The molecular structure of the high potential iron-sulfur protein isolated from Ectothiorhodospira halophila determined at 2.5-A resolution.

The molecular structure of a high potential ironsulfur protein (HiPIP) isolated from the purple photosynthetic bacterium, Ectothiorhodospira halophila strain BN9626, has been solved by x-rgy diffraction analysis to a nominal resolution of 2.5 A and refined to a crystallographic R value of 18.4% including all measured x-ray data from 30.0-to 2.5-A resolution.Crystals used in the investigation contained two molecules/asymmetric unit and belonged to the ?pace group P Z 1 with unit ceJl dimensions of a = 60.00A, b = 31.94A, c = 40.27A, and B = 100.5'.An interpretable electron density map, obtained by combining x-ray data from one isomorphous heavy atom derivative with non-crystallographic symmetry averaging and solvent flattening, clearly showed that this high potential ironsulfur protein contains 71 amino acid residues, rather than 70 as originally reported.As in other bacterial ferredoxins, the [4Fe-4S] cluster adopts a cubane-like conformation and is ligated to the protein via four cysteinyl sulfur ligands.The overall secondary structure of the E. halophila HiPIP is characterized by a series of Type I and Type I1 turns allowing the polypeptide chain to wrap around the [4Fe-4S] prosthetic group.The hydrogen bonding pattern around the cluster is nearly identical to that originally observed in the 85-amino acid residue Chromatiurn uinosum HiPIP and consequently, the 240 mV difference in redox potentials between these two proteins cannot be simply attributed to hydrogen bonding patterns alone.The high potential iron-sulfur proteins, referred to by the acronym HiPIPs, are a class of [4Fe-4S] ferredoxins commonly found in the purple phototrophic bacteria (Bartsch, 1978).These proteins undergo a reversible one-electron transfer reaction at a characteristically high oxidation-reduction midpoint potential of between +50 to +450 mV and are paramagnetic in the oxidized state (Meyer et al., 1983).As a class of ferredoxins, the HiPIPs show significant variation in their amino acid sequences, in their sizes, and in their oxidation-reduction potentials (Tedro et al., 1985; Meyer et al., 1983).The [4Fe-4S] prosthetic groups observed in the HiPIPs are widely distributed in nature and are found, for example,