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

Insights into protein adaptation to a saturated salt environment from the crystal structure of a halophilic 2Fe-2S ferredoxin

Haloarcula marismortui is an archaebacterium that flourishes in the world's saltiest body of water, the Dead Sea. The cytosol of this organism is a supersaturated salt solution in which proteins are soluble and active. The crystal structure of a 2Fe-2S ferredoxin from H. marismortui determined at 1.9 A is similar to those of plant-type 2Fe-2S ferredoxins of known structure, with two important distinctions. The entire surface of the protein is coated with acidic residues except for the vicinity of the iron-sulphur cluster, and there is an insertion of two amphipathic helices near the N-terminus. These form a separate hyperacidic domain whose postulated function to provide extra surface carboxylates for solvation. These data and the fact that bound surface water molecules have on the average 40% more hydrogen bonds than in a typical non-halophilic protein crystal structure support the notion that haloadaptation involves better water binding capacity.

Structural stability of halophilic proteins

An examination of halobacterial amino acids exchanges as they appear in the known Spirulina platensis [2Fe-2S] ferredoxin tertiary structure indicated that most of the additional acidic residues of the halophiles occurred on the external surface of the alga structure; however, further negative changes were not placed in the ferredoxin active site region. A statistical investigation of the amino acid compositions of seven halophile and nonhalophile protein counterparts indicated that the bulkiness of amino acids used by halophiles is considerably reduced and that the overall hydrophobicity of halophilic and non halophilic molecules was essentially the same. It is suggested that the principal mode of structural stabilization for halophilic proteins is effective competition with the cytoplasmic salt for water through utilization of many external carboxyl groups of glutamic and aspartic acids. A reduction is residue bulkiness would prevent inactivation in the presence of the high molarity, antichaotropic KCl. Halophilic functionality is preserved through avoidance of additional negative charge at the active site surface.

Amino acid sequence of 2Fe-2S ferredoxin from an extreme halophile, Halobacterium of the Dead Sea

The primary structure of the 2Fe-2S ferredoxin from Halobacterium of the Dead Sea was determined and it consisted of 128 amino acid residues including an N epsilon-acetyllysyl residue. Due to a high degree of sequence homology between this ferredoxin and the one from Halobacterium halobium, all tryptic peptides could be aligned in order. Only 20 amino acid differences were observed between these two halobacterial ferredoxins. The distribution of cysteinyl residues involved in the iron chelation was similar to that of chloroplast-type ferredoxins.