13C-NMR study of labeled vinyl groups in paramagnetic myoglobin derivatives

13C direct detected experiments: optimization for paramagnetic signals

To optimize 13C direct detected experiments for the observation of signals close to a paramagnetic center, we have assessed the sensitivity of different sequences based on CO-Cali coherence transfer. Features of CACO experiments were tested for Calbindin D9k, in which one of the two native Ca2+ ions is replaced by the paramagnetic Ce3+ ion. We have studied the comparison of single vs multiple quantum coherence transfer evolution as well as the influence of in-phase vs anti-phase detection of 13CO signals and finally the comparison of a coherence transfer step based on a CyO in plane with respect to a Cy ali in plane. The acquisition of the anti-phase component of the signal, accomplished by the removal of the last refocusing steps, allowed the identification of some signals unobserved with other pathways. The structural dependency of paramagnetism-induced nuclear relaxation is such that the identification of the most suitable coherence transfer pathway is not known "a priori" but it is driven by the relative proximity of Cali and CO to the paramagnetic center.

Determination of haem electronic structure in cytochrome b5 and metcyanomyoglobin

The paramagnetic shifts of 13C nuclei positioned alpha to the haems in the A and B forms of rat cytochrome b5 and in metcyanomyoglobin have been analysed in terms of molecular orbitals based on D4h symmetry with a rhombic perturbation. The contribution to the 13C shifts from pseudocontact interactions is calculated from parameters obtained for a metal-centred dipolar shift tensor by fitting 1H shifts. The effect of electron delocalisation onto the vinyl groups of these haems b is separated with reference to the shifts of the vinyl beta carbons. In each case, it was found that the orientation of the magnetic axes in the plane of the haem is rotated away from the iron-nitrogen vectors in the opposite sense to the rotation of the rhombic perturbation and the molecular orbitals. The orientation of the orbitals is closely aligned with the normal to the single His ligand in metcyanomyoglobin, and with the average of the two normals in the bis-His cytochrome b5. It is concluded that the in-plane anisotropy of haems b is dominated by the orientation of the axial ligands in a similar manner to that in haems c and that the approximations used are weakened, but not invalidated, by the presence of partially conjugated vinyl groups.

Assignment of hyperfine-shifted heme carbon resonances in ferricytochrome b5

The reverse detection heteronuclear multiple quantum coherence, HMQC, study of native bovine ferricytochrome b5 has provided the complete assignment of hyperfine shifted resonances of heme carbons attached proton(s). The dominant delocalized pi-spin density to vinyl groups gives rise to contact shifts which have opposite direction for a carbon and its attached proton(s). The most hyperfine shifted 13C heme signals are mainly generated from 3rd heme pyrrole ring substituents which identifies that the molecular orbital for facile electron transfer is oriented to exposed heme edge. Magnetic/electronic asymmetry of heme induced by two axial His makes spread the hyperfine shifted heme carbon resonances over the range of 280 ppm at 25 degrees C, which would be the more sensitive probe than those of proton resonances in characterizing the nature of heme electronic structure of ferricytochrome b5.

Two-dimensional NMR characterization of the deoxymyoglobin heme pocket

Traditionally, assigning the heme protein resonances has relied heavily on the comparison of spectra arising from protein reconstituted with specifically deuterated hemes and the native form. Such an approach can identify tentatively the broad, overlapping signals in the Fe(II) high-spin heme protein spectra. Although 2D NMR studies have reported alternative approaches to detect and assign paramagnetic signals, their effectiveness is limited primarily to Fe(III) low-spin systems and still depends upon isotopic labeling results to be definitive. For deoxymyoglobin, the reported 2D techniques have not produced any spin correlation maps. Nevertheless, our study demonstrates that the deoxymyoglobin spin correlations are indeed detectable and that a complete heme assignment, except for the meso protons, is achievable with only 2D NMR and saturation-transfer techniques. The 2D maps improve the spectral resolution dramatically and permit a comprehensive analysis of the deoxymyoglobin signals' temperature dependence, which supports the hypothesis that the electronic orbital ground state has contributions from both 5E and 5B2. The results also indicate a structural perturbation in the vicinity of the 2 vinyl group as the protein undergoes the transition from oxy- to deoxymyoglobin state and a significant contribution from zero field splitting. Moreover, saturation-transfer experiments show that NMR can observe directly oxygen binding kinetics.

1H-NMR study of reduced heme proteins myoglobin and cytochrome P450

The 1H-NMR spectra of deoxymyoglobin and reduced cytochrome P450 are analyzed by NOE spectroscopy. Progress has been made in the assignment of the hyperfine-shifted signals of deoxymyoglobin. The nuclear longitudinal-relaxation-time values indicate short electron-relaxation times whereas Curie relaxation contributes significantly to the signals linewidths. For reduced cytochrome P450 the linewidths are larger due to the Curie-relaxation contribution in a large protein. Therefore, the spectral information is poor. The electron-relaxation rates are discussed in terms of possible electronic structure.

