Assignment of hyperfine shifted resonances in high-spin forms of cytochrome c peroxidase by reconstitutions with deuterated hemins.

Assignments of hyperfine shifted proton resonances for the high-spin forms of cytochrome c peroxidase (EC 1.11.1.5) have been made (cytochrome c peroxidase, cytochrome c peroxidase-F) employing the technique of reconstituting the apoprotein with specifically deuterated protohemin IX derivatives. The results show that the heme methyl group pattern differs significantly from similar assignments made for metmyoglobin. In cytochrome c peroxidase the methyl pattern is 5 greater than 1 greater than 8 greater than 3. For cytochrome c peroxidase-F the pattern is 5 greater than 8 greater than 1 greater than 3, but the resonances are not shifted as far downfield and they exhibit a narrower spread. For myoglobin the relative methyl ordering has previously been shown to be 8 greater than 5 greater than 3 greater than 1. Several conclusions have been reached, including confirmation of the essential correspondence between the solution- and crystal-derived data for several heme crevice structural features. The pH dependence of the cytochrome c peroxidase-F methyl resonances is also presented and is shown to differ from native peroxidase. For cytochrome c peroxidase-F smooth, continuous titrations are observed with no evidence of the second conformation which was found for the native enzyme.

1H-NMR assignments and the dynamics of interconversion of the isomeric forms of cytochrome b5 in solution.

Cytochrome b5 reconstituted with specifically deuterated hemins has led to the assignment of the resolved 6,7 beta-propionate protons and heme meso protons. Freshly reconstituted cytochrome b5 contains a mixture of two isomers in an approx. 1:1 ratio. As time proceeds the minor isomer decreases in intensity until the equilibrium ratio, approx. 8:1, of the two isomers is reached. The rate of the heme disorder kinetics was investigated for cytochrome b5 as a function of pH, oxidation state and 2,4 heme substitutents. Comparison of the kinetic data for cytochrome b5 with that obtained for other b-type heme proteins supports the proposal that the heme disorder arises from a 180 degree rotation of the heme about the alpha, gamma-meso axis. Computer-difference methods allow the spectra of the two individual isomers to be generated. Comparison of the NMR spectral parameters for the two individual isomers indicates small structural differences for amino acid side-chain orientations.

1H-NMR heme resonance assignments by selective deuteration in low-spin complexes of ferric hemoglobin A.

The heme methyl and vinyl alpha-proton signals have been assigned in low-spin ferric cyanide and azide ligated derivatives of the intact tetramer of hemoglobin A, as well as the isolated chains, by reconstituting the proteins with selectively deuterated hemins. For the hemoglobin cyanide tetramer, assignment to individual subunits was effected by forming hybrid hemoglobins possessing isotope-labeled hemins in only one type of subunit. The heme methyl contact shift pattern has 1-methyl and 5-methyl shifts furthest downfield in both chains and the individual subunits of the intact hemoglobin in both the cyanide- and azide-ligated species, which is consistent with a dominant rhombic perturbation due to the proximal His-F8 imidazole pi bonding in the known structure for human adult hemoglobin. The individual chain and subunit assignments confirm that the detailed electronic/magnetic properties of the heme pocket are essentially unaltered upon assembling the R-state tetramer from the isolated subunits.

Structural and electronic properties of the liver fluke hemoglobin heme cavity by nuclear magnetic resonance: hemin isotope labeling.

Reconstitution of liver fluke (Dicrocoelium dendriticum) apo-hemoglobin with hemins selectively deuterated at specific positions has permitted the assignment of several heme resonances in the proton nuclear magnetic resonance spectrum of the Met-aquo and Met-cyano forms of the holoprotein. It was established that in the Met-aquo form the meso protons resonate at positions characteristic of a six-co-ordinated in-plane iron. From this, we deduced that the Met-aquo species retains a bound water molecule at pH values as low as 4.5. The orientation of the proximal histidine imidazole ring with respect to the heme group in the cavity was determined through the identification of the heme methyl signals and the analysis of the hyperfine shift pattern in the Met-cyano hemoglobin proton nuclear magnetic resonance spectrum. Compared to sperm whale myoglobin, the heme appears to be rotated by 180 degrees about the alpha, gamma meso-axis. Protein isomers with the heme group in a reversed orientation were not detected, even shortly after reconstitution. In the Met-cyano form, the resonances most affected by the Bohr transition were shown to arise from the heme propionates.

A fiber-optic sensor system for monitoring chlorinated hydrocarbon pollutants.

We have developed and field-tested a fiber-optic chemical sensor system for use in environmental monitoring and remediation. The system detects chlorinated hydrocarbon pollutants with colorimetry, and is based on an irreversible chemical reaction between the target compound and a specific reagent. The reaction products are detected by their absorption at 560 nm and can be monitored remotely with optical fibers. Continuous measurements are made possible by renewing the reagent from a reservoir with a miniature pumping system. The sensor has been evaluated against gas chromatography standards and has demonstrated accuracy and sensitivity (5 ppbw) sufficient for the environmental monitoring of trichloroethylene and chloroform. Successful preliminary field tests have been conducted in a variety of contamination monitoring scenarios.

