Optical Rotatory Dispersion of Cytochrome c

The conformational manifold of ferricytochrome c explored by visible and far-UV electronic circular dichroism spectroscopy

The oxidized state of cytochrome c is a subject of continuous interest, owing to the multitude of conformations which the protein can adopt in solution and on surfaces of artificial and cell membranes. The structural diversity corresponds to a variety of functions in electron transfer, peroxidase and apoptosis processes. In spite of numerous studies, a comprehensive analysis and comparison of native and non-native states of ferricytochrome c has thus far not been achieved. This results in part from the fact that the influence of solvent conditions (i.e., ionic strength, anion concentration, temperature dependence of pH values) on structure, function and equilibrium thermodynamics has not yet been thoroughly assessed. The current study is a first step in this direction, in that it provides the necessary experimental data to compare different non-native states adopted at high temperature and alkaline pH. To this end, we employed visible electronic circular dichroism (ECD) and absorption spectroscopy to probe structural changes of the heme environment in bovine and horse heart ferricytochrome c as a function of temperature between 278 and 363 K at different neutral and alkaline pH values. A careful selection of buffers enabled us to monitor the partial unfolding of the native state at room temperature while avoiding a change to an alkaline state at high temperatures. We found compelling evidence for the existence of a thermodynamic intermediate of the thermal unfolding/folding process, termed III h, which is structurally different from the alkaline states, IV 1 and IV 2, contrary to current belief. At neutral or slightly acidic pH, III h is populated in a temperature region between 320 and 345 K. The unfolded state of the protein becomes populated at higher temperatures. The ECD spectra of the B-bands of bovine and horse heart cytochrome c (pH 7.0) exhibit a pronounced couplet that is maintained below 343 K, before protein unfolding replaces it by a rather strong positive Cotton band. A preliminary vibronic analysis of the B-band profile reveals that the couplet reflects a B-band splitting of 350 cm (-1), which is mostly of electronic origin, due to the internal electric field in the heme cavity. Our results suggest that the conformational transition from the native state, III, into a thermally activated intermediate state, III h, does not substantially affect the internal electric field and causes only moderate rearrangements of the heme pocket, which involves changes, rather than a rupture, of the Fe (3+)-M80 linkage. In the unfolded state, as well as in the alkaline states IV and V, the band splitting is practically eliminated, but the positive Cotton effect observed for the B-band suggests that the proximal environment, encompassing H18 and the two cysteine residues 14 and 17, is most likely still intact and covalently bound to the heme chromophore. Both alkaline states IV and V were found to melt via intermediate states. Unfolded states probed at neutral and alkaline pH can be discriminated, owing to the different intensities of the Cotton bands of the respective B-band transitions. Differences between the ECD intensities of the B-bands of the different unfolded states and alkaline states most likely reflect different degrees of openness of the corresponding heme crevice.

Heme-Peptide Models for Hemoproteins. 2. N-Acetylmicroperoxidase-8: Study of the pi-pi Dimers Formed at High Ionic Strength Using a Modified Version of Molecular Exciton Theory

AcMP8 is the Cys-14-acetylated water-soluble heme-octapeptide fragment obtained proteolytically from cytochrome c. Two successive dimerization equilibria are observed with increasing ionic strength in aqueous solution at neutral pH (part 1, preceding article). The electronic spectra of the two pi-pi dimers were extracted from the absorption envelopes at 2.01 and 4.02 M ionic strength and resolved by Gaussian analysis. The principal transitions were assigned using a tailored version of molecular exciton theory based on coupling of the main x- and y-polarized transition dipole moments of the interacting heme groups. The spectra of both pi-pi dimers indicate that the y-polarized exciton states are blue-shifted relative to the excited states of the monomer, while the x-polarized exciton states exhibit a red shift. These shifts were correctly predicted by a simple dipole-dipole coupling model. From an analysis of the resultant transition dipole moments to the exciton states with B(x)()(0,0) and B(y)()(0,0) character and the magnitudes of their red and blue exciton shifts, respectively, we have determined the dipole-dipole interaction geometries for both dimers. The principal difference between the interaction geometry in the first dimer and that in the second is a stronger interaction for the y-polarized transition dipoles and somewhat weakened interaction for the x-polarized transition dipoles. From an analysis of available crystallographic data for porphyrin and metalloporphyrin pi-pi dimers (Scheidt, W. R.; Lee, Y. J. Struct. Bonding 1987, 64, 1) and the results of our exciton model, we conclude that the origin of the coordinate system for the Soret transition dipole moments of AcMP8 is not metal-centered. Furthermore, since the true directions of the x- and y-axes of the low-symmetry heme chromophore in AcMP8 are unknown, we have not been able to determine the structures of the pi-pi dimers from a knowledge of their transition dipole-dipole interaction geometries. This study therefore highlights one of the shortfalls of molecular exciton theory.