Homology predicted structure and functional interaction of ferredoxin fromthe eukaryotic alga Chlamydomonas reinhardtii with nitrite reductase and glutamate synthase

Ferredoxin (Fd) from Chlamydomonas reinhardtii is composed of 94 amino-acid residues and a [2Fe-2S] cluster. The homology modelling technique has been used to predict the tertiary structure of C. reinhardtii Fd. The overall s tructure shows the typical fifth-stranded beta -grasp plus two additional b eta -sheets and three alpha -helices. Site-directed mutagenesis of recombin ant Fd has allowed us to obtain four point mutants and one double mutant - all mutations being located in the short alpha -helix at the carboxy-termin al segment as well as a triple mutant affected on helix alpha1. Crosslinkin g studies and measurement of enzymatic activities reveal that the residues changed are critical for the interaction of Fd with glutamate synthase (GOG AT) and nitrite reductase (NiR). Potentiometric analyses of the Fd mutants show that the replacement of glutamate in position 91 drastically changes t he redox potential value (70 mV), thereby suggesting that such a glutamate call modulate the reactivity of Fd towards its reaction partners. According to results herein presented, the reported mutations modify the electrostat ic interactions within the complex formed between Fd and GOGAT or NiR.