The allylisopropylacetamide and novonal prosthetic heme adducts.

Administration of 2-isopropyl-4-pentenamide (AIA) and 2,2-diethyl-4-pentenamide (novonal) to phenobarbital-pretreated rats gives rise to abnormal porphyrins derived from the prosthetic heme group of inactivated cytochrome P-450. The abnormal porphyrins, identified by NMR and other spectroscopic methods, are N-alkylated protoporphyrin IX derivatives in which the N-alkyl moiety is derived from the parent drug by addition of a hydroxyl group to the internal carbon and of a porphyrin nitrogen to the terminal carbon of the pi-bond. A secondary reaction of the hydroxyl with the amide group converts the N-alkyl moiety into a lactone. The indicated alkylation-lactonization sequence is supported by the fact that the AIA adduct formed under an atmosphere of 18O2 incorporates one labeled oxygen atom. The regiochemistry of heme alkylation is consistent with a previously postulated active site topology [J. Biol. Chem. 258:4202-4207 (1983)].

Proton nuclear magnetic resonance study of the electronic and molecular structure of the heme crevice in horseradish peroxidase.

High field proton nuclear magnetic resonance spectroscopy was used to investigate the electronic and molecular structure of the ferric heme in the resting state of horseradish peroxidase. Deuterium labeling of selected positions of hemin and deuterohemin which were subsequently reconstituted into apo-horseradish peroxidase yielded hyperfine shift patterns for the prosthetic group which are consistent with a ferric porphyrin exhibiting appreciable S = 3/2 character in a quantum mixed spin state. All resolved resonances with significant hyperfine shifts can be accounted for by the porphyrin and a proximal histidyl imidazole, although a sixth ligand from the protein cannot be definitely eliminated. The extremely slow exchange rate with bulk water of the proximal histidyl imidazole exchangeable proton and the absence of deviations from Curie behavior for the porphyrin vinyl and propionic acid proton hyperfine shifts indicate a buried heme crevice which is more rigid than in metmyoglobin. The observation of significant deviations from Curie behavior of the proximal histidyl imidazole exchangeable proton in horseradish peroxidase but not in metmyoglobins is suggested to arise from strong hydrogen bonding between the coordinated imidazole and some unspecified protein acceptor residue in the former protein.

Proton nuclear magnetic resonance characterization of heme disorder in hemoproteins.

A proton NMR method is described for determining the orientation of a porphyrin within the heme pocket of a hemoprotein. The pattern of the hyperfine-shifted heme methyl resonances in low-spin ferric model compounds is demonstrated to characteristically reflect the position of a localized low-symmetry perturbation on the pi system. The specific assignments via deuteration of the two interconvertible sets of methyl resonances observed for deuteroporphyrin-reconstituted sperm whale metmyoglobin cyanide lead to the conclusion that the low-symmetry perturbations on the heme due to the apo-protein contacts differ for the two protein components by a 180 degrees rotation about the alpha-gamma meso axis. Hence the heme in the reconstituted myoglobin is "disordered" in solution, and the altered functional properties of the reconstituted protein cannot be simply attributed to the local effect of the heme substituent. This NMR technique has applicability for determining the relative heme orientation in related hemoproteins, and may clarify the origin of doubling of heme resonances observed in several native hemoproteins.

Proton nuclear magnetic resonance investigation of the nature of solution conformational equilibria of monomeric insect deoxyhemoglobins.

The proton nuclear magnetic resonance spectra of the three monomeric deoxyhemoglobins of the insect larva Chironomus thummi thummi have been recorded, assigned, and analyzed. In the two allosteric hemoglobins, the heme methyls and vinyl protons were assigned by specific deuterium labeling. The hyperfine-shifted proximal histidyl imidazole exchangeable protons for the three native and two deuteroheme-reconstituted hemoglobins were assigned by comparison of spectra in H2O and 2H2O. Both native and reconstituted allosteric hemoglobins exhibit two sets of interconvertible resonances indicative of two heme orientations differing by a 180 degrees rotation about the alpha-gamma-meso axis, as previously found for the met-cyano analogues [La Mar, G. N., Smith, K. M., Gersonde, K., Sick, H., & Overkamp, M. (1980) J. Biol. Chem. 255, 66]. The relative pH sensitivities of the heme resonance hyperfine shifts for the two allosteric hemoglobins and the apparent pK approximately 8 indicate that the t in equilibrium r allosteric transition, as modulated by the Bohr proton, is being observed. For the native hemoglobins, the t in equilibrium r conformational transition was found to be centered at the heme periphery, with the proximal histidyl imidazole environment insensitive to both pH and the rotational position of the heme, consistent with the absence of a pH influence on the ligation on-rate. For the deuteroheme-reconstituted allosteric hemoglobins, both the heme and axial imidazole environments sense the t in equilibrium r transition, and the histidine environments for the two components for each hemoglobin can be clearly distinguished, suggesting that the ligation on-rates may depend on both pH and heme orientation.