THE IRON-SULFUR CLUSTER IN THE OXIDIZED HIGH-POTENTIAL IRON PROTEIN FROM ECTOTHIORHODOSPIRA-HALOPHILA

In our efforts to characterize oxidized high-potential iron-sulfur proteins (HiPIP), we have investigated the oxidized HiPIP II from Ectothiorhodospira halophila through H-1 NMR and molecular dynamics (MD) calculations. This protein has the most symmetric isotropic shift pattern of the beta-CH2 Protons of the liganded cysteines, four signals being upfield and four downfield. H-1 NOE, NOESY, and TOCSY results have provided the necessary key connectivities to perform the assignment of the liganded cysteines, taking advantage of the structure of the HiPIP I isoprotein. It is found that the electronic distribution within the cluster is different with respect to the Chromatium vinosum and Rhodocyclus gelatinosus systems. As in the latter systems, the cluster can be described in terms of two iron(III) and two mixed valence ions, but the two pairs are oriented in a different way within the protein frame. These results are discussed in terms of structure-function relationships. An MD approach starting from the structure of HiPIP I has provided a structural model of the present protein, which is absolutely consistent with the NMR connectivities in the surroundings of the cluster.