pH Variation of midpoint potential for three photosynthetic bacterial cytochromes c′. A link between physical and functional properties

The coordination of imidazole and substituted pyridines by the hemeoctapeptide N-acetyl-ferromicroperoxidase-8 (FeIINAcMP8)

The N-terminus acetylated ferric hemeoctapeptide from cytochrome c, N-acetylmicroperoxidase-8 (Fe(III)-NAcMP8) can be reduced by dithionite in aqueous solution to produce Fe(II)-NAcMP8. The UV-Vis spectrum has a broad Soret band and relatively poorly defined Q bands which is consistent with a mixture of a five-coordinate high spin species with His as the axial ligand and a six-coordinate, predominantly high spin species with His/H(2)O as axial ligands. There are two spectroscopically observable pK(a)s at 8.7+/-0.1 and 10.9+/-0.2 which are attributed to ionization of a heme propionic acid group and coordinated H(2)O, respectively; a pK(a) > or = 14 is due to ionization of the proximal His ligand. Equilibrium constants were determined by UV-Vis spectrophotometry at 25.0+/-0.2 degrees C and 0.5 M ionic strength (NaClO(4)) for the coordination of imidazole and a number of substituted pyridines, and complement available data for the ferric hemepeptide, allowing a comparison to be made of the affinity of an iron porphyrin with Fe in the +2 and +3 oxidation states towards these ligands. Imidazole is coordinated more strongly by the ferric porphyrin (log K=4.08) than by the ferrous porphyrin (log K=3.40). The equilibrium constants for coordination of pyridines by the ferric and ferrous porphyrins increase and decrease, respectively, with increasing ligand basicity. Values determined by cyclic voltammetry show the same dependence on the identity of the ligand. In the ferric porphyrin, the stability of the complex increases with the basicity of the ligand and hence its ability to donate electron density onto the metal. In the case of the more electron rich ferrous porphyrin, greater stability occurs with pyridine ligands that have an electron withdrawing group and hence can accept electron density from the metal. This is consistent with the midpoint reduction potentials E(1/2) of the pyridine complexes determined by cyclic voltammetry; E(1/2) is linearly dependent on, and becomes more negative with an increase in, ligand basicity. Log K for coordination of pyridines by the ferrous hemepeptide correlates well with the energy of the ligand frontier orbital with pi symmetry, suggesting that pi-bonding effects are significant in determining the strength of binding of pyridines by a ferrous porphyrin.

Cloning, heterologous expression, and characterization of recombinant class II cytochromes c from Rhodopseudomonas palustris

The cytochrome (cyt) c', cyt c(556), and cyt c(2) genes from Rhodopseudomonas palustris have been cloned; recombinant cyt c' and cyt c(556) have been expressed, purified, and characterized. Unlike mitochondrial cyt c, these two proteins are structurally similar to cyt b(562), in which the heme is embedded in a four-helix bundle. The hemes in both recombinant proteins form covalent thioether links to two Cys residues. UV/vis spectra of the Fe(II) and Fe(III) states of the recombinant cyts are identical with those of the corresponding native proteins. Equilibrium unfolding measurements in guanidine hydrochloride solutions confirm that native Fe(II)-cyt c(556) is more stable than the corresponding state of Fe(III)-cyt c(556) (DeltaDeltaG(f)(o) =22 kJ/mol).

Cytochrome c' folding triggered by electron transfer: fast and slow formation of four-helix bundles