Orientation and mobility of the heme vinyl groups in myoglobins with the aid of NOE and MATDUHM NMR

The heme vinyl substituents in a shark (Galeorhinus japonicus) myoglobin in its met-cyano form (MbCN) have been characterized by NMR and the results were compared with those of the well-studied sperm whale (Physter catodon) myoglobin. Their orientation has been inferred from NOE connectivities and the analysis of the hyperfine shifts based on the principal magnetic tensor determined by MATDUHM (Magnetic Anisotropy Tensor Orientation Determination Utilizing the Heme Methyls) [Yamamoto, Y., Nanai, N. and Chujo, R.(1990) J. Chem. Soc., Chem. Commun. 1556-1557]. It has been shown that the C3-vinyl group is oriented roughly orthogonal to the heme plane in both G. japonicus and P. catodon MbCNs at 35 degrees C and their C8-vinyl groups, on the other hand, are close to in-plane orientation. Although CO form of myoglobin (MbCO) and MbCN have been thought to be isostructural to each other, the C8-vinyl orientation for P. catodon MbCN is found to be different from the orthogonal orientation indicated in the crystal structure analysis of MbCO [Hanson, J.C. and Schoenborn, B.P. (1981) J. Mol. Biol. 153, 117-146]. Their mobility has been characterized quantitatively from the study of time-dependent NOE build-up between the selected pair of the vinyl proton resonances. It has been revealed that the heme C3- and C8-vinyl groups of approximately 1 mM G. japonicus MbCN at 45 degrees C undergo internal motion with the correlation time of 1.9 and 2.4 ns, respectively. Therefore, their oscillatory motion is faster by a factor of 4-5 compared with the protein overall tumbling. Difference in the internal mobility between the two vinyl groups in the active site of this Mb is attributed to their differential contact with the apo-protein.

NMR study of Galeorhinus japonicus myoglobin. 1H-NMR evidence for a structural alteration on the active site of G. japonicus myoglobin upon azide ion binding

The heme molecular structure of the met-azido form of the myoglobin from the shark Galeorhinus japonicus has been investigated by 1H NMR. A nuclear Overhauser effect (NOE) was clearly observed among the heme peripheral side-chain proton signals of this complex, which undergoes thermal spin equilibrium between high-spin (S = 5/2) and low-spin (S = 1/2) states, and the NOE connectivities provided the assignment of the resonances from the heme C13(1)H2 and C17(1)H2 protons. Chemical shift inequivalence of these proton resonances not only provided information about the orientation of these methylene protons with respect to the heme plane, but also allowed characterization of the time-dependent build-up of the NOE between them, which yields the correlation time for the internal motion of the inter-proton vector. The relatively large mobility found for the C17(1)H2 group suggests that the carboxyl oxygen of the heme C17 propionate is not anchored to the apo-protein by a salt bridge. It has been shown that the ferric high-spin form of G. japonicus Mb possesses a penta-coordinated heme [Suzuki, T. (1987) Biochim. Biophys. Acta 914, 170-176; Yamamoto, Y., Osawa, A., Inoue, Y., Chûjô, R. & Suzuki, T. (1990) Eur. J. Biochem. 192, 225-229] and that the conformation of both heme propionate groups is fixed with respect to the heme, as well as the apo-protein, by a salt bridge [Yamamoto, Y., Inoue, Y., Chûjô, R. & Suzuki, T. (1990) Eur. J. Biochem. 189, 567-573]. Therefore the weakening or interruption of the interaction between the C17 propionate and His FG3 upon the changes of the coordination and spin state of the heme iron, during azide ion binding to ferric high-spin G. japonicus Mb, is attributed to the displacement of the FG corner of the apoprotein away from the heme C17 propionate group. A similar structural alteration has been revealed by X-ray structural analyses of unliganded and liganded forms of ferrous hemoproteins [Baldwin, J. & Chothia, C. (1979) J. Mol. Biol. 129, 175-220; Phillips, S.E.V. (1980) J. Mol. Biol. 142, 531-554].

NMR study of Galeorhinus japonicus myoglobin. 1H-NMR study of molecular structure of the heme cavity

The molecular structure of the active site of myoglobin from the shark, Galeorhinus japonicus, has been studied by 1H-NMR. Some hyperfine-shifted amino acid proton resonances in the met-cyano form of G. japonicus myoglobin have been unambiguously assigned by the combined use of various two-dimensional NMR techniques; they were compared with the corresponding resonances in Physter catodon myoglobin. The orientations of ThrE10 and IleFG5 residues relative to the heme in G. japonicus met-cyano myoglobin were semiquantitatively estimated from the analysis of their shifts using the magnetic susceptibility tensor determined by a method called MATDUHM (magnetic anisotropy tensor determination utilizing heme methyls) [Yamamoto, Y., Nanai, N. & Chûjô, R. (1990) J. Chem. Soc., Chem. Commun., 1556-1557] and the results were compared with the crystal structure of P. catodon carbonmonoxy myoglobin [Hanson, J. C. & Schoenborn, B. P. (1981) J. Mol. Biol. 153, 117-124]. In spite of a substantial difference in shift between the corresponding amino acid proton resonances for the two proteins, the orientations of these amino acid residues relative to the heme in the active site of both myoglobins were found to be highly alike.

