1H NMR studies of oxidized high-potential iron protein from Chromatium vinosum. Nuclear Overhauser effect measurements

Exploiting Molecular Symmetry to Quantitatively Map the Excited-State Landscape of Iron-Sulfur Clusters

Cuboidal [Fe4S4] clusters are ubiquitous cofactors in biological redox chemistry. In the [Fe4S4]1+ state, pairwise spin coupling gives rise to six arrangements of the Fe valences ("valence isomers") among the four Fe centers. Because of the magnetic complexity of these systems, it has been challenging to understand how a protein's active site dictates both the arrangement of the valences in the ground state as well as the population of excited-state valence isomers. Here, we show that the ground-state valence isomer landscape can be simplified from a six-level system in an asymmetric protein environment to a two-level system by studying the problem in synthetic [Fe4S4]1+ clusters with solution C3v symmetry. This simplification allows for the energy differences between valence isomers to be quantified (in some cases with a resolution of <0.1 kcal/mol) by simultaneously fitting the VT NMR and solution magnetic moment data. Using this fitting protocol, we map the excited-state landscape for a range of clusters of the form [(SIMes)3Fe4S4-X/L]n, (SIMes = 1,3-dimesityl-imidazol-4,5-dihydro-2-ylidene; n = 0 for anionic, X-type ligands and n = +1 for neutral, L-type ligands) and find that a single ligand substitution can alter the relative ground-state energies of valence isomers by at least 103 cm-1. On this basis, we suggest that one result of "non-canonical" amino acid ligation in Fe-S proteins is the redistribution of the valence electrons in the manifold of thermally populated excited states.

The structure of iron-sulfur proteins

Ferredoxins are a group of iron-sulfur proteins for which a wealth of structural and mutational data have recently become available. Previously unknown structures of ferredoxins which are adapted to halophilic, acidophilic or hyperthermophilic environments and new cysteine patterns for cluster ligation and non-cysteine cluster ligation have been described. Site-directed mutagenesis experiments have given insight into factors that influence the geometry, stability, redox potential, electronic properties and electron-transfer reactivity of iron-sulfur clusters.

1H NMR of high-potential iron-sulfur protein from the purple non-sulfurbacterium Rhodoferax fermentans

Oxidized and reduced forms of high-potential iron-sulfur protein (HiPIP) from the purple non-sulfur photosynthetic bacterium Rhodoferax fermentans have been characterized using 1H-NMR spectroscopy. Pairwise and sequence-specific assignments of hyperfine-shifted 1H-NMR signals to protons of cysteine residues bound to the [4Fe-4S]3+/2+ cluster have been performed using one-dimensional NOE and exchange spectroscopy experiments. 1H-NMR hyperfine shifts and relaxation rates of cluster-bound Cys beta-CH2 protons indicate that in the [4Fe-4S]3+ cluster one iron ion can be formally described as Fe(III), while electron density corresponding to one electron is unevenly delocalized onto the remaining three iron ions. This delocalization is effected by means of two different electronic distributions interconverting rapidly on the NMR time scale. The mechanism of paramagnetic proton relaxation, studied by analyzing longitudinal relaxation rates of Cys beta-CH2 protons in HiPIPs from six different sources as a function of the Fe-S-C beta-C alpha dihedral angle, indicate that the major contribution is due to a dipolar metal-centered mechanism, with a non-negligible contribution from a ligand-centered dipolar mechanism which involves the 3p orbital of the Cys sulfur atom. A semi-quantitative tool for extracting structural information from relaxation time measurements is proposed.

15N resonance assignments of oxidized and reduced Chromatium vinosum high-potential iron protein

The 15N resonances in reduced and oxidized Chromatium vinosum high-potential iron protein have been assigned by use of 1H-1H COSY spectra and 1H-15N HMQC. HMQC-COSY, and HMQC-NOESY spectra. Unambiguous assignment of 70 of 85 backbone 15N resonances in the reduced protein and 62 of 85 resonances in the oxidized protein are made, as are 12 of 21 side-chain 15N resonances.

