Interaction of N,N,N',N'-tetramethyl-p-phenylenediamine with photosystem II as revealed by thermoluminescence: reduction of the higher oxidation states of the Mn cluster and displacement of plastoquinone from the Q(B) niche

Is carbonic anhydrase activity of photosystem II required for its maximum electron transport rate?

It is known, that the multi-subunit complex of photosystem II (PSII) and some of its single proteins exhibit carbonic anhydrase activity. Previously, we have shown that PSII depletion of HCO3-/CO2 as well as the suppression of carbonic anhydrase activity of PSII by a known inhibitor of α‑carbonic anhydrases, acetazolamide (AZM), was accompanied by a decrease of electron transport rate on the PSII donor side. It was concluded that carbonic anhydrase activity was required for maximum photosynthetic activity of PSII but it was not excluded that AZM may have two independent mechanisms of action on PSII: specific and nonspecific. To investigate directly the specific influence of carbonic anhydrase inhibition on the photosynthetic activity in PSII we used another known inhibitor of α‑carbonic anhydrase, trifluoromethanesulfonamide (TFMSA), which molecular structure and physicochemical properties are quite different from those of AZM. In this work, we show for the first time that TFMSA inhibits PSII carbonic anhydrase activity and decreases rates of both the photo-induced changes of chlorophyll fluorescence yield and the photosynthetic oxygen evolution. The inhibitory effect of TFMSA on PSII photosynthetic activity was revealed only in the medium depleted of HCO3-/CO2. Addition of exogenous HCO3- or PSII electron donors led to disappearance of the TFMSA inhibitory effect on the electron transport in PSII, indicating that TFMSA inhibition site was located on the PSII donor side. These results show the specificity of TFMSA action on carbonic anhydrase and photosynthetic activities of PSII. In this work, we discuss the necessity of carbonic anhydrase activity for the maximum effectiveness of electron transport on the donor side of PSII.

Re-evaluation of the side effects of cytochrome b6f inhibitor dibromothymoquinone on photosystem II excitation and electron transfer

Dibromothymoquinone (DBMIB) has been used as a specific inhibitor of plastoquinol oxidation at the Q0 binding site of the cytochrome b6f complex for 40 years. It is thought to suppress electron transfer between photosystem (PS) II and I, as well as cyclic electron transfer around PSI. However, DBMIB has also been reported to act as a quencher of chlorophyll excited states. In this study, we have re-evaluated the effects of DBMIB on chlorophyll excited states and PSII photochemistry. The results show that DBMIB significantly quenches the chlorophyll excited states of PSII antenna even at low concentration (from 0.1 μM), lowering the effective excitation rate of the actinic light. It also acts as a potent PSII electron acceptor retarding the reduction of the plastoquinone pool with almost maximal potency at 2 μM. Altogether, these results suggest that experiments using DBMIB can easily be misinterpreted and stress on the importance of taking into account all these side effects that occur in the same range of DBMIB concentration used for inhibition of plastoquinol oxidation (1 μM).

Inhibition of the water oxidizing complex of photosystem II and the reoxidation of the quinone acceptor QA- by Pb2+

The action of the environmental toxic Pb(2+) on photosynthetic electron transport was studied in thylakoid membranes isolated from spinach leaves. Fluorescence and thermoluminescence techniques were performed in order to determine the mode of Pb(2+) action in photosystem II (PSII). The invariance of fluorescence characteristics of chlorophyll a (Chl a) and magnesium tetraphenylporphyrin (MgTPP), a molecule structurally analogous to Chl a, in the presence of Pb(2+) confirms that Pb cation does not interact directly with chlorophyll molecules in PSII. The results show that Pb interacts with the water oxidation complex thus perturbing charge recombination between the quinone acceptors of PSII and the S2 state of the Mn4Ca cluster. Electron transfer between the quinone acceptors QA and QB is also greatly retarded in the presence of Pb(2+). This is proposed to be owing to a transmembrane modification of the acceptor side of the photosystem.

