Structural studies of yeast iso-1 cytochrome c mutants by resonance Raman spectroscopy

Vibrational dynamics of iron in cytochrome C

Nuclear resonance vibrational spectroscopy (NRVS) and Raman spectroscopy on (54)Fe- and (57)Fe-enriched cytochrome c (cyt c) identify multiple bands involving vibrations of the heme Fe. Comparison with predictions from Fe isotope shifts reveals that 70% of the NRVS signal in the 300-450 cm(-1) frequency range corresponds to vibrations resolved in Soret-enhanced Raman spectra. This frequency range dominates the "stiffness", an effective force constant determined by the Fe vibrational density of states (VDOS), which measures the strength of nearest-neighbor interactions with Fe. The stiffness of the low-spin Fe environment in both oxidation states of cyt c significantly exceeds that for the high-spin Fe in deoxymyoglobin, where the 200-300 cm(-1) frequency range dominates the VDOS. This situation is reflected in the shorter Fe-ligand bond lengths in the former with respect to the latter. The longer Fe-S(Met80) in oxidized cyt c with respect to reduced cyt c leads to a decrease in the stiffness of the iron environment upon oxidation. Comparison with NRVS measurements allows us to assess assignments for vibrational modes resolved in this region of the heme Raman spectrum. We consider the possibility that the 372 cm(-1) band in reduced cyt c involves the Fe-S(Met80) bond.

Influence of conserved amino acids on the structure and environment of the heme of cytochrome c2. A resonance Raman study

Resonance Raman spectra using Soret excitations of oxidized and reduced Rhodobacter capsulatus cytochrome c2 at pH 7.5 were studied. The spectra of oxidized cytochrome c2 show three components for the v10 mode at 1638, 1633, and 1629 cm(-1). The intensities of these components are sensitive to the excitation wavelength. This effect is explained in the context of a conformational equilibrium of the ferriheme between a nearly planar structure and two ruffled structures. In the case of reduced cytochrome c2, the absolute frequencies as well as the excitation-dependent frequency dispersion of the v10 mode (1618-1621 cm(-1)) indicate a displacement of the conformational equilibrium of heme toward the more planar structures. To measure the influence of some key amino acid residues on the heme-protein interaction of cytochrome c2, four site-directed mutants of Rb. capsulatus cytochrome c2 have been studied by resonance Raman spectroscopy and their spectra compared with the spectra obtained for the wild type cytochrome. The mutants studied are K14E/K32E, P35A, W67Y, and Y75F. The spectral changes induced by the mutations are interpreted in terms of alterations in the structure and/or environment of the cytochrome c2 heme in the framework of the expected role of the different amino acid residues in the stability and redox potential.

Structural and functional effects of multiple mutations at distal sites in cytochrome c

Multiple mutations at distally located sites have been introduced into yeast iso-1 cytochrome c to determine the contributions of three amino acids to the structural and functional properties of this protein. The mutant proteins, for which high-resolution structures were determined, included all possible combinations of the substitutions Arg38Ala, Asn52Ile, and Phe82Ser. Arg38, Asn52, and Phe82 are all conserved in the primary sequences of eukaryotic cytochromes c and have been shown to significantly affect several properties of these proteins including protein stability, heme reduction potential, and oxidation state dependent conformational changes. The present studies show that the structural consequences of each amino acid substitution in combinatorial mutant proteins were similar to those observed in the related single-mutant proteins, and therefore no synergistic effect between mutation sites was observed for this feature. With respect to protein stability, the effect of individual mutations can be understood from the structural changes observed for each. It is found that stability effects of the three mutation sites are independent and cumulative in multiple-mutant proteins. This reflects the independent nature of the structural changes induced at the three distally located mutation sites. In terms of heme reduction potential two effects are observed. For substitution of Phe82 by serine, the mechanism by which reduction potential is lowered is different from that occurring at either the Arg38 or the Asn52 site and is independent of residue replacements at these latter two positions. For Arg38 and Asn52, overlapping interactions lead to a higher reduction potential than expected from a strict additive effect of substitutions at these residues. This appears to arise from interaction of these two amino acids with a common heme element, namely, the heme propionate A group. The present results underscore the difficulty of predicting synergistic effects of multiple mutations within a protein.

