Multinuclear magnetic resonance studies of the 2Fe.2S* ferredoxin from Anabaena species strain PCC 7120. 3. Detection and characterization of hyperfine-shifted nitrogen-15 and hydrogen-1 resonances of the oxidized form

All the nitrogen signals from the amino acid side chains and 80 of the total of 98 backbone nitrogen signals of the oxidized form of the 2Fe.2S* ferredoxin from Anabaena sp. strain PCC 7120 were assigned by means of a series of heteronuclear two-dimensional experiments [Oh, B.-H. Mooberry, E. S., & Markley, J. L. (1990) Biochemistry (second paper of three in this issue )]. Two additional nitrogen signals were observed in the one-dimensional 15N NMR spectrum and classified as backbone amide resonances from residues whose proton resonances experience paramagnetic broadening. The one-dimensional 15N NMR spectrum shows nine resonances that are hyperfine shifted and broadened. From this inventory of diamagnetic nitrogen signals and the available X-ray coordinates of a related ferredoxin [Tsukihara, T., Fukuyama, K., Nakamura, M., Katsube, Y., Tanaka, N., Kakudo, M., Wada, K., Hase, T., & Matsubara, H. (1981) J. Biochem. 90, 1763-1773], the resolved hyperfine-shifted 15N peaks were attributed to backbone amide nitrogens of the nine amino acids that share electrons with the 2Fe.2S* center or to backbone amide nitrogens of two other amino acids that are close to the 2Fe.2S* center. The seven 15N signals that are missing and unaccounted for probably are buried under the envelope of amide signals. 1H NMR signals from all the amide protons directly bonded to the seven missing and nine hyperfine-shifted nitrogens were too broad to be resolved in conventional 2D NMR spectra.(ABSTRACT TRUNCATED AT 250 WORDS)     

Fluorine-19 nuclear magnetic resonance studies of the structure of 5-fluorouracil-substituted Escherichia coli transfer RNA

19F nuclear magnetic resonance has been used to study fully active Escherichia coli tRNA1Val in which 5-fluorouracil has replaced more than 90% of all uracil and uracil-derived modified bases. The 19F spectrum of the native tRNA contains resolved resonances for all 14 incorporated 5-fluorouracils. These are spread over a 6 ppm range, from 1.8 to 7.7 ppm downfield of the standard free 5-fluorouracil. The 19F resonances serve as sensitive monitors of tRNA conformation. Removal of magnesium or addition of NaCl produces major, reversible changes in the 19F spectrum. Most affected is the lowest field resonance (peak A) in the spectrum of the native tRNA. This shifts 2-3 ppm upfield as the Mg2+ concentration is lowered or the NaCl concentration is raised. Thermal denaturation of the tRNA results in a collapse of the spectrum to a single broad peak centered at 4.7 ppm. Study of the pH dependence of the 19F spectrum shows that five incorporated fluorouracils with 19F signals in the central, 4-5.5 ppm, region of the spectrum, peaks C, D, E, F, and H, are accessible to titration in the pH 4.5-9 range. All have pKa's close to that of free 5-fluorouridine (ca. 7.5). Evidence for a conformation change in the tRNA at mildly acidic pHs, ca. 5.5, is also presented. Four of the titratable 5-fluorouracil residues, those corresponding to peaks D, E/F, and H in the 19F spectrum of fluorine-labeled tRNAVal1, are essentially completely exposed to solvent as determined by the solvent isotope shift (SIS) on transfer of the tRNA from H2O to 2H2O. These are also the 5-fluorouracils that readily form adducts with bisulfite, a reagent that reacts preferentially with pyrimidines in single-stranded regions. On the basis of these results, resonances D, E, F, and H in the middle of the 19F spectrum are attributed to 5-fluorouracils in non-base-paired (loop) regions of the tRNA. Evidence from the ionic strength dependence of the 19F spectrum and arguments based on other recent studies with fluorinated tRNAs support earlier suggestions [Horowitz, J., Ofengand, J., Daniel, W. E., & Cohn, M. (1977) J. Biol. Chem. 252, 4418-4420] that the resonances at lowest field correspond to tertiary hydrogen-bonded 5-fluorouracils. Consideration of ring-current effects and the preferential perturbation of upfield 19F resonances by the cyclophotoaddition of 4'-(hydroxymethyl)-4,5',8-trimethylpsoralen, which is known to react most readily with pyrimidines in double-stranded regions, permits initial assignment of upfield resonances to 5-fluorouracils in helical stems.(ABSTRACT TRUNCATED AT 400 WORDS)     

