- Email: [email protected]
Dynamic processes gating electron-transfer reactions of plastocyanin with cytochromes
Journal of Inorganic Biochemistry
Q02
ELECTRON AND ENERGY TRANSFER
399
DYNAMIC PROCESSES GATING ELECTRON-TRANSFER REACTIONS OF PLASTOCYANIN WITH CYTOCHROMES Nenad M. Kosti~
Department of Chemistry, Iowa State University, Gilman Hall, Ames, IA 50011, U.S.A. We study electron-transfer reactions within the 1:1 complexes that the blue-copper protein plastocyanin forms with the heme proteins cytochromef, and cytochrome c. The ground-state reaction (eq 1) is fast in the electrostatic complexes but undetectably slow in the covalent complexes, in which the cyt(II)/pc(II) ~ cyt(III)/pc(I) (1) docking configuration of the two proteins is reinforced by noninvasive covalent cross-links [1-3]. The proteins recognize each other in one configuration but react in a different one, which optimizes the electronic coupling between the heme and copper sites [4, 5]. Cross-links prevent the rearrangement between the configurations. Thorough calculations of protein-protein interactions showed that in such studies water of hydration must be included, conformational flexibility must be allowed, and Coulombic interactions must not be overemphasized [5]. Noninvasive [6] replacement of Fe(II) with Zn(II) in cytochrome c allows study of photoinduced forward (subscripts F and f) and back (subscripts B and b) reactions between associated (eq 2) and separate (eq 3) proteins. Contributions of the unimolecular (eq 2) and bimolecular (eq 3) reactions are controlled by 3Zncyt/pc(ii)
l~ v~
Zncyt+/pc(I)
kB v~
Zncyt/pc(I)
(2)
3Zncyt + pc(H) kf v'--- Zncyt + + pc(I) 1% ~"Zncyt + pc(II) (3) adjusting the ionic strength [7, 8]. Covalent cross-links slow down, but do not abolish, the unimolecular reactions in eq 2; evidently, cytochrome c reduces the Cu(II) site from some position within the broad acidic patch on the plastocyanin surface [8, 9]. Indeed, analysis in terms of van Leeuwen theory of the dependence of the bimolecular rate constants (eq 3) on ionic strength confirmed that in the reactive configuration the basic patch surrounding the exposed heine edge in cytochrome c interacts with the acidic patch in plastocyanin [ 10]. The persistent diprotein complex at low ionic strength and the transient diprotein complex at high ionic strength are the same [11]. Because the photoinduced electron-transfer reaction with the driving force of 1.2 eV is very fast, the protein rearrangement, which is slower, becomes the rate-limiting step in the reaction k F. This rate constant decreases smoothly as the solution viscosity is raised [12, 13]. Fitting of this dependence to a model involving an unreactive initial configuration and reactive rearranged configuration yielded the rate constant for the rearrangement of 2 x 105 sl [ 12, 13]. Comparison of cupriplastocyanin and ferricytochrome b 5 as electron acceptors from 3Zncyt showed that the rearrangement of the electrostatic diprotein complex amounts to configurational fluctuation of Zncyt within the acidic patch of plastocyanin, not to the large migration of Zncyt towards the hydrophobic patch [ 14]. Analysis of the temperature effects on the rate constant k F yielded the parameters AH* and AS*. The former reflects a change in the exposed protein surface as the diprotein complex rearranges. The latter reflects tightening of the contact between the associated proteins [ 11]. Kinetic effects and noneffects of nine single and double mutations of amino acids on the plastocyanin surface revealed that, in the ratedetermining protein rearrangement, the basic patch of cytochrome c moves from a position near the center of the acidic patch to a position at or near the upper edge of this patch [ 15]. (1) L. M. Peerey, H. M. Brothers, II., J. T. Hazzard, G. Tollin, and N. M. Kosti6, Biochemistry 1991, 30, 9297-9304. (2) L. Qin and N. M. Kosti6, Biochemistry 1992, 31, 5145-5150. (3) L. Qin and N. M. Kosti6, Biochemistry 1993, 32, 6073-6080. (4) G. M. Ullmann and N. M. Kosti6, J. Am. Chem. Soc. 1995, 117, 4766-4774. (5) G. M. Ullmann, E.-W. Knapp, and N. M. Kosti6, J. Am. Chem. Soc. 1997, 119, 42-52. (6) S. Ye, C. Shen, T. M. Cotton, and N. M. Kosti6, J. Inorg. Biochem. 1997, 65, 219-226. (7) J. S. Zhou and N. M. Kosti6, J. Am. Chem. Soc. 1991, 113, 6067-6073. (8) J. S. Zhou & N. M. Kosti6, J. Am. Chem. Soc. 1991, 113, 7040-7042. (9) J. S. Zhou, H. M. Brothers II, J. P. Neddersen, L. M. Peerey, T. M. Cotton, and N. M. Kosti6, Bioconjugate Chem. 1992, 3, 382-390. (10) J. S. Zhou and N. M. Kosti6, Kosti6, Biochemistry 1993, 32, 4539-4546. (11) M. Ivkovi6 -Jensen and N. M. Kosti6, Biochemistry 1996, 35, 15095-15106. (12) J. S. Zhou and N. M. Kosti6, J. Am. Chem. Soc. 1992, 114, 3562-3563. (13) J. S. Zhou and N. M. Kosti6, J. Am. Chem. Soc. 1993, 115, 10796-10804. (14) L. Qin and N. M. Kosti6, Biochemistry 1994, 33, 12592-12599. (15) M. M. Cmogorac, C. Shen, S. Young, O. Hansson, and N. M. Kosti6, Biochemistry 1996, 35, 16465-16474.