Reduced (Fe(II)) Rhodopseudomonas palustris cytochrome c' (Cyt c') is more stable toward unfolding ([GuHCl](1/2) = 2.9(1) M) than the oxidized (Fe(III)) protein ([GuHCl](1/2) = 1.9(1) M). The difference in folding free energies (Delta Delta G(f) degrees = 70 meV) is less than half of the difference in reduction potentials of the folded protein (100 mV vs. NHE) and a free heme in aqueous solution ( approximately -150 mV). The spectroscopic features of unfolded Fe(II)-Cyt c' indicate a low-spin heme that is axially coordinated to methionine sulfur (Met-15 or Met-25). Time-resolved absorption measurements after CO photodissociation from unfolded Fe(II)(CO)-Cyt c' confirm that methionine can bind to the ferroheme on the microsecond time scale [k(obs) = 5(2) x 10(4) s(-1)]. Protein folding was initiated by photoreduction (two-photon laser excitation of NADH) of unfolded Fe(III)-Cyt c' ([GuHCl] = 2.02--2.54 M). Folding kinetics monitored by heme absorption span a wide time range and are highly heterogeneous; there are fast-folding ( approximately 10(3) s(-1)), intermediate-folding (10(2)-10(1) s(-1)), and slow-folding (10(-1) s(-1)) populations, with the last two likely containing methionine-ligated (Met-15 or Met-25) ferrohemes. Kinetics after photoreduction of unfolded Fe(III)-Cyt c' in the presence of CO are attributable to CO binding [1.4(6) x 10(3) s(-1)] and Fe(II)(CO)-Cyt c' folding [2.8(9) s(-1)] processes; stopped-flow triggered folding of Fe(III)-Cyt c' (which does not contain a protein-derived sixth ligand) is adequately described by a single kinetics phase with an estimated folding time constant of approximately 4 ms [Delta G(f) degrees = -33(3) kJ mol(-1)] at zero denaturant.

Replacement of lysine 45 by uncharged residues modulates the redox-Bohr effect in tetraheme cytochrome c3 of Desulfovibrio vulgaris (Hildenborough)

The structural basis for the pH dependence of the redox potential in the tetrahemic Desulfovibrio vulgaris (Hildenborough) cytochrome c3 was investigated by site-directed mutagenesis of charged residues in the vicinity of heme I. Mutation of lysine 45, located in the neighborhood of the propionates of heme I, by uncharged residues, namely threonine, glutamine and leucine, was performed. The replacement of a conserved charged residue, aspartate 7, present in the N-terminal region and near heme I was also attempted. The analysis of the redox interactions as well as the redox-Bohr behavior of the mutated cytochromes c3 allowed the conclusion that residue 45 has a functional role in the control of the pKa of the propionate groups of heme I and confirms the involvement of this residue in the redox-Bohr effect.

pH dependence of structural and functional properties of oxidized cytochrome c" from Methylophilus methylotrophus

Cytochrome c" from Methylophilus methylotrophus is an unusual monoheme protein that undergoes a major redox-linked change in the heme arrangement: one of the two axial histidines bound to the iron in the oxidized form is detached upon reduction and a proton is taken up. The kinetics of reduction by sodium dithionite and the spectroscopic properties of the oxidized cytochrome c" have been investigated over the pH range between 1.4 and 10.0. The rate of reduction displays proton-linked transitions of pKa congruent with 5.5 and 2.4, and a spectroscopic transition with a pKa congruent with 2.4 is also observed. The protein displays a complete reversibility after exposure to low pH, and both electronic absorption and resonance Raman spectroscopic properties suggest that the transition at lower pH brings about a drastic change in the heme coordination geometry. Circular dichroism spectra indicate that over the same proton-linked transition, the protein undergoes a marked decrease (approximately 60%) of the alpha-helical content toward a random coil arrangement, which is recovered upon increasing the ionic strength. The structural change at low pH is linked to a concerted two-proton transition, suggesting the detachment and protonation of axial histidine(s). Such kinetic and spectroscopic features along with the remarkable capacity of this protein to recover its native structure after exposure to extremely low pH values makes it a promising model for studying folding processes and stability in heme proteins.

Heme-Peptide Models for Hemoproteins. 1. Solution Chemistry of N-Acetylmicroperoxidase-8

