Electron paramagnetic resonance studies of highly anisotropic low-spin states of ferrimyoglobin derivatives

Modulation of the ligand-field anisotropy in a series of ferric low-spin cytochrome c mutants derived from Pseudomonas aeruginosa cytochrome c-551 and Nitrosomonas europaea cytochrome c-552: a nuclear magnetic resonance and electron paramagnetic resonance study

Cytochromes of the c type with histidine-methionine (His-Met) heme axial ligation play important roles in electron-transfer reactions and in enzymes. In this work, two series of cytochrome c mutants derived from Pseudomonas aeruginosa (Pa c-551) and from the ammonia-oxidizing bacterium Nitrosomonas europaea (Ne c-552) were engineered and overexpressed. In these proteins, point mutations were induced in a key residue (Asn64) near the Met axial ligand; these mutations have a considerable impact both on heme ligand-field strength and on the Met orientation and dynamics (fluxionality), as judged by low-temperature electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectra. Ne c-552 has a ferric low-spin (S = 1/2) EPR signal characterized by large g anisotropy with g(max) resonance at 3.34; a similar large g(max) value EPR signal is found in the mitochondrial complex III cytochrome c1. In Ne c-552, deletion of Asn64 (NeN64Delta) changes the heme ligand field from more axial to rhombic (small g anisotropy and g(max) at 3.13) and furthermore hinders the Met fluxionality present in the wild-type protein. In Pa c-551 (g(max) at 3.20), replacement of Asn64 with valine (PaN64V) induces a decrease in the axial strain (g(max) at 3.05) and changes the Met configuration. Another set of mutants prepared by insertion (ins) and/or deletion (Delta) of a valine residue adjacent to Asn64, resulting in modifications in the length of the axial Met-donating loop (NeV65Delta, NeG50N/V65Delta, PaN50G/V65ins), did not result in appreciable alterations of the originally weak (Ne c-552) or very weak (Pa c-551) axial field but had an impact on Met orientation, fluxionality, and relaxation dynamics. Comparison of the electronic fingerprints in the overexpressed proteins and their mutants reveals a linear relationship between axial strain and average paramagnetic heme methyl shifts, irrespective of Met orientation or dynamics. Thus, for these His-Met axially coordinated Fe(III), the large g(max) value EPR signal does not represent a special case as is observed for bis-His axially coordinated Fe(III) with the two His planes perpendicular to each other.

Reversible and irreversible hemichrome generation by the oxygenation of nitrosylmyoglobin

The repeated oxygenation/reduction/nitrosylation of nitrosylmyoglobin produces low-spin ferric heme hemichromes which have been characterized by electron spin resonance spectroscopy. The predominant myoglobin hemichrome is a chemically reversible dihistidyl complex identified by the g values 1.53, 2.21, and 2.97. Also present is a low-spin ferric hydroxide derivative which is represented by the g values 1.83, 2.18, and 2.59. The formation of these species goes undetected by UV-vis spectroscopy, but the oxygenation of myoglobin to metmyoglobin is correlated with complete conversion of nitric oxide to nitrate which is released following a clear induction period. These results are interpreted in terms of the intermediates generated during the MbNO oxygenation reaction.

The cytochrome bc1 complex from Rhodovulum sulfidophilum is a dimer with six quinones per monomer and an additional 6-kDa component

A highly active, large-scale preparation of cytochrome bc1 complex has been obtained from the photosynthetic purple bacterium Rhodovulum (Rhv.) sulfidophilum. It has been characterized using mass spectrometry, quinone and lipid analysis as well as inhibitor binding. About 35 mg of pure complex can be obtained from 1 g of membrane protein. EPR spectroscopy and optical titrations have been used to obtain the redox midpoint potentials of the cofactors. The Em-value of 310 mV for the Rieske protein is the most positive midpoint potential for this protein in a bc1 complex so far. The bc1 complex from Rhv. sulfidophilum is very stable and consists of three subunits and a 6-kDa polypeptide. The complex appears as a dimer in solution and contains six quinone molecules per monomer which are tightly bound. EPR spectroscopy shows that the Q(o) site is highly occupied. High detergent concentrations convert the complex into an inactive, monomeric form that has lost the Rieske protein as well as the quinones and the 6-kDa component.

