Mössbauer characterization of the tetraheme cytochrome c3 from Desulfovibrio baculatus (DSM 1743). Spectral deconvolution of the heme components

Role of calcium in metalloenzymes: effects of calcium removal on the axial ligation geometry and magnetic properties of the catalytic diheme center in MauG

MauG is a diheme enzyme possessing a five-coordinate high-spin heme with an axial His ligand and a six-coordinate low-spin heme with His-Tyr axial ligation. A Ca(2+) ion is linked to the two hemes via hydrogen bond networks, and the enzyme activity depends on its presence. Removal of Ca(2+) altered the electron paramagnetic resonance (EPR) signals of each ferric heme such that the intensity of the high-spin heme was decreased and the low-spin heme was significantly broadened. Addition of Ca(2+) back to the sample restored the original EPR signals and enzyme activity. The molecular basis for this Ca(2+)-dependent behavior was studied by magnetic resonance and Mössbauer spectroscopy. The results show that in the Ca(2+)-depleted MauG the high-spin heme was converted to a low-spin heme and the original low-spin heme exhibited a change in the relative orientations of its two axial ligands. The properties of these two hemes are each different than those of the heme in native MauG and are now similar to each other. The EPR spectrum of Ca(2+)-free MauG appears to describe one set of low-spin ferric heme signals with a large g(max) and g anisotropy and a greatly altered spin relaxation property. Both EPR and Mössbauer spectroscopic results show that the two hemes are present as unusual highly rhombic low-spin hemes in Ca(2+)-depleted MauG, with a smaller orientation angle between the two axial ligand planes. These findings provide insight into the correlation of enzyme activity with the orientation of axial heme ligands and describe a role for the calcium ion in maintaining this structural orientation that is required for activity.

Resilience of the iron environment in heme proteins

Conformational flexibility is essential to the functional behavior of proteins. We use an effective force constant introduced by Zaccai, the resilience, to quantify this flexibility. Site-selective experimental and computational methods allow us to determine the resilience of heme protein active sites. The vibrational density of states of the heme Fe determined using nuclear resonance vibrational spectroscopy provides a direct experimental measure of the resilience of the Fe environment, which we compare quantitatively with values derived from the temperature dependence of atomic mean-squared displacements in molecular dynamics simulations. Vibrational normal modes in the THz frequency range dominate the resilience. Both experimental and computational methods find a higher resilience for cytochrome c than for myoglobin, which we attribute to the increased number of covalent links to the peptide in the former protein. For myoglobin, the resilience of the iron environment is larger than the average resilience previously determined for hydrogen sites using neutron scattering. Experimental results suggest a slightly reduced resilience for cytochrome c upon oxidation, although the change is smaller than reported in previous Mössbauer investigations on a bacterial cytochrome c, and is not reproduced by the simulations. Oxidation state also has no significant influence on the compressibility calculated for cyt c, although a slightly larger compressibility is predicted for myoglobin.

Thermodynamic and kinetic characterisation of individual haems in multicentre cytochromes c3

The characterisation of individual centres in multihaem proteins is difficult due to the similarities in the redox and spectroscopic properties of the centres. NMR has been used successfully to distinguish redox centres and allow the determination of the microscopic thermodynamic parameters in several multihaem cytochromes c(3) isolated from different sulphate-reducing bacteria. In this article we show that it is also possible to discriminate the kinetic properties of individual centres in multihaem proteins, if the complete microscopic thermodynamic characterisation is available and the system displays fast intramolecular equilibration in the time scale of the kinetic experiment. The deconvolution of the kinetic traces using a model of thermodynamic control provides a reference rate constant for each haem that does not depend on driving force and can be related to structural factors. The thermodynamic characterisation of three tetrahaem cytochromes and their kinetics of reduction by sodium dithionite are reported in this paper. Thermodynamic and kinetic data were fitted simultaneously to a model to obtain microscopic reduction potentials, haem-haem and haem-proton interacting potentials, and reference rate constants for the haems. The kinetic information obtained for these cytochromes and recently published data for other multihaem cytochromes is discussed with respect to the structural factors that determine the reference rates. The accessibility for the reducing agent seems to play an important role in controlling the kinetic rates, although is clearly not the only factor.

Mössbauer and electron paramagnetic resonance studies of the cytochrome bf complex