(19)F NMR studies of the leucine-isoleucine-valine binding protein: evidence that a closed conformation exists in solution

The leucine-isoleucine-valine binding protein (LIV) found in the periplasmic space of E. coli has been used as a structural model for a number of neuronal receptors. This "venus fly trap" type protein has been characterized by crystallography in only the open form. Herein we have labeled LIV with 5-fluorotryptophan (5F-Trp) and difluoromethionine (DFM) in order to explore the structural dynamics of this protein and the application of DFM as a potential (19)F NMR structural probe for this family of proteins. Based on mass spectrometric analysis of the protein overproduced in the presence of DFM, approximately 30% of the five LIV methionine residues were randomly substituted with the fluorinated analog. Urea denaturation experiments imply a slight decrease in protein stability when DFM is incorporated into LIV. However, the fluorinated methionine did not alter leucine-binding activity upon its incorporation into the protein. Binding of L-leucine stabilizes both the unlabeled and DFM-labeled LIV, and induces the protein to adopt a three-state unfolding model in place of the two-state process observed for the free protein. The (19)F NMR spectrum of DFM-labeled LIV gave distinct resonances for the five Met residues found in LIV. 5F-Trp labeled LIV gave a well resolved spectrum for the three Trp residues. Trp to Phe mutants defined the resonances in the spectrum. The distinct narrowing in line width of the resonances when ligand was added identified the closed form of the protein.     

Proton nuclear magnetic resonance characterization of the oxidized intermediates of cytochrome c peroxidase

Oxidation of cytochrome c peroxidase with hydrogen peroxide to form the initial oxidized intermediate, cytochrome c peroxidase compound I, drastically alters the proton hyperfine nmr spectrum. In contrast to studies of horseradish peroxidase, where the spectrum of horseradish peroxidase compound I is similar to that of the native protein, cytochrome c peroxidase compound I exhibits only broad resonances near 17 and 30 ppm from 2,2-dimethyl-2-silapentane-5-sulfonate. No unique resonances attributable to cytochrome c peroxidase compound II could be identified. These results define the molecular conditions for which resolved hyperfine resonances of the iron(IV) states of heme proteins may be observed when the data presented here are compared with the data from horseradish peroxidase. Oxidation of cytochrome c peroxidase while it is complexed to ferricytochrome c reveals that the heme resonances of cytochrome c are not influenced by the oxidation state of cytochrome c peroxidase.     

Assignment of resonances in the Escherichia coli 5 S RNA fragment proton NMR spectrum using uniform nitrogen-15 enrichment

The downfield proton NMR spectrum of aqueous uniformly nitrogen-15 enriched 5 S RNA fragment is presented. Selective nitrogen-15 decoupling difference proton spectroscopy revealed nitrogen-15 chemical shifts of fragment imino nitrogens. Nitrogen chemical shifts of nucleic acid guanine and uracil imino nitrogens have separate small ranges. Nitrogen-15 and proton chemical shift correlation by the heteronuclear decoupling permitted the identification of the base type of some previously unassigned imino proton resonances in the 5 S RNA fragment spectrum. Corresponding resonances in the natural isotopic abundance 5 S RNA fragment spectrum are assigned to base types by comparison with the enriched sample spectrum.     

Proton magnetic resonance spectra of adrenodoxin: features of the aromatic region

