The structural organization of the PsaC protein in photosystem i from single crystal EPR and x-ray crystallographic studies

In Photosystem I (PS I) the terminal electron accepters, F(A) and F(B), are iron-sulfur (4Fe-4S) centers, which are bound to the stromal subunit PsaC. The orientation of PsaC is determined relative to the whole PS I complex (see Schubert, W.-D. et al. (1995) in From Light to Biosphere (Mathis, P. ed.), Vol. II, pp. 3-10, Kluwer) from which a molecular model for the structure of PsaC within PS I is derived. Two strategies are followed: (i) PS I single crystal EPR data on the orientation of the g tensors of both F(A)^- and F(B)^- relative to each other and relative to the crystal axes (see preceding paper) are used in conjuction with the central structural part of the bacterial 2 [Fe₄S₄] ferredoxins, the cysteine binding motifs of which are known to be homologous to those of PsaC; (ii) the same core structure is fitted into the intermediate resolution electron density map of PS I. The PsaC orientation obtained both ways agree well. The local twofold symmetry axis inherent to the ferredoxin model leaves a twofold ambiguity in the structural conclusion. Deviations from this C₂-symmetry in the amino acid sequence of PsaC are analyzed with respect to observable properties which would resolve the remaining structural ambiguity. Arguments both for and against F(A) being the distal iron-sulfur center (to F(x)) are discussed.