The Photosystem I reaction center is a membrane-bound, multiprotein complex containing a primary electron donor (P-700), a primary electron acceptor (A0), an intermediate electron acceptor (A1) and three membrane-bound iron-sulfur centers (FX, FB, and FA). 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 FX without the need for prereduction of FA and FB. 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+ FX- 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+ FX- reaction deteriorates, leading to the rise of the long-lived P-700 triplet formed from the P-700+AO- backreaction; (iv) the 'slow' kinetic phase correlates with the redox and ESR properties of FA and/or FB, 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 Fx, but also residual FA and FB, on the reaction center; and (vii) the apoproteins of FA and FB 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 FA and FB are located on a low-molecular-weight polypeptide and FX is depicted as a [2Fe-2S] cluster shared between the two high-molecular-weight polypeptides Photosystem I-A1 and Photosystem I-A2.
All Science Journal Classification (ASJC) codes
- Cell Biology