A heme c-peptide model system for the resonance Raman study of c-type cytochromes: characterization of the solvent-dependence of peptide-histidine-heme interactions

The visible absorption and Soret-excited resonance Raman spectra of ferrous microperoxidase-8 [MP8(II)], an octapeptide containing a heme c, are reported. These spectroscopies indicate that MP8(II), dissolved in aqueous buffered solutions, forms low-spin six-coordinated complexes in the 7-14 pH range. Intermolecular bonding interactions of MP8(II) in water account for this behavior. On the contrary, when the hemopeptide is dispersed in aqueous solutions containing detergent or an alcohol, the spectroscopic data show that the iron atom of MP8(II) is essentially high-spin five-coordinated in accordance with a monomeric structure of MP8(II). In addition to a high-spin signature to the heme skeletal modes, the high-frequency regions of resonance Raman spectra characterize an electronic influence of the thioether bridges on the frequency of stretching modes of C beta-C beta bonds (nu 2, nu 11, and nu 29). On the other hand, the low-frequency Raman spectra of monomeric MP8(II) at pH 7.5 present significant differences in the 150-250-cm-1 regions depending upon the solvent composition (pH, presence or absence of detergent, alcohol). These effects are attributed to frequency variations of the Fe-N(His)-involving mode which indicate changes in the H-bonding interactions of the axial His and therefore solvent-dependent changes of the octapeptide conformation. Our resonance Raman data further show that the axial His of monomeric MP8(II) could be totally deprotonated in aqueous cetyltrimethylammonium bromide solution at very alkaline pH (pKa = 13.3). The vibrational data (100-1700 cm-1) obtained for the various monomeric forms of MP8(II) are expected to be useful for determining the heme structure and environment in reduced c'-type cytochromes. Comparisons of resonance Raman data with X-ray crystallographic data available for different hemoproteins allow us to evaluate the ionization and H-bonding states of the His bound to the high-spin five-coordinated hemes. These data are discussed in terms of proximal influence of protein-His-heme interactions on the determination and the regulation of a particular biological function.

Laser flash photolysis studies of electron transfer mechanisms in cytochromes: an aromatic residue at position 82 is not required for cytochrome c reduction by flavin semiquinones or electron transfer from cytochrome c to cytochrome oxidase

The influence of an aromatic side chain at position 82 of yeast iso-1-cytochrome c on the kinetics of its electron transfer reactions has been investigated using laser flash photolysis methods to compare a series of site-specific mutant cytochromes in their reduction by free flavin semiquinone and in electron transfer from reduced cytochrome to bovine cytochrome c oxidase. Although small (approximately 10%) but significant differences are observed between some of the mutants (S82, Y82, I82) and wild-type (F82) or G82 cytochrome in the second-order rate constant for reduction by lumiflavin semiquinone, these do not correlate with side-chain aromaticity. In the reaction between the ferrocytochromes and cytochrome c oxidase, significantly larger deviations from exponentiality are found for those mutants having aliphatic residues at position 82 than for wild type or Y82. We interpret the nonexponential behavior in terms of multiple orientations of the cytochromes within the oxidase binding site; the extent to which this occurs is apparently influenced by the character of the residue at position 82. However, a comparison of the average rate constants for electron transfer to cytochrome oxidase for the various mutants reveals that all are closely comparable to WT, except for I82 which is significantly slower (approximately threefold). These results, combined with those obtained previously from steady-state kinetic and thermodynamic measurements, suggest that the observed differences among the mutants are due to alterations in the mode of binding of the cytochrome to the oxidase, rather than to a specific requirement for the presence of an aromatic group at position 82.