Iron-sulfur clusters site-directed mutagenesis

From One Type to Another Type of Fe-S Cluster. Iron-sulfur clusters, such as [2Fe-2S], [3Fe-4S], and [4Fe-4S], are found in proteins see Iron-Sulfur Proteins). One of the most interesting aspects of the iron-sulfur proteins is their ready conversion from one type to another through chemical oxidation and reduction, pH changes, or site-directed mutagenesis.More importantly, several studies have indicated that some of the cluster conversions are physiologically relevant in that they play important roles in regulation of enzyme activity. 

As mentioned above, a 1 1 stoichiometric relationship between the photooxidized donor, P700 and the reduced terminal acceptors has not yet been established for the PS-I reaction center. As previously noted, the total amount of the recognized terminal acceptors reduced at 15 K is, on average, approximately 74% of the P700 photooxidized. Even more intriguing, in the reconstituted PS-I complexes from either the Cys-14->Asp or Cys-51->Asp mutant PsaC protein, the extent of photoreduction of each intact iron-sulfur cluster at 15 K remained nearly the same as in the wild-type preparation. The presence of the other cluster that was made photochemically inactive by site-directed mutagenesis apparently had no effect on the behavior of the unaltered cluster. 

The problem with the application of fliis principle in proteins is spectral crowding for proton interactions and, combined with this, only partial definition of hyperfine lines over the whole range of flie EPR spectrum. This makes direct tensor determination somewhat ambiguous, but in connection with known molecular stractures a unique solution can typically be found. A very important asset in this respect is site-directed mutagenesis, by which specific amino acid residues can be changed. For most of the examples of iron-sulfur clusters discussed, a full analysis was possible even without this tool. 

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