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A map of dielectric heterogeneity in a membrane protein: the hetero-oligomeric cytochrome b6f complex.

Hasan SS, Zakharov SD, Chauvet A, Stadnytskyi V, Savikhin S, Cramer WA - J Phys Chem B (2014)

Bottom Line: Kinetic data imply that the most probable pathway for transfer of the two electrons needed for quinone oxidation-reduction utilizes this intramonomer heme pair, contradicting the expectation based on heme redox potentials and thermodynamics, that the two higher potential hemes bn on different monomers would be preferentially reduced.Energetically preferred intramonomer electron storage of electrons on the intramonomer b-hemes is found to require heterogeneity of interheme dielectric constants.Relative to the medium separating the two higher potential hemes bn, a relatively large dielectric constant must exist between the intramonomer b-hemes, allowing a smaller electrostatic repulsion between the reduced hemes.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences and ‡Department of Physics, Purdue University , West Lafayette, Indiana 47907, United States.

ABSTRACT
The cytochrome b6f complex, a member of the cytochrome bc family that mediates energy transduction in photosynthetic and respiratory membranes, is a hetero-oligomeric complex that utilizes two pairs of b-hemes in a symmetric dimer to accomplish trans-membrane electron transfer, quinone oxidation-reduction, and generation of a proton electrochemical potential. Analysis of electron storage in this pathway, utilizing simultaneous measurement of heme reduction, and of circular dichroism (CD) spectra, to assay heme-heme interactions, implies a heterogeneous distribution of the dielectric constants that mediate electrostatic interactions between the four hemes in the complex. Crystallographic information was used to determine the identity of the interacting hemes. The Soret band CD signal is dominated by excitonic interaction between the intramonomer b-hemes, bn and bp, on the electrochemically negative and positive sides of the complex. Kinetic data imply that the most probable pathway for transfer of the two electrons needed for quinone oxidation-reduction utilizes this intramonomer heme pair, contradicting the expectation based on heme redox potentials and thermodynamics, that the two higher potential hemes bn on different monomers would be preferentially reduced. Energetically preferred intramonomer electron storage of electrons on the intramonomer b-hemes is found to require heterogeneity of interheme dielectric constants. Relative to the medium separating the two higher potential hemes bn, a relatively large dielectric constant must exist between the intramonomer b-hemes, allowing a smaller electrostatic repulsion between the reduced hemes. Heterogeneity of dielectric constants is an additional structure-function parameter of membrane protein complexes.

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Summary of possible electrontransfer routes and heme reductionstates in the b6f complex.Two different models are considered: (A) n1–n2 model: the doublyreduced state of lowest energy of the dimer corresponds to two electronsresiding on the two bn hemes belongingto different subunits (this state produces a weak negative CD signal(Figure 3). (B) n–p model: the lowestdoubly reduced state of the dimer corresponds to two electrons residingon the bn and bp hemes belonging to the same subunit (in this state, the amplitudeof the positive CD signal is significantly larger than that of anyother heme pair). The sequence of four electron transfer events inthese two models is illustrated. Reduced hemes are shown as red spheres.State N0 in panels A and B denotes fully oxidized hemesin dimeric complex. Ni represent statesof the dimeric complex in which subscript “i” represents the number of reduced hemes. States N1 and N3 are bypassed if dithionite acts as a 2 e– donor. (C) Summary of conceivable two electron half-reduced states,of which the three states marked by “X” are inferredto be substantially less probable, although they have been documentedto exist (refs (14), (15), (17), (19), (21), and (26)).
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fig5: Summary of possible electrontransfer routes and heme reductionstates in the b6f complex.Two different models are considered: (A) n1–n2 model: the doublyreduced state of lowest energy of the dimer corresponds to two electronsresiding on the two bn hemes belongingto different subunits (this state produces a weak negative CD signal(Figure 3). (B) n–p model: the lowestdoubly reduced state of the dimer corresponds to two electrons residingon the bn and bp hemes belonging to the same subunit (in this state, the amplitudeof the positive CD signal is significantly larger than that of anyother heme pair). The sequence of four electron transfer events inthese two models is illustrated. Reduced hemes are shown as red spheres.State N0 in panels A and B denotes fully oxidized hemesin dimeric complex. Ni represent statesof the dimeric complex in which subscript “i” represents the number of reduced hemes. States N1 and N3 are bypassed if dithionite acts as a 2 e– donor. (C) Summary of conceivable two electron half-reduced states,of which the three states marked by “X” are inferredto be substantially less probable, although they have been documentedto exist (refs (14), (15), (17), (19), (21), and (26)).

Mentions: “n–n” model (Figure 5A): the lowest doubly reducedstate of the dimer corresponds to two electrons residing on the two bn hemes on the electrochemically negative sideof the membrane that belong to different subunits. In that case, threeelectron transfer steps are needed to observe a significant increasein the CD signal that would arise from a reduced n–p pair.


