Inhibition of photosynthetic oxygen evolution by protonophoric uncouplers

Abstract

The protonophoric uncouplers carbonyl cyanide m-chlorophenylhydrazone (CCCP), 2,3,4,5,6-pentachlorophenol (PCP) and 4,5,6,7-tetrachloro-2-trifluoromethylbenzimidazole (TTFB) inhibited the Hill reaction with K3[Fe(CN)6] (but not with SiMo) in chloroplast and cyanobacterial membranes (the I50 values were approx. 1-2, 4-6 and 0.04-0.10 μM, respectively). The inhibition is due to oxidation of the uncouplers on the Photosystem II donor side (ADRY effect) and their subsequent reduction on the acceptor side, ie. to the formation of a cyclic electron transfer chain around Photosystem II involving the uncouplers as redox carriers. The relative amplitude of nanosecond chlorophyll fluorescence in chloroplasts was increased by DCMU or HQNO and did not change upon addition of uncouplers, DBMIB or DNP-INT; the HQNO effect was not removed by the uncouplers. The uncouplers did not inhibit the electron transfer from reduced TMPD or duroquinol to methylviologen which is driven by Photosystem I. These data show that CCCP, PCP and TTFB oxidized on the Photosystem II donor side are reduced by the membrane pool of plastoquinone (Qp) which is also the electron donor for K3 [Fe(CN)6] in the Hill reaction as deduced from the data obtained in the presence of inhibitors. Inhibition of the Hill reaction by the uncouplers was maximum at the pH values corresponding to the pK of these compounds. It is suggested that the tested uncouplers serve as proton donors, and not merely as electron donors on the oxidizing side of Photosystem II.

Kinetics of Electron Transfer from QA to QB in Photosystem II

“…The ADRY agents prevented the accumulation of plastoquinol during the flash series but not the functioning of the two-electron gate, supporting earlier evidence that the electrons reducing the high S-states are ultimately derived from the plastoquinone pool ( , ). This may seem inconsistent with the persistence of the ADRY effect in the presence of DCMU under repetitive flash conditions in the presence of divalent cations and a high concentration of ferricyanide ().…”

Photoreduction of silicomolybdate in chloroplasts by agents accelerating the deactivation reactions of the water‐oxidizing system

“…The same effects were caused by the uncouplers, PCP and TTFB [20]. In conjunction with inhibitor study data, these results were interpreted as an indication that the tested uncouplers, redox active compounds, shunt the non-cyclic electron transfer to FeCy at the level of the membrane plastoquinone pool and thereby establish cyclic electron transfer around PSII [19,20].…”

“…The diffusion mobility and binding/release kinetics of the ADRY agent molecules seem to determine the occurrence of the residual 02 evolution when 'the enzyme' is saturated with substrate (with the ADRY agent molecules). Thus, ADRY agents at low concentrations are catalysts of the cyclic electron transfer around PSII [3,13,19,20], and at high concentrations they appear to serve as electron donors for non-cyclic electron transfer involving PSII.…”

Multiple fates of non‐mature lumenal proteins in thylakoids

“…We wished to determine whether ATP induces degradation of the protein in the absence of light, and whether the light‐dependent degradation requires electron transport itself, PMF or ATP hydrolysis, or a combination of some or all of them. As shown in Figure (c), non‐mature PetE2MV‐ARAF was stable in the presence of ATP in the dark (lane 2), and its light‐induced degradation was inhibited by the electron transport inhibitors 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) and methyl viologen (Mould and Robinson, ; Samuilov et al ., ) (lane 4), the ionophores nigericin and valinomycin (Vredenberg and Bulychev, ) (lane 5), and a non‐hydrolyzable ATP analog (adenosine 5′–[β,γ‐imido]triphosphate, AMP–PNP; Sabbert et al ., ) (lane 6). These results indicate that the degradation activity requires light‐driven PMF and ATP hydrolysis.…”