Positive charges of polyamines protect PSII in isolated thylakoid membranes during photoinhibitory conditions

The effects of the positive charges of amines such as spermine (SPM), putrescine (PUT) and methylamine (MET) on the protection of PSII against excessive illumination were investigated in isolated thylakoid membranes. Under photoinhibition conditions, water oxidation, the kinetics of the Chl fluorescence rise and charge recombination in PSII were affected. A low concentration of SPM (1 mM) added before photoinhibition produced a significant improvement of F(v)/F(0), the oxygen yield and the amplitude of the B-band of thermoluminescence compared with the other amines. Amongst the amines studied, only SPM could protect the photosynthetic apparatus under photoinhibition conditions. This protection was probably provided by the polycationic nature of SPM (four positive charges at physiological pH), which can stabilize surface-exposed proteins of PSII through electrostatic interaction.

Inhibition of photosynthetic oxygen evolution and electron transfer from the quinone acceptor QA- to QB by iron deficiency

The effect of iron deficiency on photosynthetic electron transport in Photosystem II (PS II) was studied in leaves and thylakoid membranes of lettuce (Lactuca sativa, Romaine variety) plants. PS II electron transport was characterized by oxygen evolution and chlorophyll fluorescence parameters. Iron deficiency in the culture medium was shown to affect water oxidation and the advancement of the S-states. A decrease of maximal quantum yield of PS II and an increase of fluorescence intensity at step J and I of OJIP kinetics were also observed. Thermoluminescence measurements revealed that charge recombination between the quinone acceptor of PS II, Q(B), and the S(2) state of the Mn-cluster was strongly perturbed. Also the dark decay of Chl fluorescence after a single turnover white flash was greatly retarded indicating a slower rate of Q(A)(-) reoxidation.

The oxidation/reduction kinetics of the plastoquinone pool controls the appearance of the I-peak in the O-J-I-P chlorophyll fluorescence rise: effects of various electron acceptors

Quantitative analysis of the fluorescence induction (FI) rise was used in this study to elucidate the complex effects of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) on thylakoids. Reduced TMPD molecules, responsible for the ADRY agent effect, caused an increase in the amplitude of the O-J rise. Also, only oxidized TMPD molecules were shown to have the ability to bind the Q(B) pocket of photosystem II (PSII). On the other hand, the I-P rise was slowed in proportion with the oxidized TMPD concentration, inducing the clear appearance of the I-peak. While this property was previously thought to be unique to TMPD, this study shows that some artificial electron acceptors of PSII, silicomolybdate, 2,5-dichloro-p-benzoquinone, and phenyl-p-benzoquinone, have a similar effect. These results demonstrated a major role of the oxido-reduction kinetics of the PQ-pool in the resolution of J-I and I-P phases in the FI of isolated thylakoids.

Spermine and spermidine inhibition of photosystem II: Disassembly of the oxygen evolving complex and consequent perturbation in electron donation from TyrZ to P680+ and the quinone acceptors QA− to QB

Polyamines are implicated in plant growth and stress response. However, the polyamines spermine and spermidine were shown to elicit strong inhibitory effects in photosystem II (PSII) submembrane fractions. We have studied the mechanism of this inhibitory action in detail. The inhibition of electron transport in PSII submembrane fractions treated with millimolar concentrations of spermine or spermidine led to the decline of plastoquinone reduction, which was reversed by the artificial electron donor diphenylcarbazide. The above inhibition was due to the loss of the extrinsic polypeptides associated with the oxygen evolving complex. Thermoluminescence measurements revealed that charge recombination between the quinone acceptors of PSII, QA and QB, and the S2 state of the Mn-cluster was abolished. Also, the dark decay of chlorophyll fluorescence after a single turn-over white flash was greatly retarded indicating a slower rate of QA- reoxidation.