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Photosystem 2 mediated electron transport and phosphorylation with ferricyanide and dibromothymoquinone the uncoupling activity of dibromothymoquinone

1976; Elsevier BV; Volume: 7; Issue: 3 Linguagem: Inglês

10.1016/0304-4211(76)90131-0

1878-2310

Gozal Ben‐Hayyim, Ziwia Drechsler, Joseph Neumann,

Photoreceptor and optogenetics research

(1) In lettuce chloroplasts, ferricyanide can be photoreduced at two sites in the electron transport chain. At pH 7 and below, ferricyanide is preferentially photoreduced by Photosystem 2 (PS2), whereas at higher pH values it is photoreduced primarily by Photosystem 1 (PS1). (2) When ferricyanide is photoreduced by Photosystem 2 only (either at low pH or at any pH in KCN treated chloroplasts), the ATP/e2 value is reduced from 1.0 to about 0.5. (3) Dibromothymoquinone (DBMIB) while inhibiting electron flow to PS1 acts as an electron acceptor for PS2. At pH 6.0 addition of DBMIB to ferricyanide causes an increase in electron flow and an inhibition of ΔpH. The former is due to the fact that both ferricyanide and DBMIB are photoreduced, whereas the latter is probably due to the fact that (a) DBMIB is reduced inside the thylakoid, and reoxidized outside the thylakoid, dissipating the proton gradient formed by its reduction and (b) DBMIB reduction competes to some extent with ferricyanide reduction. Addition of DBMIB to ferricyanide causes a decrease in the P/e2 ratio; consequently true values for P/e2 at “coupling site II” cannot be obtained in the presence of ferricyanide plus DBMIB, but can be obtained with ferricyanide in KCN-treated chloroplasts or in lettuce chloroplasts with ferricyanide alone, at low pH values.