Acceptor and donor-side interactions of phenolic inhibitors in Photosystem II

Abstract

Certain phenolic compounds represent a distinct class of Photosystem (PS) II Q B site inhibitors. In this paper, we report a detailed study of the effects of 2,4,6-trinitrophenol (TNP) and other phenolic inhibitors, bromoxynil and dinoseb, on PS II energetics. In intact PS II, phenolic inhibitors bound to only 90–95% of Q B sites even at saturating concentrations. The remaining PS II reaction centers (5–10%) showed modified Q A to Q B electron transfer but were sensitive to urea/triazine inhibitors. The binding of phenolic inhibitors was 30- to 300-fold slower than the urea/triazine class of Q B site inhibitors, DCMU and atrazine. In the sensitive centers, the S 2Q A − state was 10-fold less stable in the presence of phenolic inhibitors than the urea/triazine herbicides. In addition, the binding affinity of phenolic herbicides was decreased 10-fold in the S 2Q A − state than the S 1Q A state. However, removal of the oxygen-evolving complex (OEC) and associated extrinsic polypeptides by hydroxylamine (HA) washing abolished the slow binding kinetics as well as the destabilizing effects on the charge-separated state. The S 2-multiline electron paramagnetic resonance (EPR) signal and the ‘split’ EPR signal, originating from the S 2Y Z state showed no significant changes upon binding of phenolic inhibitors at the Q B site. We thus propose a working model where Q A redox potential is lowered by short-range conformational changes induced by phenolic inhibitor binding at the Q B niche. Long-range effects of HA-washing eliminate this interaction, possibly by allowing more flexibility in the Q B site.