The (57)Fe-enriched cytochrome bf complex has been isolated from hydrocultures of spinach. It has been studied at different redox states by optical, EPR, and Mössbauer spectroscopy. The Mössbauer spectrum of the native complex at 190 K with all iron centers in the oxidized state reveals the presence of four different iron sites: low-spin ferric iron in cytochrome b [with an isomer shift (delta) of 0.20 mm/s, a quadrupole splitting (DeltaE(Q)) of 1.77 mm/s, and a relative area of 40%], low-spin ferric iron of cytochrome f (delta = 0.26 mm/s, DeltaE(Q) = 1.90 mm/s, and a relative area of 20%), and two high-spin ferric iron sites of the Rieske iron-sulfur protein (ISP) with a bis-cysteine and a bis-histidine ligated iron (delta(1) = 0.15 mm/s, DeltaE(Q1) = 0.70 mm/s, and a relative area of 20%, and delta(2) = 0.25 mm/s, DeltaE(Q2) = 0.90 mm/s, and a relative area of 20%, respectively). EPR and magnetic Mössbauer measurements at low temperatures corroborate these results. A crystal-field analysis of the EPR data and of the magnetic Mössbauer data yields estimates for the g-tensors (g(z)(), g(y)(), and g(x)()) of cytochrome b (3.60, 1.35, and 1.1) and of cytochrome f (3.51, 1.69, and 0.9). Addition of ascorbate reduces not only the iron of cytochrome f to the ferrous low-spin state (delta = 0.43 mm/s, DeltaE(Q) = 1.12 mm/s at 4.2 K) but also the bis-histidine coordinated iron of the Rieske 2Fe-2S center to the ferrous high-spin state (delta(2) = 0.73 mm/s, DeltaE(Q2) = -2.95 mm/s at 4.2 K). At this redox step, the Mössbauer parameters of cytochrome b have not changed, indicating that the redox changes of cytochrome f and the Rieske protein did not change the first ligand sphere of the low-spin ferric iron in cytochrome b. Reduction with dithionite further reduces the two hemes of cytochrome b to the ferrous low-spin state (delta = 0.49 mm/s, DeltaE(Q) = 1.08 mm/s at 4.2 K). The spin Hamiltonian analysis of the magnetic Mössbauer spectra at 4.2 K yields hyperfine parameters of the reduced Rieske 2Fe-2S center in the cytochrome bf complex which are very similar to those reported for the Rieske center from Thermus thermophilus [Fee, J. A., Findling, K. L., Yoshida, T., et al. (1984) J. Biol. Chem. 259, 124-133].

Mössbauer characterization of Paracoccus denitrificans cytochrome c peroxidase. Further evidence for redox and calcium binding-induced heme-heme interaction

Mössbauer and electron paramagnetic resonance (EPR) spectroscopies were used to characterize the diheme cytochrome c peroxidase from Paracoccus denitrificans (L.M.D. 52.44). The spectra of the oxidized enzyme show two distinct spectral components characteristic of low spin ferric hemes (S = 1/2), revealing different heme environments for the two heme groups. The Paracoccus peroxidase can be non-physiologically reduced by ascorbate. Mössbauer investigation of the ascorbate-reduced peroxidase shows that only one heme (the high potential heme) is reduced and that the reduced heme is diamagnetic (S = 0). The other heme (the low potential heme) remains oxidized, indicating that the enzyme is in a mixed valence, half-reduced state. The EPR spectrum of the half-reduced peroxidase, however, shows two low spin ferric species with gmax = 2.89 (species I) and gmax = 2.78 (species II). This EPR observation, together with the Mössbauer result, suggests that both species are arising from the low potential heme. More interestingly, the spectroscopic properties of these two species are distinct from that of the low potential heme in the oxidized enzyme, providing evidence for heme-heme interaction induced by the reduction of the high potential heme. Addition of calcium ions to the half-reduced enzyme converts species II to species I. Since calcium has been found to promote peroxidase activity, species I may represent the active form of the peroxidatic heme.

Cytochrome c6 from Monoraphidium braunii. A cytochrome with an unusual heme axial coordination

A soluble monoheme c-type cytochrome (cytochrome c6) has been isolated from the green alga Monoraphidium braunii. It has a molecular mass of 9.3 kDa, an isoelectric point of 3.6 and a reduction potential of 358 mV at pH 7. The determined amino acid sequence allows its classification as a class-I c-type cytochrome. The ferric and ferrous cytochrome forms and their pH equilibria have been studied using 1H-NMR, ultraviolet/visible, EPR and Mössbauer spectroscopies. The pH equilibria are complex, several pKa values and pH-dependent forms being observed. The amino acid sequence, the reduction-potential value and the visible and NMR spectroscopies data in the pH range 4-9 indicate that the heme iron has a methionine-histidine axial coordination. However, the EPR and Mössbauer data obtained for the ferricytochrome show that in this pH range two distinct forms are present: form I, gz = 3.27, gy = 2.05 and gx = 1.05; form II, gz = 2.95, gy = 2.29 and gx = 1.43. While form I has crystal-field parameters typical of a methionine-histidine coordination, those associated with form II would suggest a histidine-histidine axial ligation. This possibility was extensively analyzed by spectroscopic methods and by chemical modification of a histidine residue. It was concluded that form II actually corresponds to an unusual type of methionine-histidine axial coordination. Straightforward examples of this type of coordination have recently been found in other c-type hemeproteins [Teixeira, M., Campos, A. P., Aguiar, A. P., Costa, H. S., Santos, H., Turner, D. L. & Xavier, A. V. (1993) FEBS Lett. 317, 233-236], corroborating our proposal. Since both forms, with very distinct crystal-field parameters, are shown to have the same reduction potential, it may be concluded that the axial and rhombic distortions of the heme-iron ligand field cannot be directly correlated with the heme-reduction potential. The pH-dependence studies have also shown that the form I and form II are interconvertible, with pKa approximately 5. To establish a possible physiological significance for this process, in particular for the interaction of the cytochrome with the membrane-bound electron-transfer complexes b6f and photosystem I, the effect of surfactants on the spectroscopic characteristics of cytochrome c6 has been studied.

Pitfalls in assigning heme axial coordination by EPR. c-Type cytochromes with atypical Met-His ligation

Different monohemic c-type cytochromes were analyzed by visible, EPR and 1H NMR spectroscopies. While the visible and NMR data show unambiguously that the heme iron has a Met-His heme axial coordination, the EPR data indicate an axial ligand field typical of that for a bis-histidinyl ligation. The validity of the widely used EPR methods for the determination of the heme iron axial coordination, based on the crystal field parameters (tetragonality and rhombicity), is questioned.