An improved method for the preparation of the heme octapeptide acetyl-MP8, obtained by proteolysis of horse heart cytochrome c, is described. AcMP8 obeys Beer's law at pH 7.0 in aqueous solution up to a concentration of 3 x 10(-)(5) M. The self-association constant measured at 25 degrees C (log K(D) = 4.04) is an order of magnitude lower than that for MP8, reflecting the role of the N-acetyl protecting group in abolishing intermolecular coordination. However, AcMP8 does form pi-stacked dimers in aqueous solution with increasing ionic strength. A more weakly packed pi-pi dimer reaches a maximum abundance at approximately 3 M ionic strength, but a more tightly packed dimer is favored at &mgr; > 3 M. An equilibrium model based on charge neutralization by specific binding of Na(+) ions gives a total molecular charge of 3- for AcMP8 at pH 7.0 and a self-association constant log K(D) = 4.20. AcMP8 exhibits six spectroscopically active pH-dependent transitions. The Glu-21 c-terminal carboxylate binds to the heme iron at low pH (pK(a) = 2.1) but is substituted by His-18 (pK(a) = 3.12) as the pH increases. The two heme propanoic acid substituents ionize with pK(a)'s of 4.95 and 6.1. This is followed by ionization of iron-bound water with a pK(a) = 9.59, DeltaH = 48 +/- 1 kJ mol(-)(1), and DeltaS = -22 +/- 3 J K(-)(1) mol(-)(1). The electronic spectra indicate that AcMP8 is predominantly in the S = (5)/(2) state at pH 7.0, while the hydroxo complex at pH 10.5 corresponds to an equilibrium mixture of S = (5)/(2) and S = (1)/(2) states at 25 degrees C. In the final transition, His-18 ionizes to form the S = (1)/(2) histidinate complex with a pK(a) of 12.71. AcMP8 is relatively stable under alkaline conditions, dimerizing slowly at high pH (k = 2.59 +/- 0.14 M(-)(1) s(-)(1)) to form a high-spin &mgr;-oxo-bridged species. The pH-dependent behavior of AcMP8 in the presence of excess 3-cyanopyridine, however, is markedly different. At low pH, AcMP8 simultaneously binds the exogenous ligand and the Glu-21 c-terminal carboxylate with a pK(a) < 2. His-18 replaces the carboxylate ligand at higher pH (pK(a) = 2.60), and both heme propanoic acid groups ionize with a mean pK(a) = 5.10. Unlike AcMP8.OH(-), the axial histidine of the 3-CNPy complex ionizes at near neutral pH (pK(a) = 7.83), prior to being replaced by OH(-) (pK(a) = 10.13). The sixth transition in the AcMP8/3-CNPy system produces the bis(hydroxo) complex (pK(a) > 13).

Concerted movement of side chains in the haem vicinity observed on ligand binding in cytochrome c' from rhodobacter capsulatus

We have determined the structure of n-butylisocyanide-bound Rhodobacter capsulatus cytochrome c'. This is the first example of a ligand-bound structure of a class IIa cytochrome c. Compared with the structure of native cytochrome c', there are significant conformational changes of amino acid residues in the haem vicinity, accompanied by a rearrangement of the hydrogen bonding pattern. The results suggest that rearrangements resulting from ligand binding could drive dimer dissociation in some species and also that the haem propionate may participate in proton transfer.

Regulation of the redox order of four hemes by pH in cytochrome c3 from D. vulgaris Miyazaki F

The assignment of 1H-NMR signals of the heme methyl and propionate groups of cytochrome c3 of D. vulgaris Miyazaki F was performed. The heme assignment was revised for hemes 2 and 3 (sequential heme numbering). Namely, heme 4 is mainly reduced at first with hemes 1, 2 and 3 following it in this order. The p2H titration of heme methyl signals in four macroscopic oxidation states was performed in the p2H range of 5.2 to 9.0. While the heme methyl resonances in the fully oxidized state showed just small changes with p2H, most resonances in the intermediate oxidation states revealed clear p2H dependence. In particular, the methyl resonances of heme 1 shifted significantly in the acidic region. Then, the chemical shifts of beta-CH2 (next to the carboxyl group) of all propionate groups in the fully oxidized state were observed at various p2H in the range of 4.5 to 9.0. Only the propionate group at C-13 (IUPAC-IUB nomenclature) of heme 1 showed a clear change in this p2H range, its titration curve being similar to those of the methyl resonances of heme 1 in the intermediate oxidation states. pKa of the propionate group was 5.95 +/- 0.05. Analysis of the microscopic formal redox potentials was carried out for the observations at p2H 5.2, 7.1 and 9.0. The redox potentials of heme 1 showed the most remarkable p2H dependence, resulting in the change of the order of the redox potentials of four hemes. A significant change was also found in the interacting potential between hemes 1 and 2. In the light of the p2H-titration experiments, the propionate at C-13 of heme 1 was identified as the most plausible ionizable group responsible for the p2H dependence of microscopic redox potentials of heme 1 in the acidic region.