A map of dielectric heterogeneity in a membrane protein: the hetero-oligomeric cytochrome b6f complex.

Hasan SS, Zakharov SD, Chauvet A, Stadnytskyi V, Savikhin S, Cramer WA - J Phys Chem B (2014)

Summary of possible electrontransfer routes and heme reductionstates in the b6f complex.Two different models are considered: (A) n1–n2 model: the doublyreduced state of lowest energy of the dimer corresponds to two electronsresiding on the two bn hemes belongingto different subunits (this state produces a weak negative CD signal(Figure 3). (B) n–p model: the lowestdoubly reduced state of the dimer corresponds to two electrons residingon the bn and bp hemes belonging to the same subunit (in this state, the amplitudeof the positive CD signal is significantly larger than that of anyother heme pair). The sequence of four electron transfer events inthese two models is illustrated. Reduced hemes are shown as red spheres.State N0 in panels A and B denotes fully oxidized hemesin dimeric complex. Ni represent statesof the dimeric complex in which subscript “i” represents the number of reduced hemes. States N1 and N3 are bypassed if dithionite acts as a 2 e– donor. (C) Summary of conceivable two electron half-reduced states,of which the three states marked by “X” are inferredto be substantially less probable, although they have been documentedto exist (refs (14), (15), (17), (19), (21), and (26)).
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Related In: Results  -  Collection

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fig5: Summary of possible electrontransfer routes and heme reductionstates in the b6f complex.Two different models are considered: (A) n1–n2 model: the doublyreduced state of lowest energy of the dimer corresponds to two electronsresiding on the two bn hemes belongingto different subunits (this state produces a weak negative CD signal(Figure 3). (B) n–p model: the lowestdoubly reduced state of the dimer corresponds to two electrons residingon the bn and bp hemes belonging to the same subunit (in this state, the amplitudeof the positive CD signal is significantly larger than that of anyother heme pair). The sequence of four electron transfer events inthese two models is illustrated. Reduced hemes are shown as red spheres.State N0 in panels A and B denotes fully oxidized hemesin dimeric complex. Ni represent statesof the dimeric complex in which subscript “i” represents the number of reduced hemes. States N1 and N3 are bypassed if dithionite acts as a 2 e– donor. (C) Summary of conceivable two electron half-reduced states,of which the three states marked by “X” are inferredto be substantially less probable, although they have been documentedto exist (refs (14), (15), (17), (19), (21), and (26)).
Mentions: “n–n” model (Figure 5A): the lowest doubly reducedstate of the dimer corresponds to two electrons residing on the two bn hemes on the electrochemically negative sideof the membrane that belong to different subunits. In that case, threeelectron transfer steps are needed to observe a significant increasein the CD signal that would arise from a reduced n–p pair.

Bottom Line: Kinetic data imply that the most probable pathway for transfer of the two electrons needed for quinone oxidation-reduction utilizes this intramonomer heme pair, contradicting the expectation based on heme redox potentials and thermodynamics, that the two higher potential hemes bn on different monomers would be preferentially reduced.Energetically preferred intramonomer electron storage of electrons on the intramonomer b-hemes is found to require heterogeneity of interheme dielectric constants.Relative to the medium separating the two higher potential hemes bn, a relatively large dielectric constant must exist between the intramonomer b-hemes, allowing a smaller electrostatic repulsion between the reduced hemes.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences and ‡Department of Physics, Purdue University , West Lafayette, Indiana 47907, United States.

ABSTRACT
The cytochrome b6f complex, a member of the cytochrome bc family that mediates energy transduction in photosynthetic and respiratory membranes, is a hetero-oligomeric complex that utilizes two pairs of b-hemes in a symmetric dimer to accomplish trans-membrane electron transfer, quinone oxidation-reduction, and generation of a proton electrochemical potential. Analysis of electron storage in this pathway, utilizing simultaneous measurement of heme reduction, and of circular dichroism (CD) spectra, to assay heme-heme interactions, implies a heterogeneous distribution of the dielectric constants that mediate electrostatic interactions between the four hemes in the complex. Crystallographic information was used to determine the identity of the interacting hemes. The Soret band CD signal is dominated by excitonic interaction between the intramonomer b-hemes, bn and bp, on the electrochemically negative and positive sides of the complex. Kinetic data imply that the most probable pathway for transfer of the two electrons needed for quinone oxidation-reduction utilizes this intramonomer heme pair, contradicting the expectation based on heme redox potentials and thermodynamics, that the two higher potential hemes bn on different monomers would be preferentially reduced. Energetically preferred intramonomer electron storage of electrons on the intramonomer b-hemes is found to require heterogeneity of interheme dielectric constants. Relative to the medium separating the two higher potential hemes bn, a relatively large dielectric constant must exist between the intramonomer b-hemes, allowing a smaller electrostatic repulsion between the reduced hemes. Heterogeneity of dielectric constants is an additional structure-function parameter of membrane protein complexes.

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