A MODEL FOR THE PHOTOSYSTEM-II REACTION-CENTER CORE INCLUDING THE STRUCTURE OF THE PRIMARY DONOR P-680

For a detailed understanding of the function of photosystem II (PSII), a molecular structure is needed, The crystal structure has not yet be en determined, but the PSII reaction center proteins D1 and D2 show ho mology with the L and M subunits of the photosynthetic reaction center from purple bacteria, We have modeled important parts of the D1 and D 2 proteins on the basis of the crystallographic structure of The react ion center from Rhodopseudomonas viridis. The model contains the centr al core of the PSII reaction center, including the protein regions for the transmembrane helices B, C, D, and E and loops B-C and C-D connec ting the helices, In the model, four chlorophylls, two pheophytins, an d the nonheme Fe2+ ion are included. We have applied techniques from c omputational chemistry that incorporate statistical data on side-chain rotameric states from known protein structures and that describe inte ractions within the model using an empirical potential energy function . The conformation of chlorophyll pigments in the model was optimized by using exciton interaction calculations in combination with potentia l energy calculations to rind a solution that agrees with experimental ly determined exciton interaction energies, The model is analyzed and compared with experimental results for tile regions of Pb-680, the red ox active pheophytin, the acceptor side Fe2+, and the tyrosyl radicals Tyr(D) and Tyr(Z), P-680 is proposed to be a weakly coupled chlorophy ll a pair which makes three hydrogen bonds with residues on the D1 and D2 proteins. In the model the redox-active pheophytin is hydrogen bon ded to D1-Glu130 and possibly also to D1-Tyr126 and D1-Tyr147. Tyr(D) is hydrogen bonded to D2-His190 and also interacts with D2-Gln165. Tyr (Z) is bound in a hydrophilic environment which is partially constitut ed by D1-Gln165, D1-Asp170, D1-Glu189, and D1-His190, These polar resi dues are most likely involved in proton transfer from oxidized Tyr(Z) or in metal binding.