Characterization of the [3Fe-4S] and [4Fe-4S] Clusters in Unbound PsaC Mutants C14D and C51D. Midpoint Potentials of the Single [4Fe-4S] Clusters Are Identical to F_A and F_B in Bound PsaC of Photosystem I

In a previous paper we showed that the C51D mutant of PsaC contains a [3Fe-4S] cluster in the F_A site and a [4Fe-4S] cluster in the F_B site and that the C14D mutant contains an uncharacterized cluster in the F_B site and a [4Fe-4S] cluster in the F_A site [Zhao, J. D., Li, N., Warren, P. V., Golbeck, J. H., & Bryant, D. A. (1992) Biochemistry 31, 5093–5099]. In this paper we describe the electrochemical and electron spin resonance properties of the recombinant C14D and C51D holoproteins after in vitro reinsertion of the iron-sulfur clusters. Unbound PsaC shows no significant resonances in the oxidized state, but the unbound C14D and C51D mutant proteins show an intense set of resonances at g ∼ 2.02 and 1.99 characteristic of an oxidized [3Fe-4S]^1+/0 cluster. The E_m′ values for the [3Fe-4S]^1+/0 clusters in C14D (Fb*) and C51D (F_A*) are −98 mV, and both represent one-electron transfers. After reduction with dithionite at pH 10.0, wild-type PsaC shows a broad set of resonances resulting from the superposition of F_A⁻ and F_B⁻ characterized by a low-field peak at an apparent g value of 2.051 and a high-field trough at an apparent g value of 1.898. The F_B resonances in C51D were slightly narrower, with a low-field peak at an apparent g value of 2.039 and high-field trough at an apparent g value of 1.908. The F_A resonances in C14D were somewhat broader, with a low-field peak at an apparent g value of 2.042 and a high-field trough at an apparent g value of 1.898. The E_m′ value for F_A in C14D is −515 mV, and the E_m′ value for F_B in C51D is −580 mV; both represent one-electron transfers. These values are nearly identical to the E_m′ values determined for the F_A and F_B clusters in PsaC bound to the photosystem I complex. This result indicates that the midpoint potentials of F_A and F_B are determined solely by the primary amino acid sequence of PsaC and not by interaction with PsaD or with the photosystem I core.