A Map of
Dielectric Heterogeneity in a Membrane Protein:
the Hetero-Oligomeric Cytochrome <i>b</i><sub>6</sub><i>f</i> Complex
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Abstract
The
cytochrome <i>b</i><sub>6</sub><i>f</i> complex,
a member of the cytochrome <i>bc</i> family that
mediates energy transduction in photosynthetic and respiratory membranes,
is a hetero-oligomeric complex that utilizes two pairs of <i>b</i>-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 <i>b</i>-hemes, <i>b</i><sub>n</sub> and <i>b</i><sub>p</sub>, 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 <i>b</i><sub>n</sub> on different monomers
would be preferentially reduced. Energetically preferred intramonomer
electron storage of electrons on the intramonomer <i>b</i>-hemes is found to require heterogeneity of interheme dielectric
constants. Relative to the medium separating the two higher potential
hemes <i>b</i><sub>n</sub>, a relatively large dielectric
constant must exist between the intramonomer <i>b</i>-hemes,
allowing a smaller electrostatic repulsion between the reduced hemes.
Heterogeneity of dielectric constants is an additional structure–function
parameter of membrane protein complexes