Solution structural characteristics of cyanometmyoglobin: resonance assignment of heme cavity residues by two-dimensional NMR

Steady-state nuclear Overhauser effects (NOE), two-dimensional (2D) nuclear Overhauser effect spectroscopy (NOESY), and 2D spin correlation spectroscopy (COSY) have been applied to the fully paramagnetic low-spin, cyanide-ligated complex of sperm whale ferric myoglobin to assign the majority of the heme pocket side-chain proton signals and the remainder of the heme signals. It is shown that the 2D NOESY map reveals essentially all dipolar connectivities observed in ordinary 1D NOE experiments and expected on the basis of crystal coordinates, albeit often more weakly than in a diamagnetic analogue. For extremely broad (approximately 600-Hz) and rapidly relaxing (Tf1 approximately 3 ms) signals which show no NEOSY peaks, we demonstrate that conventional steady-state NOEs obtained under very rapid pulsing conditions still allow detection of the critical dipoar connectivities that allow unambiguous assignments. The COSY map was found to be generally less useful for the hyperfine-shifted residues, with cross peaks detected only for protons greater than 6 A from the iron. Nevertheless, numerous critical COSY cross peaks between strongly hyperfine-shifted peaks were resolved and assigned. In all, 95% (53 of 56 signals) of the total proton sets within approximately 7.5 A of the iron, the region experiencing the strongest hyperfine shifts and paramagnetic relaxation, are now unambiguously assigned. Hence it is clear that the 2D methods can be profitably applied to paramagnetic proteins. The scope and limitations of such application are discussed. The resulting hyperfine shift pattern for the heme confirmed expectations based on model compounds. In contrast, while exhibiting fortuitous 1H NMR spectral similarities, a major discrepancy was uncovered between the hyperfine shift pattern of the axially bound (F8 histidyl) imidazole in the protein and that of the imidazole in a relevant model compound [Chacko, V.P., & La Mar, G. N. (1982) J. Am. Chem. Soc. 104, 7002-7007], providing direct evidence for a protein-based deformation of axial bonding in the protein.

Determination of the functionally important heme peripheral vinyl group orientation in paramagnetic hemoprotein by 2D NMR

2D NMR spectroscopies have been successfully used to characterize the heme peripheral vinyl groups in paramagnetic hemoprotein in spite of the difficulties from the rapid paramagnetic relaxation and the low digital resolution of the 2D NMR map. The scalar coupling network system among the vinyl protons is clearly identified in the COSY spectra from its characteristic cross-peak pattern and the dipolar coupling connectivities of the vinyl proton resonances with other heme side-chain proton resonances not only provide the specific assignment of vinyl beta-proton resonances but also allow the determination of the vinyl group orientation with respect to the heme plane.

Natural abundance 13C-NMR study of paramagnetic horse heart ferricytochrome c cyanide complex: assignment of hyperfine shifted heme methyl carbon resonances

Hyperfine shifted heme methyl carbon resonances of paramagnetic horse heart ferricytochrome c cyanide complex (Cyt-c(CN)) have been observed for the first time in the natural abundance 13C-NMR spectrum and assigned using 1H-13C heteronuclear chemical shift correlated spectroscopy (1H-13C COSY). Individual heme methyl carbon NMR signal assignment permits a direct comparison between the hyperfine shifts of heme methyl carbon and attached methyl proton resonances which provides a useful information on the delocalization mechanism of the unpaired spin from the pi-conjugated system of porphyrin ring into the peripheral methyl side chains.

Assignment of hyperfine shifted haem methyl carbon resonances in paramagnetic low-spin met-cyano complex of sperm whale myoglobin

The hyperfine shifted resonances arising from all four individual haem carbons of the paramagnetic low-spin met-cyano complex of sperm whale myoglobin have been clearly identified and assigned for the first time with the aid of 1H-13C heteronuclear chemical shift correlated spectroscopy. Alteration of the in-plane symmetry of the electronic structure of haem induced by the ligation of proximal histidyl imidazole spreads the haem carbon resonances to 32 ppm at 22 degrees C, indicating the sensitivity of those resonances to the haem electronic/molecular structure. Those resonances are potentially powerful probes in characterizing the nature of haem electronic structure.