Structural Basis of Cyanobacterial Photosystem II Inhibition by the Herbicide Terbutryn

Abstract

Herbicides that target photosystem II (PSII) compete with the native electron acceptor plastoquinone for binding at the Q B site in the D1 subunit and thus block the electron transfer from Q A to Q B . Here, we present the first crystal structure of PSII with a bound herbicide at a resolution of 3.2 Å . The crystallized PSII core complexes were isolated from the thermophilic cyanobacterium Thermosynechococcus elongatus. The used herbicide terbutryn is found to bind via at least two hydrogen bonds to the Q B site similar to photosynthetic reaction centers in anoxygenic purple bacteria. Herbicide binding to PSII is also discussed regarding the influence on the redox potential of Q A , which is known to affect photoinhibition. We further identified a second and novel chloride position close to the water-oxidizing complex and in the vicinity of the chloride ion reported earlier (Guskov, A., Kern, J., Gabdulkhakov, A., Broser, M., Zouni, A., and Saenger, W. (2009) Nat. Struct. Mol. Biol. 16, 334 -342). This discovery is discussed in the context of proton transfer to the lumen.The process of photosynthesis converts solar energy into biochemically amenable energy. A distinction is made between oxygenic and anoxygenic photosynthesis. In the latter sulfur compounds, hydrogen gas or organic materials serve as electron source. In contrast, oxygenic photosynthesis in higher plants, algae, and cyanobacteria uses water as an electron source and generates molecular oxygen, thereby maintaining the level of oxygen in the atmosphere. Water oxidation takes place at the large homodimeric protein-cofactor complex photosystem II (PSII), 7 a light-driven water:plastoquinone oxidoreductase harbored in the thylakoid membrane (1-3). The structure of the PSII core complex (PSIIcc) from the thermophilic cyanobacterium Thermosynechococcus elongatus is known from x-ray crystallographic studies at a current resolution of 2.9 Å (4, 5). The photochemical reaction center (RC) in PSII is of type II and structurally related to the RC of purple bacteria (pbRC) (6), which perform anoxygenic photosynthesis. The PSII-RC contains four chlorophyll a (Chla) molecules (P D1 , P D2 , Chl D1 , and Chl D2 ), two pheophytins (Pheo D1 and Pheo D2 ), and two plastoquinones (PQ) (Q A and Q B ) with a nonheme iron in between. These cofactors are embedded in a heterodimeric protein matrix formed by subunits D1 and D2 (systematic names: PsbA and PsbD, respectively) and are arranged in two pseudo-C2 symmetric branches with respect to a 2-fold rotation axis, which crosses the non-heme iron and is oriented normal to the membrane plane.Photons from sunlight are collected by antenna proteins of PSII, and the excitation energy is transferred to the RC, where it gives rise to the formation of the radical pair P D1 ⅐ϩ Pheo D1 ⅐Ϫ and subsequent electron transfer to the fixed single-electron transmitter Q A . The electron hole at P D1 ⅐ϩ is able to abstract electrons via the redox-active tyrosine Y Z (Tyr-161A) from the heteronuclear Mn 4 Ca cluster located at the lumenal (d...

Native-like Photosystem II Superstructure at 2.44 Å Resolution through Detergent Extraction from the Protein Crystal

“…No electron density is observed at the plastoquinone binding site Q C identified previously (Guskov et al, 2009). Two chloride ions near the Mn 4 CaO 5 cluster ( Figure S2a) occupy the positions Cl 1 (Cl 1A in Broser et al, 2011) and Cl 2 (as identified by Umena et al, 2011). No electron density is observed at the Cl 1B position (Broser et al, 2011) and at Cl 3 near PsbU .…”

“…Two chloride ions near the Mn 4 CaO 5 cluster ( Figure S2a) occupy the positions Cl 1 (Cl 1A in Broser et al, 2011) and Cl 2 (as identified by Umena et al, 2011). No electron density is observed at the Cl 1B position (Broser et al, 2011) and at Cl 3 near PsbU . Three additional Ca 2+ ion positions previously found in the presence of 10 mM CaCl 2 are unoccupied, which is likely due to the absence of calcium in our transformation buffer.…”

“…X-ray structures of the dimeric PSII core complex (dPSIIcc) from thermophilic cyanobacteria have been reported with resolutions up to 1.9 Å (Guskov et al, 2009Umena et al, 2011) (Figure 1). In addition, structures of a monomeric form (mPSIIcc) , herbicide-inhibited dPSIIcc (Broser et al, 2011), and dPSIIcc with Ca 2+ substituted by Sr 2+ (Koua et al, 2013) were reported. In all cases, bDM was used as detergent, while the recent structures of Thermosynechococcus vulcanus PSIIcc feature an additional detergent, n-heptyl-b-thioglucoside (HTG) Koua et al, 2013).…”

Effect of pretilachlor on nitrogen uptake and assimilation by the cyanobacterium Desmonostoc muscorum PUPCCC 405.10

“…Monosulfuron exhibited its mode of action on acetolactate synthase enzyme in cyanobacteria (Shen et al 2009). Terbutryn was found to bind via two hydrogen bonds to Q B site in the D1 subunit of PS-II in thermophilic cyanobacterium Thermosynechococcus elongatus (Broser et al 2011). The concentration-dependent increase in generation time indicated herbicide affected cell division of Desmonostoc muscorum PUPCCC 405.10.…”

<strong>LIGAND-BASED PHARMACOPHORE MODEL AND QSAR STUDIES ON HERBICIDES TARGETING PHOTOSYSTEM II FROM CHLAMYDOMONAS REINHARDTII</strong>

“…[3, 4] Computational methods such as ligand-based pharmacophores and quantitative structureactivity relationship (QSAR) are widely used in the discovery of new chemicals for pharmaceutical and agrochemical fields.…”