Involvement of a labile axial histidine in coupling electron and proton transfer in Methylophilus methylotrophus cytochrome c''

Methylophilus methylotrophus cytochrome c'' is an unusual monohaem protein (15 kDa) undergoing a redox-linked spin-state transition [Santos, H. & Turner, D. L. (1988) Biochim. Biophys. Acta 954, 277-286]. The midpoint redox potential of cytochrome c" was measured over the pH range 4-10. The pH dependence of the midpoint redox potential was interpreted in terms of a model that considers the redox-state dependence of the ionization of two distinct and non-interacting protonated groups in the protein. This analysis led to the following pKa values within the pH range studied: pKa10 = 6.4, pKa1r = 5.4 and pKa2r = 8.1. Proton-NMR spectroscopy was used to assist the characterization of the two ionizing groups responsible for the observed redox-Bohr effect: the group ionizing with a lower pKar was assigned to a haem propionic acid substituent and the other to the axial histidine ligand which becomes detached upon reduction, which has a pKa0 too low to be measured. It is shown that M. methylotrophus cytochrome c" is able to couple electron and proton transfer in the physiological pH range through a mechanism involving reversible change in the haem-iron coordination. Possible implications for the physiological role of the protein are discussed.

NOE and two-dimensional correlated 1H-NMR spectroscopy of cytochrome c' from Chromatium vinosum

1H two-dimensional (nuclear Overhauser effect spectroscopy (NOESY) and two-dimensional correlated spectroscopy (COSY) spectra of cytochrome c' from Chromatium vinosum have been obtained. The protein is of medium size (Mr 28,000), essentially high spin (S = 5/2) although some quantum mechanical spin admixing with S = 3 2 may be present. Under these circumstances NOESY cross peaks have been revealed between geminal protons (alpha-CH2 propionate and beta-CH2 protons of the bound histidine) and between alpha-CH2 propionate protons and the heme methyl groups. COSY maps have confirmed the geminal nature of the proton pairs, even with a linewidth as large as 900 Hz; the J value is about 12 Hz. This assignment has rationalized on a sound basis the biochemical behavior of this protein with pH and has showed the utility of this kind of spectroscopy for the other cytochromes c' structures and analogous systems.

Cyanide-linked dimer-monomer equilibrium of Chromatium vinosum ferric cytochrome c'

Cyanide binding to Chromatium vinosum ferricytochrome c' has been studied to further investigate possible allosteric interactions between the subunits of this dimeric protein. Cyanide binding to C. vinosum cytochrome c' appears to be cooperative. However, the cyanide binding reaction is unusual in that the overall affinity of cyanide increases as the concentration of cytochrome c' decreases and that cyanide binding causes the ligated dimer to dissociate to monomers as shown by gel-filtration chromatography. Therefore, the cyanide binding properties of C. vinosum ferricytochrome c' are complicated by a cyanide-linked dimer to monomer dissociation equilibrium of the complexed protein. The dimer to monomer dissociation constant is 20-fold smaller than that for CO linked dissociation constant of ferrocytochrome c'. Furthermore, the pH dependence of both the intrinsic equilibrium binding constant and the dimer to monomer equilibrium dissociation constant was investigated over the pH range of 7.0 to 9.2 to examine the effect of any ionizable groups. The equilibrium constants did not exhibit a significant pH dependence over this pH range.

1H NMR studies of Chromatium vinosum cytochrome c'

The cytochrome c' from Chromatium vinosum has been studied through 1H NMR in the pH range 4-11 in both the oxidized and the reduced forms. The 1H NMR spectra are similar to those of the other cytochrome c' systems. Three pKa values of 5.1, 7.0, and 9.2 have been observed for the oxidized species and tentatively assigned to the two carboxylate propionic residues of the heme moiety and to the iron-coordinated histidine 125, respectively. The spectra are consistent with an essentially S = 5/2 state in all the pH ranges investigated. Some evidence is provided for conformational flexibilities. Among the oxidized cytochromes c' the present one is capable of binding cyanide, giving rise to a low spin state. The reduced species is a typical high spin iron(II) system.