Q02
ELECTRON AND ENERGY TRANSFER
399
DYNAMIC PROCESSES GATING ELECTRON-TRANSFER REACTIONS OF PLASTOCYANIN WITH CYTOCHROMES Nenad M. Kosti~
Department of Chemistry, Iowa State University, Gilman Hall, Ames, IA 50011, U.S.A. We study electron-transfer reactions within the 1:1 complexes that the blue-copper protein plastocyanin forms with the heme proteins cytochromef, and cytochrome c. The ground-state reaction (eq 1) is fast in the electrostatic complexes but undetectably slow in the covalent complexes, in which the cyt(II)/pc(II) ~ cyt(III)/pc(I) (1) docking configuration of the two proteins is reinforced by noninvasive covalent cross-links [1-3]. The proteins recognize each other in one configuration but react in a different one, which optimizes the electronic coupling between the heme and copper sites [4, 5]. Cross-links prevent the rearrangement between the configurations. Thorough calculations of protein-protein interactions showed that in such studies water of hydration must be included, conformational flexibility must be allowed, and Coulombic interactions must not be overemphasized [5]. Noninvasive [6] replacement of Fe(II) with Zn(II) in cytochrome c allows study of photoinduced forward (subscripts F and f) and back (subscripts B and b) reactions between associated (eq 2) and separate (eq 3) proteins. Contributions of the unimolecular (eq 2) and bimolecular (eq 3) reactions are controlled by 3Zncyt/pc(ii)
l~ v~
Zncyt+/pc(I)
kB v~
Zncyt/pc(I)
(2)
3Zncyt + pc(H) kf v'--- Zncyt + + pc(I) 1% ~"Zncyt + pc(II) (3) adjusting the ionic strength [7, 8]. Covalent cross-links slow down, but do not abolish, the unimolecular reactions in eq 2; evidently, cytochrome c reduces the Cu(II) site from some position within the broad acidic patch on the plastocyanin surface [8, 9]. Indeed, analysis in terms of van Leeuwen theory of the dependence of the bimolecular rate constants (eq 3) on ionic strength confirmed that in the reactive configuration the basic patch surrounding the exposed heine edge in cytochrome c interacts with the acidic patch in plastocyanin [ 10]. The persistent diprotein complex at low ionic strength and the transient diprotein complex at high ionic strength are the same [11]. Because the photoinduced electron-transfer reaction with the driving force of 1.2 eV is very fast, the protein rearrangement, which is slower, becomes the rate-limiting step in the reaction k F. This rate constant decreases smoothly as the solution viscosity is raised [12, 13]. Fitting of this dependence to a model involving an unreactive initial configuration and reactive rearranged configuration yielded the rate constant for the rearrangement of 2 x 105 sl [ 12, 13]. Comparison of cupriplastocyanin and ferricytochrome b 5 as electron acceptors from 3Zncyt showed that the rearrangement of the electrostatic diprotein complex amounts to configurational fluctuation of Zncyt within the acidic patch of plastocyanin, not to the large migration of Zncyt towards the hydrophobic patch [ 14]. Analysis of the temperature effects on the rate constant k F yielded the parameters AH* and AS*. The former reflects a change in the exposed protein surface as the diprotein complex rearranges. The latter reflects tightening of the contact between the associated proteins [ 11]. Kinetic effects and noneffects of nine single and double mutations of amino acids on the plastocyanin surface revealed that, in the ratedetermining protein rearrangement, the basic patch of cytochrome c moves from a position near the center of the acidic patch to a position at or near the upper edge of this patch [ 15]. (1) L. M. Peerey, H. M. Brothers, II., J. T. Hazzard, G. Tollin, and N. M. Kosti6, Biochemistry 1991, 30, 9297-9304. (2) L. Qin and N. M. Kosti6, Biochemistry 1992, 31, 5145-5150. (3) L. Qin and N. M. Kosti6, Biochemistry 1993, 32, 6073-6080. (4) G. M. Ullmann and N. M. Kosti6, J. Am. Chem. Soc. 1995, 117, 4766-4774. (5) G. M. Ullmann, E.-W. Knapp, and N. M. Kosti6, J. Am. Chem. Soc. 1997, 119, 42-52. (6) S. Ye, C. Shen, T. M. Cotton, and N. M. Kosti6, J. Inorg. Biochem. 1997, 65, 219-226. (7) J. S. Zhou and N. M. Kosti6, J. Am. Chem. Soc. 1991, 113, 6067-6073. (8) J. S. Zhou & N. M. Kosti6, J. Am. Chem. Soc. 1991, 113, 7040-7042. (9) J. S. Zhou, H. M. Brothers II, J. P. Neddersen, L. M. Peerey, T. M. Cotton, and N. M. Kosti6, Bioconjugate Chem. 1992, 3, 382-390. (10) J. S. Zhou and N. M. Kosti6, Kosti6, Biochemistry 1993, 32, 4539-4546. (11) M. Ivkovi6 -Jensen and N. M. Kosti6, Biochemistry 1996, 35, 15095-15106. (12) J. S. Zhou and N. M. Kosti6, J. Am. Chem. Soc. 1992, 114, 3562-3563. (13) J. S. Zhou and N. M. Kosti6, J. Am. Chem. Soc. 1993, 115, 10796-10804. (14) L. Qin and N. M. Kosti6, Biochemistry 1994, 33, 12592-12599. (15) M. M. Cmogorac, C. Shen, S. Young, O. Hansson, and N. M. Kosti6, Biochemistry 1996, 35, 16465-16474.