Paramagnetic NMR Spectroscopy Is a Tool to Address Reactivity, Structure, and Protein–Protein Interactions of Metalloproteins: The Case of Iron–Sulfur Proteins

The study of cellular machineries responsible for the iron–sulfur (Fe–S) cluster biogenesis has led to the identification of a large number of proteins, whose importance for life is documented by an increasing number of diseases linked to them. The labile nature of Fe–S clusters and the transient protein–protein interactions, occurring during the various steps of the maturation process, make their structural characterization in solution particularly difficult. Paramagnetic nuclear magnetic resonance (NMR) has been used for decades to characterize chemical composition, magnetic coupling, and the electronic structure of Fe–S clusters in proteins; it represents, therefore, a powerful tool to study the protein–protein interaction networks of proteins involving into iron–sulfur cluster biogenesis. The optimization of the various NMR experiments with respect to the hyperfine interaction will be summarized here in the form of a protocol; recently developed experiments for measuring longitudinal and transverse nuclear relaxation rates in highly paramagnetic systems will be also reviewed. Finally, we will address the use of extrinsic paramagnetic centers covalently bound to diamagnetic proteins, which contributed over the last twenty years to promote the applications of paramagnetic NMR well beyond the structural biology of metalloproteins.

The NMR contribution to protein-protein networking in Fe-S protein maturation

Iron–sulfur proteins were among the first class of metalloproteins that were actively studied using NMR spectroscopy tailored to paramagnetic systems. The hyperfine shifts, their temperature dependencies and the relaxation rates of nuclei of cluster-bound residues are an efficient fingerprint of the nature and the oxidation state of the Fe–S cluster. NMR significantly contributed to the analysis of the magnetic coupling patterns and to the understanding of the electronic structure occurring in [2Fe–2S], [3Fe–4S] and [4Fe–4S] clusters bound to proteins. After the first NMR structure of a paramagnetic protein was obtained for the reduced E. halophila HiPIP I, many NMR structures were determined for several Fe–S proteins in different oxidation states. It was found that differences in chemical shifts, in patterns of unobserved residues, in internal mobility and in thermodynamic stability are suitable data to map subtle changes between the two different oxidation states of the protein. Recently, the interaction networks responsible for maturing human mitochondrial and cytosolic Fe–S proteins have been largely characterized by combining solution NMR standard experiments with those tailored to paramagnetic systems. We show here the contribution of solution NMR in providing a detailed molecular view of “Fe–S interactomics”. This contribution was particularly effective when protein–protein interactions are weak and transient, and thus difficult to be characterized at high resolution with other methodologies.

The IR-¹⁵N-HSQC-AP experiment: a new tool for NMR spectroscopy of paramagnetic molecules

A crucial factor for the understanding of structure-function relationships in metalloproteins is the identification of NMR signals from residues surrounding the metal cofactor. When the latter is paramagnetic, the NMR information in the proximity of the metal center may be scarce, because fast nuclear relaxation quenches signal intensity and coherence transfer efficiency. To identify residues at a short distance from a paramagnetic center, we developed a modified version of the ¹⁵N-HSQC experiment where (1) an inversion recovery filter is added prior to HSQC, (2) the INEPT period has been optimized according to fast relaxation of interested spins, (3) the inverse INEPT has been eliminated and signals acquired as antiphase doublets. The experiment has been successfully tested on a human [Fe₂S₂] protein which is involved in the biogenesis of iron-sulfur proteins. Thirteen HN resonances, unobserved with conventional HSQC experiments, could be identified. The structural arrangement of the protein scaffold in the proximity of the Fe/S cluster is fundamental to comprehend the molecular processes responsible for the transfer of Fe/S groups in the iron-sulfur protein assembly machineries.

Structure and dynamics of reduced Bacillus pasteurii cytochrome c: oxidation state dependent properties and implications for electron transfer processes

The solution structure of reduced Bacillus pasteurii cytochrome c, which has only 71 amino acids, has been determined by NMR to an RMSD of 0.46 +/- 0.08 A for all backbone atoms and 0.79 +/- 0.08 A for all heavy atoms and refined through restrained energy minimization. The target function out of 1645 constraints is 0.52 +/- 0.11 A(2), and the penalty function is 66 +/- 12 kJ mol(-)(1). The structure appears very similar to that in the oxidized state, only Trp87 and the propionates showing significant differences. The mobility was investigated through (15)N R(1) and R(2) relaxation rates, (15)N-(1)H NOE, and (1)H/(2)H exchange. It is found that the oxidized form is generally more mobile than the reduced one. By comparing the redox-state dependence of the structural/dynamic properties of Fe-S proteins, cytochrome c, and blue copper proteins, hints are provided for a better comprehension of the electron transfer processes.