A spectroscopic investigation of the structure and redox properties of Escherichia coli cytochrome b-562

The six-coordinate monohaem ferricytochrome b-562 from Escherichia coli exhibits two haem-linked pH-dependent transitions detected by NMR and optical spectroscopy. Only one of these transitions, that of the Fe(III)-coordinated His-102, is detected by EPR and MCD; the ionisation of a haem propionate is not. Both ionisations are redox-state-dependent and the midpoint redox potential of the protein is markedly pH-dependent. Over the pH range 5.0 to 8.5 the potential drops from 260 mV to 110 mV and at least five single proton ionisations are responsible for this. In addition to the two spectroscopically identified ferricytochrome ionisations, there are at least three unidentified ionisations, two of which occur in the ferrous protein. From a consideration of the X-ray structure, together with NMR data, it seems probable that at least one of these ionisations involves an amino acid carboxylate. The X-ray structure also suggests that the relatively low pKa of His-102 is a result of its proximity to Arg-98. However, an appreciable interaction between these groups requires that the solution conformation differs slightly from the X-ray structure. The fast rate of electron self-exchange, over 4 X 10(6) M-1 X s-1 at 315 K and pH* 7, may be a reflection of the fact that, as shown by the X-ray structure, a large amount of the haem and axial histidine ligand are exposed at the molecular surface with an asymmetric distribution of charged groups surrounding them.

Cytochromes c' in their reaction with ethyl isocyanide

The binding of ethyl isocyanide (EIC) to a representative number of cytochromes c' is demonstrated. Spectroscopic and equilibrium constants have been measured and compared for the binding of EIC to cytochromes c' from the photosynthetic bacteria Chromatium vinosum, Rhodopseudomonas palustris, Rhodospirillum rubrum, and Rhodopseudomonas sphaeroides. While the absorption spectra of the EIC complexes resemble those of EIC complexes of other high-spin hemoproteins, the Soret half band widths and extinction coefficients per heme exhibit more than a 2-fold difference with the values of C. vinosum being most similar to those of Rh. sphaeroides and of Rh. palustris similar to those of Rs. rubrum. The cytochromes exhibit binding equilibria consistent with the ligation of one molecule of EIC per heme in contrast to the reported binding of more than one molecule of CO per heme. The binding constants exhibit more than a 1000-fold difference with the values of C. vinosum being closely similar to those of Rh. sphaeroides and of Rh. palustris similar to those of Rs. rubrum. The lack of correlation between EIC and CO binding properties indicates that electronic factors do not determine the difference in EIC binding properties. The observed correlation between the extinction coefficients, half band widths, and equilibrium constants for EIC complex formation provides the first spectroscopic evidence that the differences in binding properties are associated with sterically hindered ligation to the heme. Although the differences in binding properties provide evidence of steric hindrance, the EIC binding constants for particular cytochromes c' indicate that the distal heme binding site is more accessible than previously indicated.(ABSTRACT TRUNCATED AT 250 WORDS)

NMR studies of electron transfer mechanisms in a protein with interacting redox centres: Desulfovibrio gigas cytochrome c3

The proton NMR spectra of the tetrahaem cytochrome c3 from Desulfovibrio gigas were examined while varying the pH and the redox potential. The analysis of the NMR reoxidation pattern was based on a model for the electron distribution between the four haems that takes into account haem-haem redox interactions. The intramolecular electron exchange is fast on the NMR time scale (larger than 10(5) s-1). The NMR data concerning the pH dependence of the chemical shift of haem methyl resonances in different oxidation steps and resonance intensities are not compatible with a non-interacting model and can be explained assuming a redox interaction between the haems. A complete analysis at pH* = 7.2 and 9.6, shows that the haem-haem interacting potentials cover a range from -50 mV to +60 mV. The midpoint redox potentials of some of the haems, as well as some of their interacting potentials, are pH-dependent. The physiological relevance of the modulation of the haem midpoint redox potentials by both the pH and the redox potential of the solution is discussed.