Electron Donation in Photosystem II

The pathways of electron donation in Photosystem II (PS II) as studied by electron paramagnetic resonance (EPR) and time‐resolved optical spectroscopy are discussed. The EPR studies have defined two competing pathways of electron donation in PS II, from cytochrome b₅₅₉ and from the Mn site of the oxygen‐evolving center. The kinetics of re‐reduction of the primary electron donor of PS II (P₆₈₀). as measured by optical spectroscopy, are re‐evaluated in light of the EPR results. We propose that the 35‐μs kinetic component is due to the reduction of chlorophyll, an alternate electron donor on the cytochrome b₅₅₉ pathway, rather than to the reduction of P₆₈₀. The chlorophyll/cytochrome b₅₅₉ pathway has been proposed to be part of a cyclic electron transfer pathway around PS II; we suggest that photooxidation of chlorophyll is the first step leading to photoinhibition and that cytochrome b₅₅₉ serves to protect PS II from photoinhibition by rapidly re‐reducing the oxidized chlorophyll (Thompson, L.K.; Brudvig, G.W. Biochemistry, 1988, 27: 6653). These results and proposals are summarized in an overall scheme of electron transfer pathways and rates in PS II.