Backbone dynamics of plastocyanin in both oxidation states. Solution structure of the reduced form and comparison with the oxidized state

A model-free analysis based on (15)N R(1), (15)N R(2), and (15)N-(1)H nuclear Overhauser effects was performed on reduced (diamagnetic) and oxidized (paramagnetic) forms of plastocyanin from Synechocystis sp. PCC6803. The protein backbone is rigid, displaying a small degree of mobility in the sub-nanosecond time scale. The loops surrounding the copper ion, involved in physiological electron transfer, feature a higher extent of flexibility in the longer time scale in both redox states, as measured from D(2)O exchange of amide protons and from NH-H(2)O saturation transfer experiments. In contrast to the situation for other electron transfer proteins, no significant difference in the dynamic properties is found between the two redox forms. A solution structure was also determined for the reduced plastocyanin and compared with the solution structure of the oxidized form in order to assess possible structural changes related to the copper ion redox state. Within the attained resolution, the structure of the reduced plastocyanin is indistinguishable from that of the oxidized form, even though small chemical shift differences are observed. The present characterization provides information on both the structural and dynamic behavior of blue copper proteins in solution that is useful to understand further the role(s) of protein dynamics in electron transfer processes.

Extension of transverse relaxation-optimized spectroscopy techniques to allosteric proteins: CO- and paramagnetic fluoromet-hemoglobin [beta (15N-valine)]

We present the first steps in applying transverse relaxation-optimized spectroscopy (TROSY) techniques to the study of allosterism. Each beta-chain of the hemoglobin (Hb) tetramer has 17 valine residues. We have (15)N-labeled the beta-chain Val residues and detected 16 of the 17 (1)H-(15)N correlation peaks for beta-chain Val of the R state CO-Hb structure by using the TROSY technique. Sequence-specific assignments are suggested, based mainly on analysis of the (1)H pseudocontact-shift increments produced by oxidizing the diamagnetic R state HbCO to the paramagnetic R state fluoromet form. When possible, we support these assignments with sequential nuclear Overhauser effect (NOE) information obtained from a two-dimensional [(1)H,(1)H]-NOESY-TROSY experiment (NOESY, NOE spectroscopy). We have induced further the R-T conformational change by adding the allosteric effector, inositol hexaphosphate, to the fluoromet-Hb sample. This change induces substantial increments in the (1)H and (15)N chemical shifts, and we discuss the implication of these findings in the context of the tentative sequence assignments. These preliminary results suggest that amide nitrogen and amide proton chemical shifts in a selectively labeled sample are site-specific probes for monitoring the allosteric response of the ensemble-averaged solution structure of Hb. More important, the chemical-shift dispersion obtained is adequate to permit a complete assignment of the backbone (15)N/(13)C resonances upon nonselective labeling.

New applications of paramagnetic NMR in chemical biology

The methodological accessibility to solution structure and dynamic investigation of paramagnetic metallobiomolecules has afforded the ability to tackle the redox pairs of electron transfer proteins of which at least one is paramagnetic, to study the orientation effects of high magnetic fields on paramagnetic biomolecules, and finally to study the role of metal-based cofactors in protein folding and stability.

Probing the backbone dynamics of oxidized and reduced rat microsomal cytochrome b5 via 15N rotating frame NMR relaxation measurements: biological implications

Rotating frame 15N relaxation NMR experiments have been performed to study the local mobility of the oxidized and reduced forms of rat microsomal cytochrome b5, in the microsecond to millisecond time range. Measurements of rotating frame relaxation rates (R1rho) were performed as a function of the effective magnetic field amplitude by using off-resonance radio frequency irradiation. Detailed analysis of the two data sets resulted in the identification of slow motions along the backbone nitrogens for both oxidation states of the protein. The local mobility of reduced and oxidized cytochrome b5 turned out to be significantly different; 28 backbone nitrogens of the oxidized form were shown to participate in a conformational exchange process, while this number dropped to 12 in the reduced form. The correlation time, tauex, for the exchange processes could be determined for 21 and 9 backbone nitrogens for oxidized and reduced cytochrome b5, respectively, with their values ranging between 70 and 280 microseconds. The direct experimental evidence provided in this study for the larger mobility of the oxidized form of the protein is consistent with the different backbone NH solvent exchangeability recently documented for the two oxidation states [Arnesano, F., et al. (1998) Biochemistry 37, 173-184]. Our experimental observations may have significant biological implications. The differential local mobility between the two oxidation states is proposed to be an important factor controlling the molecular recognition processes in which cytochrome b5 is involved.

Solution structure of oxidized cytochrome c6 from the green alga Monoraphidium braunii

Cytochrome c6 from Monoraphidium braunii, an 89-amino acid electron transfer protein, has been investigated by NMR in solution, in its oxidized form, at pH 7 and 300 K. By using a combination of COSY, TOCSY, and NOESY experiments, 84% of the proton resonances have been assigned. A total of 1668 experimental NOE constraints, 1109 of which were meaningful, together with 288 pseudocontact shifts, have been used to determine the structure in solution. This is represented as a family of 40 structures which have been energy minimized. The rmsd values with respect to the mean structure are 0.57 +/- 0.08 and 0.94 +/- 0.09 A for the backbone and heavy atoms, respectively. The structure has been found to be very similar to that of the reduced form, except for a rearrangement in propionate 7, a feature which has been observed in all c-type cytochromes investigated so far. Such a feature could be relevant for the efficiency of the electron transfer pathway with either the oxidizing or the reducing partners. Other differences in the oxidation states have been noted in the region proposed to be involved in the interaction with the physiological partners.

