1!• The primary e- donor P680 is formed by the Chl a pair PD1/PD perpendicular to the membrane plane • Monomeric ChlD1 and ChlD2 are analagous to accessory BChls of purple bacteria • The P680 excited state is de- localized over the four Chls • PheoD1 and PheoD2 follow co- factor branches • Only ChlD1 and PheoD1 active in charge separation • Site of tightly bound QA is on D2 near nonheme iron • P680+ located at PD1 close to immediate e- donor TyrZ, bridg- ing the oxygen evolving complex (OEC) • ChlzD1 and ChlzD2 bridge CP43 and CP47 in exciton transfer to P680 and participate in photoprotective e- transfer pathway with cytochrome b-559 and nearby β-carotene • D1 Tyr161 (Yz) is the immediate e- donor to P680+ situated between P680 and the OEC b!!OEC • Contains 20 subunits – mass of 350 kD • 1.9-Å structure revealed detailed architecture of water splitting catalytic center (OEC) – all metal atoms of Mn4CaO5 cluster and ligands • >1,300 H2O molecules seen-- in H-bonded net- works as possible H+ channels • 5 α-helices of D1 (green) and D2 (maroon) reaction center polypeptides in semi-circle along two-fold symmetry axis • 6 α-helices assigned to the CP43 (purple) and CP47 (gold) -- PSII core LH complexes arranged as trimers of dimers (13 and 16 Chl a, respectively) • One cytochrome b-559 (light green), 19 of 20 subunits resolved OEC!PsbO -- Mn-stablizing protein!PsbU!PsbV!Peripheral subunits!Stroma!2!• Oxygen evolving complex (OEC) – a cubane containing 3 Mn atoms, 1 Ca ion and 5 oxygen atoms forming oxo bridges • One additional Mn is attached to the cluster through two O’s • OEC contains 4 coordinated H2O molecules • Dangler Mn is only Mn with an attached H2O molecule • Makes it a likely candidate for involvement in O2 formation either directly -- by forming an O-O bond with the other water or an O in the cube/or --indirectly by replacing cube O’s which may form an O-O bond with each other Bond lengths (Å) • Seven ligands to Ca • Six ligands to each Mn • The ligation pattern and geometric positions of the metal atoms may have important consequences for maintaining OEC struct-ure and mechanisms of water splitting and O–O bond formation3!Blue Chl –co-ordinated to CP43-Asn 39!Green Chls –coordinated to His!Orange Chls –coordinated to H2O!For D1/D2, note semi-circul-ar arrangement around 2-fold symmetry axis!CP 43 and CP47 are trimers of α-helical dimers !• Most of antenna Chls of CP43 and CP47 (dark green) are arranged in 2 layers on opposite side of membrane in van der Waals contact • ChlzD1 and ChlzD2 are not optimized to receive energy from CP43 and CP47 (slow energy trapping by