Question

Macías-Rubalcava et al., in J. Photochem. Photobiol. (2017) B 166:35-43, investigated the mechanism of action of several compounds that might serve as new herbicides. One of these new compounds (#7) inhibited photophosphorylation. The data below show the effects of compound 7 on uncoupled electron transport in isolated illuminated chloroplasts. The scheme below is a linear version of the Z-scheme, showing the sequence of electron transport through PSII and PSI. Arrows show the sites of electron donation and acceptance for various artificial donors/acceptors. Vertical dashed lines show sites of inhibition by Tris and DCMU. - DCPIPM % Activity DPC- DCPIP H, O H, O SIMO DCBQ 0 50 250 300 100 150 200 Compound 7 (M) SIMODCBQ OEC QA - 95 pot) -W -- RO Tris DCMU DCPIP MV

Based on these data, predict the site of action of the new compound. Explain your reasoning.

Match the items in the left column to the appropriate blanks in the sentences on the right.

Answer 1

From the above graph it is clearly visible that compound #7 has an inhibitory effect on various electron donating and accepting mechanisms in the photophosphorylation. To analyze the graph, first see which curve is decreasing at a higher rate i.e., curve of H2O->DCBQ is decreasing at a higher rate when compared to other curves and hence it is the most inhibited reaction in the pathway. The topmost curve i.e., DCPIP->MV is almost linear and hence it is the least inhibited reaction. Hence the site of action of the new compound is on the electron donation between Q_Aand Q_B as the electron donations between Q_B to DCPIP is partially inhibited and also to DCBQ is extensively inhibited and hence the Q_B is most likely to be the site of action of the new compound and also Q_A to SiMO is also being inhibited. Hence the quinone cycle of Q_Aand Q_B is the site of action of the new compound.

Compound 7 acts at OEC, most likely at the water to DCBQ.

PSI is not a target, since electron transport from DCPIP to MV was not significantly affected.

The most pronounced inhibitory effect was on electron transport from water -> DCBQ and water -> SiMO, both partial reactions involving OEC.

The DCP -> DCPIP reaction was partially inhibited, suggesting that compound 7 also inhibits PSII between P680 and Q_B

_{If you have any queries, kindly comment before giving a thumbs up. Thank you.}