This paper presents the first 1H-NMR spectra of the aromatic region of adrenodoxin, a mammalian mitochondrial 2Fe-2S non-heme iron ferredoxin. One-dimensional proton NMR spectra of both reduced and oxidized adrenodoxin were recorded as a function of pH. Resonances due to two of the three histidines of adrenodoxin gave sharp signals in the one-dimensional proton NMR spectra. The pKa values of the resolved histidine resonances in the oxidized protein were 6.64 +/- 0.03 and 6.12 +/- 0.06. These values were unchanged when adrenodoxin was reduced by the addition of sodium dithionite. In addition, the oxidized protein showed a broadened histidine C-2H resonance with a pKa value of approx. 7. This resonance was not apparent in the spectra of the reduced protein. The resonances due to the single tyrosine in adrenodoxin were identified using convolution difference spectroscopy. In addition, a two-dimensional Fourier-transform double quantum filtered (proton, proton) chemical shift correlated (DQF-COSY) spectrum of oxidized adrenodoxin was obtained. The cross peaks of the resonances due to the tyrosine, the four phenylalanines, and two of the three histidines of adrenodoxin were resolved in the DQF-COSY spectrum. Reduction of the protein caused several changes in the aromatic region of the NMR spectra. The resonances assigned to the C2 proton of the histidine with a pKa of 6.6 shifted upfield approx. 0.15 ppm. In addition, when the protein was reduced one of the resonances assigned to a phenylalanine residue with a chemical shift of 7.50 ppm appeared to move downfield to 7.82 ppm.(ABSTRACT TRUNCATED AT 250 WORDS)     

A nuclear overhauser study of heme orientational isomerism in monomeric Chironomus hemoglobins

The nuclear Overhauser effect (NOE) was used to investigate heme orientation and to obtain assignments for all resolved resonances in the 1H-NMR spectrum of met-cyano Chironomus thummi thummi monomeric hemoglobins III and IV (Hb III and Hb IV). The only non-heme resolved resonance was found to be from Phe-38 (CD1), and NOE dipolar connectivity between this resonance and the heme 5- and 8-methyls was used to establish the absolute orientation of the heme for each heme-insertion isomer present. The assignments of resonances and heme disorder permitted structural comparisons between the various components, including those due to a point mutation in Hb III. Finally, the characteristic differences of NOE patterns to amino-acid protons from substituents on heme pyrroles I and II formed the basis for assigning resonances and heme orientation relative to native Hb IV for deuterohemin-reconstituted Hb IV, for which there are no X-ray data available.     

The downfield resonances in the 1H NMR spectra of Azotobacter vinelandii and Pseudomonas putida seven-iron ferredoxins

Pseudomonas putida and Azotobacter vinelandii ferredoxins each contain one [4Fe-4S] cluster and one [3Fe-4S] cluster. Their polypeptide chains are nearly identical, differing by only 15 residues out of a total of 106. T1 measurements and temperature dependence studies of the 1H NMR spectrum of each ferredoxin demonstrate that all six resolved downfield resonances are near an iron-sulfur center. The five most downfield resonances are shown to arise from protons on cysteinyl beta-carbons by incorporation of cysteine deuterated at the beta-carbon into cell protein. The sixth peak (10.5 ppm) is shown to be a non-cysteinyl proton. This peak resolves into two resonances of approximately equal intensity at temperatures below 15 degrees or above 25 degrees C. A nuclear Overhauser effect observed between the two downfield-most resonances of A. vinelandii ferredoxin indicates that they originate from a geminal pair of beta-cysteinyl protons. An Overhauser effect observed between the resonances at 22.3 and 15.7 ppm, in conjunction with other results, implies that the resonance at 22.3 ppm arises from a beta-proton on the 3Fe-center-bound Cys16, while the resonance at 15.7 ppm arises from Cys45 beta-proton, which is bound to the 4Fe center. The five most downfield resonances are pH-dependent. The sixth peak (10.5 ppm in P. putida ferredoxin) is pH-independent. Possible origins for the observed pH dependencies are discussed.     

Comparison of histidine proton magnetic resonances of human carbonmonoxyhaemoglobin in different buffers

We have recorded the C-2 proton resonances of the histidines of carbonmonoxyhaemoglobin A and of four abnormal human HbCOs in different buffers and at different concentrations of haemoglobin. Resonance H assigned by Perutz et al. (1985) to His HC3(146) beta, is present at both pH 7.30 and pH 6.90, but somewhat broadened when recorded in 5 to 10% HbCO A in 0.1 M-bis-Tris. The broadening disappears on tenfold dilution of the Hb with bis-Tris and the resonance then stands out sharply. Resonance H is absent at both Hb concentrations in HbCO Cowtown (His HC3(146) beta----Leu). HbCO Fort de France (His CD3(45) alpha----Arg) in 0.1 M-bis-Tris of pH 6.90 has a spectrum similar to that of HbCO A. In the same buffer a resonance marked L by Russu et al. (1982) is absent from the spectrum of Hb Abbruzzo (His H21(143) beta----Arg), whereas resonance H is present. Hb Barcelona contains an additional histidine in position FG1(94) beta; in 0.1 M-bis-Tris buffer of pH 6.90 its resonance is not resolved and resonance H is either shifted or broadened. The resonances of both histidines are resolved in phosphate buffer. At pH 6.90, spectra in 0.1 M-bis-Tris buffer are similar to those previously recorded in 0.2 M-HEPES. Addition of 0.1 M-KCl produces marked changes. Replacement of bis-Tris by 0.2 M-KCl + 0.2 M-phosphate gives rise to a different and much better resolved spectrum.     

