Photosystem I charge separation in the absence of center A and B. III. Biochemical characterization of a reaction center particle containing P-700 and F X

Abstract

The Photosystem I reaction center is a membrane-bound, multiprotein complex containing a primary electron donor (P-700), a primary electron acceptor (A 0), an intermediate electron acceptor (A 1) and three membrane-bound iron-sulfur centers (F X, F B, and F A). We reported in part I of this series (Golbeck, J.H. and Cornelius, J.M. (1986) Biochim. Biophys. Acta 849, 16–24) that in the presence of 1% lithium dodecyl sulfate (LDS), the reaction center becomes dissociated, resulting in charge separation and recombination between P-700 and F X without the need for prereduction of F A and F B. In this paper, we report (i) the LDS-induced onset of the 1.2-ms ‘fast’ phase of the P-700 absorption transient is time-dependent, attaining a maximum 3:1 ratio of ‘fast’ to ‘slow’ kinetic phases; (ii) the ‘fast’ kinetic phase, corresponding to the P-700 + F X − backreaction, is stabilized indefinitely by dilution of the LDS-treated particle followed by ultrafiltration over a YM-100 membrane; (iii) without stabilization, the P-700 + F X − reaction deteriorates, leading to the rise of the long-lived P-700 triplet formed from the P-700 +A O − backreaction; (iv) the ‘slow’ kinetic phase correlates with the redox and ESR properties of F A and/or F B, which indicates that in a minority of particles the terminal iron-sulfur protein remains attached to the reaction center core; (v) the ultrafiltered reaction center is severely deficient in all of the low molecular-weight polypeptides, particularly the 19-kDa, 18-kDa and 12-kDa polypeptides relative to the 64-kDa polypeptide(s); (vi) the stabilized particle contains 5.8 mol labile sulfide per mol photoactive P-700, reflecting largely the iron-sulfur content of F x, but also residual F A and F B, on the reaction center; and (vii) the apoproteins of F A and F B are physically removed from the reaction center particle as indicated by the presence of protein-bound zero-valence sulfur in the YM-100 filtrate. These results are interpreted in terms of a model for Photosystem I in which F A and F B are located on a low-molecular-weight polypeptide and F X is depicted as a [2Fe-2S] cluster shared between the two high-molecular-weight polypeptides Photosystem I-A1 and Photosystem I-A2.