of the complexes 3Zncyt/pc(II) and Zncyt+/pc(I) that gate their respective reactions aresimilar but not identical. Since the back reaction of all plastocyanin variants is faster than the forwardreaction, the difference in free energy between the docking and the reactive configuration is smaller forthe back reaction than for the forward reaction. This difference is explained by the change in the electrostaticpotential on the plastocyanin surface as Cu(II) is reduced to Cu(I). It is the smaller H that makes Gsmaller for the back reaction than for the forward reaction.
Zncyt/pc(II), betweenthe cation (radical) of zinc cytochrome c (Zncyt+) and cuproplastocyanin [pc(I)], which follows it. Bothreactions occur between associated (docked) reactants, and the respective unimolecular rate constants arekF and kB. Our previous studies showed that the forward reaction is gated by a rearrangement of thediprotein complex. Now we examine the back reaction and complare the two. We study the effects oftemperature (in the range 273.3-302.9 K) and viscosity (in the range 1.00-17.4 cP) on the rate constantsand determine enthalpies (H), entropies (S), and free energies (G) of activation. We comparewild-type spinach plastocyanin, the single mutants Tyr83Leu and Glu59Lys, and the double mutantGlu59Lys/Glu60Gln. The rate constant kB for wild-type spinach plastocyanin and its mutants markedlydepends on viscosity, an indication that the back reaction is also gated. The activation parameters Hand S show that the forward and back reactions have similar mechanisms, involving a rearrangementof the diprotein complex from the initial binding configuration to the reactive configuration. Therearrangements of the complexes 3Zncyt/pc(II) and Zncyt+/pc(I) that gate their respective reactions aresimilar but not identical. Since the back reaction of all plastocyanin variants is faster than the forwardreaction, the difference in free energy between the docking and the reactive configuration is smaller forthe back reaction than for the forward reaction. This difference is explained by the change in the electrostaticpotential on the plastocyanin surface as Cu(II) is reduced to Cu(I). It is the smaller H that makes Gsmaller for the back reaction than for the forward reaction.