Nuclear Overhauser experiments at 500 MHz on the downfield proton spectra of 5S ribonucleic acid and its complex with ribosomal protein L25

The downfield (9-15 ppm) proton spectrum of Escherichia coli 5S RNA has been examined at 500 MHz by using nuclear Overhauser methods. The data confirm the existence of the terminal and procaryotic loop helices within the molecule [Fox, G. E., & Woese, C. R. (1975) Nature (London) 256, 505-506]. Very little stable, double-helical structure is detectable in the third loop of the molecule, the one comprising bases 12-68. The downfield spectrum of 5S RNA is perturbed in a highly specific manner upon addition of protein L25 to the system. The changes seen strongly suggest that the binding site for L25 on 5S RNA includes the procaryotic loop helix, but not the terminal stem helix. Similar complexes formed between L25 and the 5S RNA fragment consisting of bases 1-11, 69-87, and 89-120 show exactly the same spectral alterations. A number of downfield resonances appear in the spectra of these complexes which have no counterparts in the free RNA, suggesting the stabilization of new RNA structures by the protein. There are some indications of protein-nucleic acid nuclear Overhauser effects.     

19F NMR of 5-fluorouracil-substituted transfer RNA transcribed in vitro: resonance assignment of fluorouracil-guanine base pairs.

5-Fluorouracil is readily incorporated into active tRNA(Val) transcribed in vitro from a recombinant phagemid containing a synthetic E. coli tRNA(Val) gene. This tRNA has the expected sequence and a secondary and tertiary structure resembling that of native 5-fluorouracil-substituted tRNA(Val), as judged by 19F NMR spectroscopy. To assign resonances in the 19F spectrum, mutant phagemids were constructed having base changes in the tRNA gene. Replacement of fluorouracil in the T-stem with cytosine, converting a FU-G to a C-G base pair, results in the loss of one downfield peak in the 19F NMR spectrum of the mutant tRNA(Val). The spectra of other mutant tRNAs having guanine for adenine substitutions that convert FU-A to FU-G base pairs all have one resonance shifted 4.5 to 5 ppm downfield. These results allow assignment of several 19F resonances and demonstrate that the chemical shift of the 19F signal from base-paired 5-fluorouracil differs considerably between Watson-Crick and wobble geometry.     

Effect of magnesium ion on the structure of the 5S RNA from Escherichia coli. An imino proton magnetic resonance study of the helix I, IV, and V regions of the molecule

The imino proton nuclear magnetic resonance spectrum of Escherichia coli 5S ribonucleic acid (RNA) changes when the Mg2+ ion concentration drops below physiological levels. The transition between the physiological and low magnesium spectral forms of 5S RNA has a midpoint at approximately 0.3 mM Mg2+. Many of the most conspicuous changes observed in the downfield spectrum of 5S RNA as the magnesium concentration is reduced are due to adjustments in the structures of helices I and IV and the disappearance of resonances originating in helix V. The binding of ribosomal protein L25 to 5S RNA in the absence of magnesium stabilizes helix V structures.     

Complete assignment of the imino protons of Escherichia coli valine transfer RNA: two-dimensional NMR studies in water

The imino proton spectrum of Escherichia coli valine tRNA has been studied by two-dimensional nuclear Overhauser effect spectroscopy (NOESY) in H2O solution. The small nuclear Overhauser effects from the imino proton of an internal base pair to the imino protons of each nearest neighbor can be observed as off-diagonal cross-peaks. In this way most of the sequential NOE connectivity trains for all the helices in this molecule can be determined in a single experiment. AU resonances can be distinguished from GC resonances by the AU imino NOE to the aromatic adenine C2-H, thus leading to specific base-pair assignments. In general, the NOESY spectrum alone is not capable of assigning every imino proton resonance even in well-resolved tRNA spectra. Multiple proton peaks exhibit more than two cross-peaks, resulting in ambiguous connectivities, and coupling between protons with similar chemical shifts produces cross-peaks that are incompletely resolved from the diagonal. The sequence of the particular tRNA determines the occurrence of the latter problem, which can often be solved by careful one-dimensional experiments. The complete imino proton assignments of E. coli valine tRNA are presented.     

