PsaC subunit of photosystem I: Electronic structure of iron-sulfur clusters and NMR solution structure

1H NMR spectra of fully reduced, unbound PsaC could be recorded and a unique assignment of all 18 observable hyperfine-shifted signals through a de novo 1D NOE analysis was obtained. This enabled us to sequence-specifically assign the signals of three cysteines from each of the two reduced [4Fe-4S] clusters and to obtain the preferential localization of mixed- and equal-valence pairs in F_A− and F_B−. In F_A− the mixed-valence pair is ligated by cysteines 47 and 53; the equal valence pair is ligated by cysteines 20 and 50. In F_B− the mixed-valence pair is ligated by cysteines 10 and 16; the equal valence pair is ligated by cysteines 13 and 57. To study changes, it is necessary to know the three-dimensional structure of both bound and unbound PsaC. The Clostridium acidi urici ferredoxin structure has served as a model for the refinement of a portion of the electron density map in a 4 Å resolution X-ray structure of PS I (Klukas, O., Schubert, W. D., Jordan, P., Krauss, N., Fromme, P., Witt, H. T. and Saenger, W. (1999) J. Biol. Chem., 274, 7351–60). Thus a model for PsaC bound to PS I exists. However, no X-ray structure of unbound PsaC is available. We present in this work the three-dimensional NMR solution structure of recombinant, oxidized, unbound PsaC from Synechococcus sp. PCC 7002. Significant differences are outlined between the unbound PsaC structure presented here and the available X-ray structure of bound PsaC as an integral part of the whole cyanobacterial PS I complex. These differences mainly concern the arrangement of the N- and C-termini with respect to the [4Fe-4S] core domains. These structural changes result in a concerted movement of the N- and C-termini of PsaC away from the F_A binding site. Instead of the expected [3Fe-4S] cluster, chemical rescue by an external thiolate ligand allows for formation of a [4Fe-4S] cluster in the alanine and glycine mutants. To support this hypothesis, we reconstituted C13G/C33S using 2-mercaptoethanol, 4-fluorothiophenol and 2,2,2-trifluoroethanethiol as the rescue ligands to F_B. Proving the chemical rescue hypothesis. We also have shown that both aryl and acyl thiolates can serve as external thiolate ligands to protein-bound [4Fe-4S] clusters. (Abstract shortened by UMI.)...