Characterization of the unbound 2[Fe4S4]-ferredoxin-like photosystem I subunit PsaC from the cyanobacterium Synechococcus elongatus

Recombinant PsaC was reconstituted in vitro and investigated by UV/vis, EPR, and H-1 NMR spectroscopy. Its UV/vis and EPR spectroscopic properties correspond to those of the wild-type protein. Fast repetition 1D and 2D H-1 NMR spectra allowed the sequence-specific assignment of the hyperfine-shifted proton resonances of the cluster-ligating resonances, taking advantage also of chemical shift analogies with other 4 and 8 Fe ferredoxins and a structural model for PsaC. The C-alpha-C-beta-S-Fe dihedral angles of the cluster ligands could be estimated from the chemical shifts and relaxation properties of their beta CH2 protons. All NMR-derived structural information on PsaC confirms its similarity to smaller 8Fe ferredoxins serving as electron transfer proteins in solution. Partial reduction of PsaC leads to an intermediate species with strongly exchange broadened H-1 NMR resonances. The intermolecular electron exchange rate is estimated to be in the 10(2)-10(4) s(-1) range, the intramolecular electron exchange rate between the two [Fe4S4] clusters to be higher than 10(4) s(-1). The consequences of these findings for the electron transfer in photosystem I are discussed.