Photophosphorylation uncouplers

As pointed out previously, most herbicides have been discovered using random screening programs rather than from an applied rational approach to herbicide design, target, and synthesis. A few attempts to rationally design herbicides chose Inhibition of sites of photophosphorylation uncouplers (284). glycol ate oxidase (285). oxidation of Indoleacetic acid (lAA) by peroxidase (286). and secondary plant metabolism, I.e., phenlyalanine ammonia-lyase (2SZ)... 

FIGURE 19-19 Two chemical uncouplers of oxidative phosphorylation. Both DNP and FCCP have a dissociable proton and are very hydrophobic. They carry protons across the inner mitochondrial membrane, dissipating the proton gradient. Both also uncouple photophosphorylation (see Fig. 19-57). 

Uncouplers. Uncouplers dissociate electron transport from photophosphorylation. Both noncyclic and cyclic phosphorylation are inhibited, but electron transport reactions are either unaffected or stimulated. Because uncouplers relieve the inhibition of electron transport imposed by energy transfer inhibitors, they are considered to act at a site closer to the electron transport chain than the site of phosphate uptake. In Figure 2, they are shown (site 2) as dissipating some form of conserved energy represented as on the noncyclic and cyclic ATP-gener-ating pathways. Perfluidone is the only herbicide identified to date that functions as a pure uncoupler at pH 8.0 (2). Compounds that uncouple photophosphorylation also uncouple mitochondrial oxidative phosphorylation. 

Inhibitory Uncouplers. Inhibitory uncouplers inhibit the reactions affected by both electron transport inhibitors and uncouplers. Hence, they inhibit basal, methylamine-uncoupled, and coupled electron transport with ferricyanide as electron acceptor and water as the electron donor, much like electron transport inhibitors. Coupled noncyclic photophosphorylation is inhibited and the phosphorylation reaction is slightly more sensitive than the reduction of ferricyanide. Cyclic photophosphorylation is also inhibited. NADP reduction, when photosystem II is circumvented with ascorbate + DPIP, is not inhibited however, the associated phosphorylation is inhibited. Inhibitory uncouplers act at both sites 1 and 2 (Figure 2). 

Tricolorin A (46) and G (47) are prototype members of this class of amphiphilic glycoconjugates (24). They constitute the allelochemical principles of Ipomoea tricolor Cav., a plant used in traditional agriculture in Mexico as a cover crop to protect sugar cane against invasive weeds. Their molecular mechanism of action likely involves the inhibition of the FT-ATPase of the plasma membrane, an enzyme that plays a crucial role in plant cell physiology. Moreover, 46 acts as a natural uncoupler of photophosphorylation in spinach chloroplasts. This compound also displays general cytotoxicity against several... 

Although the control rates were rather low in the latter case, both experiments provide support for the artificial channel approach in algae. Whereas the addition of CCCP also increases H2 production in cyanobacteria, the uncoupling of electron transport and photophosphorylation per se does not seem to be involved in the me chanism.284... 

When the rate of ATP synthesis is plotted versus the total electrochemical gradient which serves as its driving force, a non-linear curve is obtained with little or no ATP synthesis until a threshold value of about 120-150 mV is reached, and a close to linear dependence thereafter. This behavior has been demonstrated in all of the ATP-synthesizing systems photophosphorylation, post-illumination phosphorylation and acid-base phosphorylation, and under a variety of energy-limiting conditions such as light intensity, inhibitors, uncouplers, etc. 

The halftime for the PPj-induced changes is around 0.4 s which is one-tenth of the half-time when ATP is used as the energy donor [4]. The extent of the changes obtained with PPj is usually about two to three times that obtained with ATP. The effects of PPj and ATP are additive [2], The reoxidation of h-type cytochrome starts when the concentration of PP falls below 25 juM [4], Uncouplers of photophosphorylation and electron transport inhibitors abolish both the PPj and the ATP-in-duced reactions. In line with the results with PP synthesis and hydrolysis, the PPj-induced reactions are not inhibited by oligomycin. 

Ammonia and simple aliphatic amines were among the first uncouplers reported for photophosphorylation by Krogmann, Jagendorf and Avron and by Good respectively. Ammonia and simple aliphatic amines in their lipid-soluble, unprotonated forms can freely permeate the thylakoid membrane. Once inside, they can take up protons and be converted to the corresponding ammonium ions [Fig. 10 (B)]. To maintain electrical neutrality, the accumulation of ammonium ions results in the influx of anions. The accumulation of ammonium salts inside consequently results in an osmotic influx of water and swelling of the thylakoids. 

Other hand, H+-uptake, basal and uncoupled electron transport were not affected. The acetylation of 26a yielded piquerol diacetate (26b) increasing its lipophilicity and photophosphorylation capacity [53]. 

The electron transport inhibitors do not directly affect photophosphorylation or interfere with mitochondrial electron transport and phosphorylation ( 1), However, the inhibitory uncouplers, in addition to interfering with electron transport in thylakoids, uncouple photophosphorylation and oxidative phosphorylation, and inhibit mitochondrial electron transport. 

The atebrin/phosphorylation ratio (Table II, last column) relates inhibition of photophosphorylation to dissipation of the postulated energized state (A pH) of the thylakoid membrane. A low ratio suggests that the two responses are correlated. However, the high ratios obtained for ioxynil and propanil suggest that these two herbicides act as electron transport inhibitors rather than as uncouplers. 

Bromoxynil is a special case because the parent compound bromoxynil-octanoate, although being a baseline toxicant, is highly toxic due to its high hydrophobicity (Fig. 10). The hydrolysis product bromoxynil is the active ingredient and has a specific mode of action Bromoxynil is a potent inhibitor of Photosystem II and is also an uncoupler of photophosphorylation (i.e. destroys the electrochemical proton gradient formed in the electron transport... 

In some experiments we saw not only a enhancement of the photophosphorylation onset lag but also a slow down of the ATP synthesis speed. This would be expected, if pyridine had also uncoupling properties (data not shown but see the same behaviour eg. in ref. 9, fig. 1). 

The above results demonstrate that the fluorescence decrease upon addition of uncouplers or inhibitors of the mitochondrial oxidases can be attributed to a transition to state II it occurs with similar kinetics, it is accompanied by a decrease in the ratio of F685nro/F7i5nnj fluorescence emissions at 77K and an increased phosphorylation of the LHC subunits and it is not observed in cytbg/f mutants which are blocked in state I. The mechanism by which ATP controls the state transitions remains to be elucidated. We conclude that ATP deprivation causes a reorganization of the photosynthetic apparatus suitable for an increase in PSI-mediated cyclic photophosphorylation and favours ATP synthesis versus NADPH production... 

Lipophilic tertiary amines (t-amines) are effective inhibitors of photophosphorylation (1-3) Bound up with the inhibition, light-induced uptake of (l C)-methylamine and quenching of 9 afflinoacridine (9 AA) fluorescence disappear. This indicates that inhibition of ATP synthesis may be related to a decline of the transthylakoid proton gradient, ApH (4). IntriguingTy, a strong pH-dependent control of electron flow was maintained in the presence of t-amines (3 5) despite of the low ApH Therefore, t-amines were termed selective uncouplers (1). 

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