Assignment of acyl chain resonances from membranes of mammalian cells by two-dimensional NMR methods

Two-dimensional nuclear magnetic resonance (NMR) methods have been successfully used to assign resonances in the 1H NMR spectrum of intact viable rat mammary adenocarcinoma cells. Two-dimensional scalar-correlated spectroscopy identifies connectivities for resonances of the lipid acyl chains in the plasma membrane of these cells. We expect that two-dimensional scalar-correlated methods may be of general use for providing unequivocal assignments in the complex and often poorly resolved 1H NMR spectra of cells.     

1H NMR studies of transfer RNA III: the observed and the computed spectra of the hydrogen-bonded NH resonances of baker''s yeast transfer-RNA Phe.

The hydrogen-bonded NH resonances of Baker's yeast tRNAphe in H2O solution with Mg++ have been measured by a 360 MHz spectrometer at 23 degrees C. Totally, fifteen peaks and one shoulder can be resolved which represent 25 +/- 1 protons. Based on the refined atomic coordinates of the tRNAphe in the orthorhombic crystal, on the recent advances in the distance dependence of the ring-current magnetic field effects and on the adopted values for the isolated hydrogen-bonded NH resonances, a computed spectrum consisting of 23 protons was constructed. A quantitative comparison by computer was made between the computed spectrum and the spectrum simulated from the observed spectrum. These two spectra are closely similar but not identical. We suggest that the conformation of yeast tRNAphe in aqueous solution is closely similar but not identical to that found in the crystal, especially in the T psi C region and D region. Also the NH resonances in 3-4 proposed hydrogen bonds (most likely for tertiary structure) may exchange very rapidly in aqueous solution.     

Imino proton NMR assignments and ion-binding studies on Escherichia coli tRNA3Gly

The imino region of the proton NMR spectrum of Escherichia coli tRNA3Gly has been assigned mainly by sequential nuclear Overhauser effects between neighbouring base pairs and by comparison of assignments of other tRNAs. The effects of magnesium, spermine and temperature on the 1H and 31P NMR spectra of this tRNA were studied. Both ions affect resonances close to the G15 . C48 tertiary base pair and in the ribosylthymine loop. The magnesium studies indicate the presence of an altered tRNA conformer at low magnesium concentrations in equilibrium with the high magnesium form. The temperature studies show that the A7 . U66 imino proton (from a secondary base pair) melts before some of the tertiary hydrogen bonds and that the anticodon stem does not melt sequentially from the ends. Correlation of the ion effects in the 1H and 31P NMR spectra has led to the tentative assignment of two 31P resonances not assigned in the comparable 31P NMR spectrum of yeast tRNAPhe. 31P NMR spectra of E. coli tRNA3Gly lack resolved peaks corresponding to peaks C and F in the spectra of E. coli tRNAPhe and yeast tRNAPhe. In the latter tRNAs these peaks have been assigned to phosphate groups in the anticodon loop. Ion binding E. coli tRNA3Gly and E. coli tRNAPhe had different effects on their 1H NMR spectra which may reflect further differences in their charge distribution and conformation.     

A nuclear Overhauser effect study of the active site of myeloperoxidase. Structural similarity of the prosthetic group to that on lactoperoxidase