Electron paramagnetic resonance investigations of exogenous ligand complexes of low-spin ferric chloroperoxidase: further support for endogenous thiolate ligation to the heme iron

Previous spectroscopic studies of chloroperoxidase have provided evidence for endogenous thiolate sulfur donor ligation to the central heme iron of the enzyme. This conclusion is further supported by recent DNA sequence data which revealed the existence of a third cysteine residue (in addition to a disulfide pair detected earlier) in the protein available for coordination to the heme iron. Thus, chloroperoxidase shares many spectroscopic properties with cytochrome P-450, the only other known thiolate-ligated heme protein. Surprisingly, a previous electron paramagnetic resonance (EPR) study of low-spin ferric chloroperoxidase-ligand complexes (Hollenberg, P.F., Hager, L.P., Blumberg, W.E. and Peisach, J. (1980) J. Biol. Chem. 255, 4801-4807) was unable to provide clear support for the presence of a thiolate ligand, although sulfur coordination was implicated. This was, in part, because an insufficient number of complexes was examined. In this work, we have significantly expanded upon the previous EPR study by using an extensive variety of over twenty exogenous ligands including carbon, nitrogen, oxygen, phosphorus and sulfur donors. Crystal field analysis, using the procedure of Blumberg and Peisach, of the present data in comparison with data for analogous complexes of cytochrome P-450-CAM, thiolate-ligated heme model systems, and myoglobin, is clearly indicative of endogenous thiolate ligation for chloroperoxidase. In addition, the UV-visible absorption and EPR spectral data suggest that a carboxylate ligand is a possible candidate for the endogenous sixth ligand to the heme iron that is responsible for the reversible conversion of ferric chloroperoxidase from high-spin to low-spin at low temperatures (less than 200 K).

Correlations among parameters that describe low-spin ferric heme complexes

The g values from low-spin ferric hemes can be related through the t2g hole model to rhombic (V/lambda) and tetragonal (delta/lambda) ligand field components and to the lowest Kramer's doublet energy (E/lambda). The latter is also a measure of unpaired electron sharing among the iron 3d (t2g) orbitals. For a series of ligands (X), there is a monotonic increase in myoglobin complex (Mb . X) [E/lambda] values with nonheme hexacoordinate metal complex (M . X6) [eg-t2gPg] orbital separations. As the aqueous solution pKa values of the sulfurous or nitrogenous ligands in model heme complexes increase, values of V/lambda and delta/lambda increase linearly, but those of [E/lambda] decrease linearly. The greater the electron-acceptor ability of the ligand, as suggested by its position in the spectrochemical series or its pKa, the more the unpaired electron sharing among the heme t2g orbitals increases. The rate of change of [E/lambda] with V/lambda and the pKa is different with sulfurous and nitrogenous ligands, and the magnitude of both rates increases with two sulfurs less than sulfur and nitrogen less than two nitrogens bound to the heme. The maximum magnitude of this rate with V/lambda for cytochrome P-450 is four times less than that for myoglobin, which may explain, in part, the differences in ligand binding between these two hemeproteins. The perturbation of [E/lambda], V/lambda, and delta/lambda induced by strain of iron-ligand bonds is quantitated for several hemeproteins and heme models. In addition, energy level comparisons suggest that the largest-magnitude g value falls approximately along the iron-chlorin ring normal. This suggestion implies that the electron distribution of the iron at the catalytic sites of cytochrome P-450 and certain chlorin-containing enzymes is in some way similar, but distinct from that at the transport site of myoglobin.