Study Of The Mechanism Of Electron Transfer Of The Qb Site In Photosystemll Of Higher Plants

Photosystemll (PSII), a pigment protein complex located in the thylakoid membranes of photosynthetic organisms, performs light-induced electron and proton transfer and efficient water-splitting reactions, leading to the formation of molecular oxygen. It is the most sensitive part of plant under stress conditions and plays an irreplaceable role. Non-cycle electron transfer of photosynthesis began with PSII. The QB site is the final sites of the electron transfer in PSII, and also is the target site of most herbicides, so it is particularly important to research the electron transfer of QB site.We first prepared PSII membrane fragments with highly oxygen evolving activity from spinach, then screened a series of electron acceptors which can maintain the oxygen evolving activity of PSII. There are hydrophobic and hydrophilic electron acceptors. Rates of oxygen evolution were determined in the presence of various concentrations of hydrophobic and hydrophilic electron acceptors, and the two kinetic parameters, Vmax and Km, were estimated from double reciprocal plots. These results offer a simple and convenient proceture to estimate the affinities of electron acceptors to the related binding sites of PSII. Because of their hydrophobicity and structural similarities with PQ, the hydrophobic quinones can be used as exogenous electron acceptors of PSII and recover the oxygen evolution activity. The properties, number and position of substituents of benzoquinones had efffect on the ability of electron accept. Results of dynamic analysis showed that PPBQ and chloranil can accept electrons mainly on one site in PSII, while DCBQ, MBQ and BQ can accept electrons on two sites in PSII. The hydrophilic electron acceptors, K.3[Fe(CN)6] and DCIP can also act as exogenous electron acceptors of PSII and maintain the oxygen evolving activity. Results of dynamic analysis showed that the hydrophilic K3[Fe(CN)6] and DCIP can accept electrons on two sites in PSII.Investigate the competitive relationship of DCMU and electron acceptors to further analyze the binding sites of electron acceptors. The results showed that the hydrophobic electron acceptors classified into two categories. DCBQ, BQ, MBQ can enter the QB site and accept electrons directly in their high concentrations, while at the low concentrations, they accept electrons mainly from the PQ sites. The other category, PPBQ and chloranil, mainly accept electrons from the PQ sites, and little from QB sites. The hydrophilic electron acceptors, K3[Fe(CN)6] and DCIP, they can accept electrons from both PQ and QB sites. When they act as the exogenous electron acceptors, there is little effect to oxygen evolution activity of PSII, the oxygen evolution activity is low.