A Map of Dielectric Heterogeneity in a Membrane Protein: the Hetero-Oligomeric Cytochrome b 6 f Complex

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

Structural aspects of the cytochromeb 6 f complex; structure of the lumen-side domain of cytochromef

The following findings concerning the structure of the cytochrome b(6)f complex and its component polypeptides, cyt b(6), subunit IV and cytochrome f 1. Comparison of the amino acid sequences of 13 and 16 cytochrome b(6) and subunit IV polypeptides, respectively, led to (a) reconsideration of the helix lengths and probable interface regions, (b) identification of two likely surface-seeking helices in cyt b(6) and one in SU IV, and (c) documentation of a high degree of sequence invariance compared to the mitochondrial cytochrome. The extent of identity is particularly high (88% for conserved and pseudo- conserved residues) in the segments of cyt b(6) predicted to be extrinsic on the n-side of the membrane. 2. The intramembrane attractive forces between trans-membrane helices that normally stabilize the packing of integral membrane proteins are relatively weak. 3. The complex isolated in dimeric form has been visualized, along with isolated monomer, by electron microscopy. The isolated dimer is much more active than the monomer, is the major form of the complex isolated and purified from chloroplasts, and is inferred to be a functional form in the membrane. 4. The isolated cyt b(6)f complex contains one molecule of chlorophyll a. 5. The structure of the 252 residue lumen-side domain of cytochrome f isolated from turnip chloroplasts has been solved by X-ray diffraction analysis to a resolution of 2.3 Å