The low-spin, cyanide-ligated ferric complex of the intact bovine granulocyte myeloperoxidase (MPO-CN) has been studied by proton nuclear magnetic resonance utilizing the nuclear Overhauser effect (NOE). This is the largest globular protein (approximately 1.5 x 10(5) for the intact alpha 2 beta 2 tetrameric species) for which successful NOEs have been observed without serious interference of spin diffusion, and demonstrably confirms the utility of such studies on large paramagnetic as compared to diamagnetic proteins. The 1H NMR spectrum of MPO-CN is found to have a remarkable similarity in the number, resonance pattern, and metal ion-induced relaxation properties of the resolved, hyperfine-shifted resonances to those reported earlier for the analogous complex of bovine lactoperoxidase (LPO-CN); moreover, the interproton connectivities between pairs of hyperfine-shifted proton sets, as reflected by the NOEs, are also essentially the same (Thanabal, V., and La Mar, G. N. (1989) Biochemistry 28, 7038-7044). Since the extracted prosthetic group of lactoperoxidase is a porphyrin with proposed functionalization of the 8-methylene group (Nichol, A. W., Angel, L. A., Moon, T., and Clezy, P. S. (1987) Biochem. J. 247, 147-150), we interpret the resultant similarity in 1H NMR spectral parameters for LPO-CN and MPO-CN as indicating that the prosthetic groups in MPO and LPO are very similar, and hence likely both porphyrins with a similarly functionalized periphery that allows covalent linkage to the protein matrix. The hyperfine shift pattern of the broadest resolved lines lead to their assignment to the axial histidyl imidazole side chain. Two pairs of resonances are found to have similar relaxation properties and/or dipolar as similarly shifted resonances that arise from a distal His and Arg in horseradish peroxidase (as also found in LPO-CN), and suggest that MPO also possesses these catalytically functional residues in the distal heme pocket.     

Magnetic resonance study of the distribution of 2,2,6,6-tetramethylpiperidine-N-oxyl in phosphatidylcholine bilayers.

With the aid of paramagentic praseodymium ions the resonances at 60 MHz of the inward and outward facing choline methyl protons of sonicated egg yolk phosphatidylcholine vesicles were resolved. The subsequent addition of 2,2,6,6,-tetramethylpiperidine-N-oxyl (TEMPO) to the vesicle suspension broadened the inner and outer resonances equally. TEMPO easily penetrates the egg yolk phosphatidylcholine vesicles and has free access to the entire lipid volume above the gel to liquid crystalline transition temperature. The electron spin resonance (ESR) spectrum of TEMPO in the egg yolk phosphatidylcholine suspension exhibits features indicating that TEMPO dissolves principally in the hydrocarbon portion of the egg yolk phosphatidylcholine bilayer. The egg yolk phosphatidylcholine methylene chain proton resonances are also broadened by TEMPO notably to a greater extent than the choline methyl resonances. These data indicate that TEMPO should be more sensitive to the melting behavior of the fatty acyl chains of phospholipid suspensions than to the polar head groups.     

NMR spectroscopic studies of the hydrogenosomal [2Fe-2S] ferredoxin from Trichomonas vaginalis: hyperfine-shifted 1H resonances

The hyperfine-shifted 1H NMR resonances of oxidized and reduced Trichomonas vaginalis ferredoxin, a functionally unique [2Fe-2S] ferredoxin, have been studied. The oxidized protein spectrum displayed a pattern of six broad upfield-shifted resonances between 13 and 40 ppm with chemical shifts distinct from those of other [2Fe-2S] ferredoxins. All hyperfine 1H resonances of the oxidized ferredoxin displayed anti-Curie temperature dependences. Reduced T. vaginalis ferredoxin displayed hyperfine resonances both upfield and downfield of the diamagnetic region. These resonances showed Curie temperature dependences. Overall the hyperfine-shifted NMR spectrum of T. vaginalis ferredoxin, along with other spectroscopic properties, suggested different structural properties for the active center of oxidized hydrogenosomal ferredoxins from those of other [2Fe-2S] ferredoxins.     

Assignment of paramagnetically shifted resonances in the 1H NMR spectrum of horse ferricytochrome c.

The proton resonances of the heme, the axial ligands, and other hyperfine-shifted resonances in the 1H nuclear magnetic resonance spectrum of horse ferricytochrome c have been investigated by means of one- and two-dimensional nuclear Overhauser and magnetization transfer methods. Conditions for saturation transfer experiments in mixtures of ferro- and ferricytochrome c were optimized for the cross assignment of corresponding resonances in the two oxidation states. New resonance assignments were obtained for the methine protons of both thioether bridges, the beta and gamma meso protons, the propionate six heme substituent, the N pi H of His-18, and the Tyr-67 OH. In addition, several recently reported assignments were confirmed. All of the resolved hyperfine-shifted resonances in the spectrum of ferricytochrome c are now identified. The Fermi contact shifts experienced by the heme and ligand protons are discussed.