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) s ite and that the C14D mutant contains an uncharacterized cluster in th e 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 ele ctron spin resonance properties of the recombinant C14D and C51D holop roteins after in vitro reinsertion of the iron-sulfur clusters. Unboun d PsaC shows no significant resonances in the oxidized state, but the unbound C14D and C51D mutant proteins show an intense set of resonance s at g approximately 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 C14 D (F(B)) 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 valu e of 2.051 and a high-field trough at an apparent g value of 1.898. Th e F(B) resonances in C51D were slightly narrower, with a low-field pea k 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 C 14D 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 bo und to the photosystem I complex. This result indicates that the midpo int potentials of F(A) and F(B) are determined solely by the primary a mino acid sequence of PsaC and not by interaction with PsaD or with th e photosystem I core.