Experimental evidence for the role of buried polar groups in determining the reduction potential of metalloproteins: the S79P variant of Chromatium vinosum HiPIP

The amide group between residues 78 and 79 of Chromatium vinosum high-potential iron-sulfur protein (HiPIP) is in close proximity to the Fe4S4 cluster of this protein and interacts via a hydrogen bond with S gamma of Cys77, one of the cluster ligands, The reduction potential of the S79P variant was 104+/-3 mV lower than that of the recombinant wild-type (rcWT) HiPIP (5 mM phosphate, 100 mM NaCl, pH 7, 293 K), principally due to a decrease in the enthalpic term which favors the reduction of the rcWT protein. Analysis of the variant protein by NMR spectroscopy indicated that the substitution has little effect on the structure of the HiPIP or on the electron distribution in the oxidized cluster. Potential energy calculations indicate that the difference in reduction potential between rcWT and S79P variant HiPIPs is due to the different electrostatic properties of amide 79 in these two proteins, These results suggest that the influence of amide group 79 on the reduction potential of C, vinosum HiPIP is a manifestation of a general electrostatic effect rather than a specific interaction. More generally, these results provide experimental evidence for the importance of buried polar groups in determining the reduction potentials of metalloproteins

THE ROLE OF A CONSERVED TYROSINE RESIDUE IN HIGH-POTENTIAL IRON-SULFUR PROTEINS

Conserved tyrosine-12 of Ectothiorhodospira halophila high-potential iron sulphur protein (HiPIP) iso-I was substituted with phenylalanine (Y12F), histidine (Y12H), tryptophan (Y12W), isoleucine (Y12I), and alanine (Y12A). Variants Y12A and Y12I were expressed to reasonable levels in cells grown at lower temperatures, but decomposed during purification. Variants Y12F, Y12H, and Y12W were substantially destabilized with respect to the recombinant wild-type HiPIP (rcWT) as determined by differential scanning calorimetry over a pH range of 7.0-11.0. Characterization of the Y12F variant by NMR indicates that the principal structural differences between this variant and the rcWT HiPIP result from the loss of the two hydrogen bonds of the Tyr-12 hydroxyl group with Asn-14 O delta 1 and Lys-59 NH, respectively. The effect of the loss of the latter interaction is propagated through the Lys-59/ Val-58 peptide bond, thereby perturbing Gly-46. The Delta Delta G(D)(app) of Y12F of 2.3 kcal/mol with respect to rcWT HiPIP (25 degrees C, pH 7.0) is entirely consistent with the contribution of these two hydrogen bonds to the stability of the latter. CD measurements show that Tyr-12 influences several electronic transitions within the cluster. The midpoint reduction potentials of variants Y12F, Y12H, and Y12W were 17, 19, and 22 mV (20 mM MOPS, 0.2 M sodium chloride, pH 6.98, 25 degrees C), respectively, higher than that of rcWT HiPIP. The current results indicate that, although conserved Tyr-12 modulates the properties of the cluster, its principle function is to stabilize the HiPIP through hydrogen bonds involving its hydroxyl group and electrostatic interactions involving its aromatic ring

A serine->cysteine ligand mutation in the high potential iron-sulfur protein from Chromatium vinosum provides insight into the electronic structure of the [4Fe-4S] cluster

We have succeeded in preparing a mutant of the High Potential Iron-Sulfur Protein (HiPIP) from Chromatium vinosum in which a cysteine ligand has been replaced by a serine (C77S). Proton chemical shift data and nuclear Overhauser effects indicate that structural perturbations induced by the C77S mutation are minimal in both the oxidized and reduced forms of the HiPIP. The reduction potential of C77S is 25 mV lower than that of the wild-type HiPIP (WT) (0.2 M ionic strength, pH 4.5-9.0, 25 degrees C). Assignment of the hyperfine shifted signals in the H-1 NMR spectrum of oxidized C77S revealed that the temperature dependences of the signals associated with cluster-ligating residues 46 and 77 are Curie and and-Curie type, respectively, and are thus the reverse of those in WT. Taken together, these observations indicate that the iron bound to Ser-77 is less reducible than the corresponding iron in WT. The results are consistent with a previous model of the electronic structure of oxidized HiPIP clusters, confirming the presence of an equilibrium between two species of differing valence distributions. The current results permit the extension of this model to predict the relative reduction potentials of the individual iron ions in the oxidized HiPIPs up to now investigated

The influence of a surface charge on the electronic and steric structure of a high potential iron-sulfur protein

The recombinant high-potential iron-sulfur protein (HiPIP) iso-I from Ectothiorhodospira halophila has been mutated at position 68. The alpha C of Val 68 is within a 0.6-nm sphere from the closest iron ion of the cluster, The valine residue has been replaced by a negatively charged glutamate residue (V68E) and by a positively charged lysine residue (V68K), With respect to the recombinant wild-type protein the reduction potentials of the V68E and V68K variants are -21+/-2 and + 29+/-2 mV respectively (200 mM NaCl, pH 7, 25 degrees C). The solution structure of the V68E mutant was solved up to a pairwise RMSD of 66 pm for backbone atoms and 138 pm for all heavy atoms. The structure of the variant is very similar to that of recombinant wild type, indicating that the observed changes in reduction potentials are largely due to the effect of the introduced charges. It is proposed that the valence distribution within the oxidized iron-sulfur cluster is affected only slightly by the change in charge at position 68, but consistently with a simple electrostatic model