Synthesis and characterization of de novo designed peptides modelling the binding sites of [4Fe–4S] clusters in photosystem I

Photosystem I (PS I) converts the energy of light into chemical energy via transmembrane charge separation. The terminal electron transfer cofactors in PS I are three low-potential [4Fe–4S] clusters named FX, FA and FB, the last two are bound by the PsaC subunit. We have modelled the FA and FB binding sites by preparing two apo-peptides (maquettes), sixteen amino acids each. These model peptides incorporate the consensus [4Fe–4S] binding motif along with amino acids from the immediate environment of the iron–sulfur clusters FA and FB. The [4Fe–4S] clusters were successfully incorporated into these model peptides, as shown by optical absorbance, EPR and Mössbauer spectroscopies. The oxidation–reduction potential of the iron–sulfur cluster in the FA-maquette is − 0.44 ± 0.03 V and in the FB-maquette is − 0.47 ± 0.03 V. Both are close to that of FA and FB in PS I and are considerably more negative than that observed for other [4Fe–4S] model systems described earlier (Gibney, B. R., Mulholland, S. E., Rabanal, F., and Dutton, P. L. Proc. Natl. Acad. Sci. U.S.A. 93 (1996) 15041–15046). Our optical data show that both maquettes can irreversibly bind to PS I complexes, where PsaC-bound FA and FB were removed, and possibly participate in the light-induced electron transfer reaction in PS I.