Proton resonance assignments for Pseudomonas aeruginosa ferrocytochrome c-551

Heme axial methionine fluxionality in Hydrogenobacter thermophilus cytochrome c552

The heme group in paramagnetic (S = 1/2) ferricytochromes c typically displays a markedly asymmetric distribution of unpaired electron spin density among the heme pyrrole beta substituents. This asymmetry is determined by the orientations of the heme axial ligands, histidine and methionine. One exception to this is ferricytochrome c(552) from Hydrogenobacter thermophilus, which has similar amounts of unpaired electron spin density at the beta substituents on all four heme pyrroles. Here, determination of the orientation of the magnetic axes and analysis of NMR line shapes for H. thermophilus ferricytochrome c(552) is performed. These data reveal that the unusual electronic structure for this protein is a result of fluxionality of the heme axial methionine. It is proposed that the ligand undergoes inversion at the pyramidal sulfur, and the rapid interconversion between two diastereomeric forms results in the unusual heme electronic structure. Thus a fluxional process for a metal-bound amino acid side chain has now been identified.

Expression of Pseudomonas stutzeri Zobell cytochrome c-551 and its H47A variant in Escherichia coli

The nirM gene encoding cytochrome c-551 from Pseudomonas stutzeri Zobell (PZ) has been expressed in Escherichia coli at levels higher than those previously reported but only under strict anaerobic growth conditions. Expression yields for wild-type cytochrome in this study typically reached 0.6 micromol per liter of saturated E. coli culture (5.5mg/L). Culture conditions investigated are compared to obtained c-551 expression levels; the results may lead to a greater understanding of the challenges encountered when expressing c-type hemoproteins in E. coli. The nirM gene was mutated to produce a histidine-47-alanine mutation of c-551 that been heterologously expressed in E. coli using optimum culture conditions and had its physiochemical properties compared to those of the wild-type protein. In PZ, the histidine-47 residue is part of a conserved hydrogen-bonding network located at the bottom of the heme crevice that also involves tryptophan-56 and a heme propionate. Ionization events within this network are experimentally demonstrated to modulate c-551 oxidation-reduction potential and its observed dependence on pH around neutrality. The redox potential of the mutant cytochrome still displays pH-dependence; however, the midpoint potential is approximately 25mV lower with respect to wild-type c-551 at neutral pH while the pK at which the heme propionate (HP-17) ionizes is lowered by 1.3 pH units. Temperature and chemical denaturant studies also show that loss of the hydrogen-bond-donating imidazole leads to a large decrease in c-551 tertiary stability.

Solution Conformation of the Met 61 to His 61 Mutant of Pseudomonas stutzeri ZoBell Ferrocytochrome c-551

The gene encoding for bacterial cytochrome c-551 from Pseudomonas stutzeri substrain ZoBell has been mutated to convert the invariant sixth ligand methionine residue into histidine, creating the site-specific mutant M61H. Proton NMR resonance assignments were made for all main-chain and most-side chain protons in the diamagnetic, reduced form at pH 9.2 and 333 K by two-dimensional NMR techniques. Distance constraints (1074) were determined from nuclear Overhauser enhancements and main-chain torsion-angle constraints (72) from scalar coupling estimates. Solution conformations for the protein were computed by the simulated annealing approach. For 28 computed structures, the root mean squared displacement from the average structure excluding the terminal residues 1, 2, 81, and 82 was 0.52 A (sigma = 0.096) for backbone atoms and 0.90 A (sigma = 0.122) for all heavy atoms. The global folding of the mutant protein is the same as for wild type. The biggest changes are localized in a peptide span over residues 60-65. The most striking behavior of the mutant protein is that at room temperature and neutral pH it exists in a state similar to the molten globular state that has been described for several proteins under mild denaturing conditions, but the mutant converts to a more ordered state at high pH and temperature.

Ionization of the heme propionate substituents in pseudomonad cytochromes c-551

The heme propionate substituents in Pseudomonas cytochrome c-551 are partially buried by folds of polypeptide in the structure of the protein, and are involved in several hydrogen bonds. The ionization behavior of these groups has been of interest because the oxidation potential of the heme changes with pH in a manner that may parallel ionization of a propionate. The ionization pKa's of these groups have been determined by following the NMR chemical shifts of nearby protons acting as probes of the ionization state of the propionates. In Pseudomonas aeruginosa c-551 the 13-propionate (IUB-IUPAC porphyrin nomenclature) has been assigned a pKa of 3.1, and the 17-propionate a pKa of 7.2. In the homologous Pseudomonas stutzeri c-551, the respective propionates both have pKa values of 3.0.

Hydrogen exchange in Pseudomonas cytochrome c-551

Hydrogen exchange rates were measured or estimated for 75 amide protons in in ferrocytochrome c-551 from Pseudomonas aeruginosa (82 residues total) at neutral pH and 300 K. Rate constants span at least eight orders of magnitude. Rate constants or limiting estimates were determined by a combination of methods relying upon 1H-NMR spectroscopy, including the direct observation in one- or two-dimensional spectra of the decrease in proton intensity for samples dissolved in deuterium oxide, or, in a few favorable cases, saturation transfer from the solvent protic water. The heme ligand residues and the thioether bridge residues were slowly exchanging backbone amides, but the slowest exchanging backbone amides were found in two clusters. One was composed of Ile-48 and Lys-49 in the last turn of what is termed the 40's helix in the protein. The second was composed of Leu-74, Ala-75, Lys-76 and Val-78 in the C-terminal alpha helix.