Differential inhibition of the plastoquinone reductase activity by weak organic acids and its relationship to the bicarbonate effect in spinach thylakoids

Bicarbonate-reversible inhibition of the electron acceptor side of photosystem II (PS II), but not of photosynthetic bacteria, has been known for a long time. In formate treated thylakoids, the (Q$\sb{\rm A}\sp-$) decays show a larger slowing down after the second and subsequent flashes than after the first flash. This suggests a blockage of protonation that stabilizes Q$\sb{\rm B}\sp-$.Measurements of the initial binding rate constant of formate/formic acid, when one of the two (HCO$\sb2\sp-$/HCOOH) equilibrium species is kept constant by changing the pH of the medium, suggest that acid, not the anion, is the binding species. This conclusion is also consistent with the preliminary data on the observed pH dependence of the binding of (halogenated)-acetates as well as (methyl) acetates.Several halogenated acetates increase the lifetime of the Q$\sb{\rm A}\sp-$ reoxidation by Q$\sb{\rm B}$/Q$\sb{\rm B}\sp-$ as well as the equilibrium (Q$\sb{\rm A}\sp-$). These effects are reversed, in different degrees, upon the bicarbonate addition. The inhibitory activity on the Q$\sb{\rm A}\sp-$-to-Q$\sb{\rm B}\sp{(-)}$ electron transfer decreases, whereas the bicarbonate reversibility increases in the following order: (1) trichloroacetic acid $>$ dichloroacetic acid $>$ monochloroacetic acid $>$ acetic acid; (2) monobromoacetic acid $\ge$ monochloroacetic acid $>$ monofluoroacetic acid $>$ acetic acid. Trichloroacetic acid, with a log P (partition coefficient) of 1.54 and a dipole moment of 2.12 Debye, almost totally blocks the Q$\sb{\rm A}\sp-$ reoxidation just as DCMU does. A correlation between the inhibitory activity and the geometry as well as the hydrophobicity of the inhibitors is observed, demonstrating the importance of the CX$\sb3$ group of these chemicals in affecting the plastoquinone reduction. Furthermore, monochloroacetic acid, with an asymmetric chlorine atom, a log P of 0.32 and a dipole moment of 3.25 Debye, converts, in a bicarbonate-reversible manner, the flash number dependence of (Q$\sb{\rm A}\sp{-}$) with the maxima occurring at even to that at odd flashes. This apparent stabilization of Q$\sb{\rm B}\sp-$, caused by MCA, is explained by a combined effect of the shift of the Q$\sb{\rm A}\sp-$Q$\sb{\rm B}{}$Q$\sb{\rm A}$Q$\sb{\rm B}\sp-$ equilibrium towards (Q$\sb{\rm A}\sp-$) and an increase of the ratio of Q$\sb{\rm B}\sp-$ to Q$\sb{\rm B}$ in dark.U of I OnlyETDs are only available to UIUC Users without author permissio