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Characteristics of electron transport in oxygen evolving photosystem ii complexes from a thermophilic cyanobacterium synechococcus sp



Characteristics of electron transport in oxygen evolving photosystem ii complexes from a thermophilic cyanobacterium synechococcus sp



Photobiochemistry & Photobiophysics 7(2): 77-90



Light-induced electron transport from water to either K3Fe(CN)6 or phenyl-p-benzoquinone was studied in O2 evolving photosystem II complexes isolated from a thermophilic cyanobacterium Synechococcus sp. described in the preceding paper (Schatz and Witt, 1984). Steady-state electron transport reflected a rate constant of k1 = 257 s-1 under optimal conditions; k1 is attributed to the rate-limiting step in PS II [photosystem II] complexes. Rates were dependent on K3Fe(CN)6 concentration, following 2nd-order kinetics with k2 = 3.6 .cntdot. 105 M-1 .cntdot. s-1. Electron transport rates showed optimal values at pH 6.5 with phenyl-p-benzoquinone and at pH 4.5 with K3Fe(CN)6. With K3Fe(CN)6 pH dependence followed a 2-step curve with inflections at pH 4.8 and pH 6.5 attributed to the titration of negative charges which control access of Fe(CN)63- to the native acceptor. Temperature optima was 40.degree.-45.degree. C with phenyl-p-benzoquinone and 45.degree.-50.degree. C with K3Fe(CN)6 and activation energy was .apprx. 38 kJ/mol. DCMU [3-(3',4'-dichlorophenyl)-1,1-dimethylurea] completely inhibited electron transport with phenyl-p-benzoquinone, but only partially with K3Fe(CN)6. K3Fe(CN)6 was reduced at least at 2 different sites. The effects of detergent concentration on electron transport rates with K3Fe(CN)6 were also studied.

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