1H-NMR investigation of oxidized and reduced high-potential iron-sulfur protein from Rhodopseudomonas globiformis

1H one-dimensional and two-dimensional NMR spectra have been recorded for the oxidized and reduced forms of the high-potential iron-sulfur protein (HiPIP) from Rhodopseudomonas globiformis which has the highest known reduction potential. The spectrum of the oxidized protein is similar to that of Chromatium vinosum and Rhodocyclus gelatinosus HiPIP but different from that of the HiPIP II from Ectothiorhodospira halophila. Surprisingly, site-specific assignment has shown that in the oxidized protein the distribution of oxidation numbers within the cluster is very similar to that found for E. halophila HiPIP II and different from that of the other two proteins. The spectrum of the reduced species is very similar to that of all other HiPIPs known to date, indicating very similar electronic and geometric structures for the reduced forms. These findings are discussed in terms of cluster structure in HiPIPs and of redox potentials.

Sequence-specific assignments of the 1H nuclear magnetic resonance spectra of reduced high-potential ferredoxin (HiPIP) from Chromatium vinosum

The 1H resonances of the high-potential [4Fe-4S]2+ ferredoxin from Chromatium vinosum have been assigned through conventional sequential methodology applied to 2D NMR spectra. Almost 80% of the residues were identified using standard 2D COSY, HOHAHA, and NOESY pulse sequences. These residues correspond to four segments of the primary structure that do not interact strongly with the iron-sulfur cluster. A minor correction to the amino acid sequence is strongly suggested by these NMR data. Additional protons more sensitive to the proximity of the cluster were assigned by a combination of NOESY experiments with fast repetition rates and short mixing times and of HOHAHA spectra recorded with reduced spin-lock duration aimed at compensating for the short relaxation rates. Hence, the contributions of 79 residues out of 85 were identified in NMR spectra, among which the assignments of 64 residues were completed. Even the fastest relaxing protons, like those of the cysteine ligands, could be correlated, partly because the strong hyperfine shifts isolate them from the crowded diamagnetic region. However, other protons, in particular those involved in NH-S hydrogen bonds with the iron-sulfur cluster, were more difficult to identify, most probably because their relatively broad signals overlapped with those of protons not or less perturbed by the active site. The availability of the major part of the 1H NMR assignments has enabled the detection and identification of many interresidue NOESY cross peaks. These data are in full agreement with the elements of secondary structure previously revealed by X-ray crystallographic analysis of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequential resonance assignments of oxidized high-potential iron-sulfur protein from Chromatium vinosum

2D NMR spectra of the high-potential iron-sulfur protein (HiPIP) from Chromatium vinosum have been used to obtain partial resonance assignments for the oxidized paramagnetic redox state of the protein. Sequence-specific assignments were made using NOESY and COSY spectra in H2O and D2O of the following backbone segments: Asn-5-Arg-33, Glu-39-Asp-45, Gly-55-Cys-63, Gly-68-Ala-78, and Leu-82-Gly-85. NOESY spectra with a spectral width wide enough to include the hyperfine-shifted resonances revealed numerous NOE contacts between these signals and those in the main envelope of the proton spectrum. With the aid of the X-ray crystal structure [Carter, C.W., Kraut, J., Freer, S. T., Xuong, N. H., Alden, R. A., & Bartsch, R. G. (1974) J. Biol. Chem. 249, 4212], these NOEs permitted seven of the nine hyperfine-shifted signals to be assigned to three of the cysteine residues liganded to the metal cluster (Cys-43, Cys-46, and Cys-77). The other two hyperfine-shifted signals produced no detectable NOEs to other resonances in the spectrum and were tentatively assigned to the remaining cysteinyl ligand (Cys-63). These assignments, in conjunction with recent theoretical models of the electronic structure of the Fe4S4 cluster [Noodleman, L. (1988) Inorg. Chem. 27, 3677; Bertini, I., Briganti, F., Luchinat, C., Scozzafava, A., & Sola, M. (1991) J. Am. Chem. Soc. 113, 1237], indicate that the iron atoms coordinated to Cys-63 and Cys-77 are those of the mixed-valence Fe(3+)-Fe2+ pair whereas Cys-43 and Cys-46 are ligands to the Fe(3+)-Fe3+ metal pair.