1H and (13)C NMR Studies of an Oxidized HiPIP

1H-(13)C HETCOR NMR spectra have been recorded for the oxidized HiPIP I from Ectothiorhodospira halophila for which an extended (1)H assignment was available. The hyperfine shifts of the alpha and beta carbons of the coordinated cysteines, as well as those of their attached protons, have been discussed in terms of the current magnetic coupling models and of the mechanisms of spin density delocalization. Through HSQC spectra preceded by a proton 180 degrees pulse, the nonselective T(1) values of the protons have been accurately obtained. It is shown how the nuclear T(1) values can be used as constraints, together with NOEs, for solution structure determination even when the present magnetic coupling scheme occurs. The oxidized cluster is shown to have an effective relaxation time much shorter than that in the reduced state.

Solution structure of reduced microsomal rat cytochrome b5

The solution structure of the major form of the reduced soluble fragment of rat microsomal cytochrome b5 has been solved through 1H-NMR spectroscopy. The protein contains 98 amino acids. Proton assignment was available for residues 1-94, except 90 [Guiles, R. D., Basus, V. J., Kuntz, I. D. & Waskell, L. (1992) Biochemistry 31, 11,365-11,375] and has been confirmed. From 1722 NOEs, of which 1203 were found to be meaningful, a family of 40 energy-minimized structures has been obtained with average backbone rmsd (for residues 5-89) of 0.078 +/- 0.018 nm and average target function of 0.0045 nm2, no distance violations being larger than 0.029 nm. The structure has been compared with the X-ray structure of the oxidized rat mitochondrial isoenzyme and with that of the highly similar bovine microsomal isoenzyme in the oxidized form. The analysis of the elements of secondary structure is instructive in terms of their stability and of their occurrence in related structures, and of the capability of NMR and X-ray spectroscopy to observe them. Some detailed structural variations are noticed among the solved structures of the various isoenzymes and between solid and solution. The structural features in solution of the residues proposed to be involved in protein-protein recognition are found to be largely conserved with respect to the solid state.

Characterization of a partially unfolded high potential iron protein

A partially unfolded state of the Fe4S4-containing high potential iron-sulfur protein from Chromatium vinosum has been detected and characterized by NMR spectroscopy following addition of a concentrated solution of guanidinium chloride to the native protein. This intermediate species (i) maintains the polymetallic center, (ii) exhibits a largely collapsed secondary structure, and (iii) undergoes fast cluster decomposition upon oxidation. This information is framed into the knowledge about this class of proteins, and the possible role of this intermediate with respect to the in vivo folding/unfolding process is discussed as well its role in the slow hydrolytic degradation characteristic of oxidized HiPIPs.

The solution structure refinement of the paramagnetic reduced high-potential iron-sulfur protein I from Ectothiorhodospira halophila by using stable isotope labeling and nuclear relaxation

The reduced high-potential iron sulfur protein I from Ectothiorhodospira halophila which contains the [4Fe-4S]2+ polymetallic center has been fully labeled with 15N and 13C. The protein is paramagnetic, the nuclear relaxation times of nuclei close to the paramagnetic ion are drastically shortened and some strategic dipolar connectivities are lost. Notwithstanding, the solution structure has been reported [Banci, L., Bertini, I., Eltis, L. D., Felli, I. C., Kastrau, D. H. W., Luchinat, C., Piccioli, M., Pierattelli, R. & Smith, M. (1994) Eur. J. Biochem. 225, 715-725]. We have performed classical HNHA, HNCA soft-COSY, soft-HCCH E. COSY and 15N-1H correlated NOESY experiments in order to obtain a set of 3J scalar coupling constants. Some experiments have been optimized to counterbalance the effect of paramagnetism. From heteronuclear single-quantum experiments preceded by a 180 degrees pulse and variable delay times, the non-selective magnetization recovery has been followed from which the contribution to dipolar relaxation of nuclei due to the interaction with the paramagnetic metal ions (rho para) has been estimated. Finally, the intensities of NOEs have been corrected for the presence of paramagnetic metal ions and these corrected values together with 3J values and rho para data have been used to obtain a well defined solution structure. The aim is that of obtaining a structure with enough constraints to be well resolved all over the protein, including the vicinity of the paramagnetic metal cluster, which is anchored to the protein through the rho para constraints. In total, 1226 corrected NOESY crosspeaks (of which 945 were found to be meaningful), 37 one-dimensional NOEs, 39 3JHNH alpha and 37 3JHNC' (providing 45 phi dihedral angle constraints) 54 3JH alpha H beta and 31 3JNH beta (providing 26 chi 1 dihedral angle constraints), 4 chi 2 dihedral angle constraints of the coordinated cysteines, obtained from the hyperfine shifts of the beta CH protons, and 58 rho para constraints, have been used for structure calculation. Restrained molecular dynamics simulations have also been performed to provide the final family of structures. This research demonstrates that stable isotope labeling provides specific advantages for the NMR investigation of paramagnetic molecules, as the small magnetic moment of heteronuclei minimizes the paramagnetic influence of unpaired electrons.