Some new inhibitors of electron transport in chloroplasts
Most herbicides acting on photosynthetic electron transport are found to be inhibitors that bind to the QB protein of photosystem II, which is believed to have a plastoquinone binding site. However, evidence is now available to suggest the existence of other plastoquinone binding sites within the electron-transfer chain of chloroplasts. The cytochrome bf complex is involved in plastoquinol oxidation (QO site) and plastoquinone reduction (QR site). Plastoquinone is also an intermediate in the electron-transfer pathway of ferredoxin-catalysed cyclic photophosphorylation. Recent evidence is available to suggest the existence of a specific ferredoxin-plastoquinone reductase (FQR) which is not associated with the cytochrome bf complex. A series of routine electron-transport assays have been developed to characterize the four plastoquinone-binding sites discussed above. Inhibitors of the QR site inhibited the slow phase of the electrochromic shift (P518s) and the re-oxidation of cytochrome b-563. QO site inhibitors affected the re-reduction of both cytochromes f and b-563, in addition to the attenuation of P518s. Cyclic electron transport systems have been set up in broken chloroplasts with either ferredoxin or 9,10-anthraquinone-2-sulphonate as cofactor. FQR inhibitors affected the former cyclic process but not the latter. Evidence was obtained to support the recent notion of the primary site of action of antimycin being at FQR rather than the QR site, which is the primary site in mitochondria. Simple analogues of antimycin such as 3,5 dihalosalicyl-N-(n-substituted) amides also inhibited the FQR. These observations indicated that the inhibitory property of antimycin is associated with the substituted aromatic moiety whilst the remaining dilactone portion provides an additional lipophilicity. The requirement of the aromatic ring for inhibitory activity was confirmed by the effects of tetrahalogenated 4-hydroxy-pyridines. These were found to act not at the QR site as reported in the literature, but at FQR. In addition to the aromatic nucleus, these FQR inhibitors required a phenolic hydroxyl group for activity. Data obtained was consistent with an obligatory, fixed stoichiometry H+/e- of three) Q cycle in the cytochrome bf complex, insensitive to antimycin. Kinetic evidence supported the existence of two quinone binding sites in this complex. Inhibition at one of these, QR site, by 2-alkyl quinoline N-oxide required a high degree of lipophilicity as well as the N-oxide and a ring hydroxyl group. Structural features of the inhibitors which appear to distinguish binding at the various sites include the number of redox active groups on the aromatic nucleus, the requirement for an electron withdrawing group ortho to the redox active group, and the requirement for the redox active group to carry a negative charge.