Preliminary X-ray diffraction studies on 2 Fe-ferredoxin from Halobacterium of the Dead Sea.

The unusual property of halophilic proteins to withstand high concentrations of salt, and in some cases the requirement of salt to maintain their conformation (Werber et al., 1978a), has prompted us to study their three-dimensional structure. Recently, a 2 Fe-ferredoxin has been isolated and purified from Halobacterium of the Dead Sea (Werber & Mevarech, 1978a), and other species of Halobacterium (Kerscher et al., 1976), where it is present in large quantities-about 1 o/o of the total protein. It was found to resemble chloroplast-type 2 Fe-ferredoxins in many properties such as optical characteristics and electron par&magnetic resonance spectrum. An immunochemical study (Geiger et al., 1978) showed that halobacterial ferredoxin cross-reacted 15% with an algal ferredoxin (from Spirulina maxima), whereas it did not cross-react with 2 Fe-ferredoxins from a class to which Escherichia coli and other bacterial 2 Fe-ferredoxins belong. There is a high degree of similarity between the sequences of the halobacterial ferredoxins and that of the blue-green alga Nostoc muscorum (Hase et al., 1978, and unpublished data). On the other hand, the halobacterial 2 Fe-ferredoxins possess some distinct characteristics: they are much more acidic than the chloroplast-type ferredoxins, as are most halophilic proteins (Werber et al., 197&z), and have a higher redox potential: -345 mV as compared to -420 mV (Werber & Mevarech, 1978a; Kerscher et al., 1976). The Mossbauer spectra for the reduced states have a narrower linewidth in the case of the halobacterial ferredoxin (Werber et al., 1978b), as well as a lower temperature required to observe its electron par&magnetic resonance signal (Kerscher et al., 1976 ; Werber et al., 19786) which imply shorter spin relaxation rates in helophilic ferredoxins, as well as possibly a weaker antiferromagnetic coupling. Finally, very distinct functional characteristics were found for these special ferredoxins: Halobacterium of the Dead Sea ferredoxin was found to be a cofactor in the reduction of nitrite to gaseous products, as part of a dissimilatory pathway of nitrate (Werber & Mevarech, 1978b), whereas Halobacterium halobium ferredoxin was found to be able to serve as a coenzyme of a-keto acid oxidoreductases (Kerscher t Oesterhelt, 1977). In order to compare the structures of halobacterial to the chloroplast-type ferredoxins, for which there are preliminary reports of crystallizations (Ogawa et al., 1977 ; Kunita et al., 1978), we were interested in obtaining suitable crystals for the X-ray structure determination of ferredoxin from the Halobacterium of the Dead Sea. Our task was rendered easier by the availability of several hundred milligrams of very pure ferredoxin, which was obtained by the original purification procedure (Werber & Mevarech, 1978a). We report here the crystallization of 2 Fe-ferredoxin from Halobacterium of the