Cl- channel inhibitors of the arylaminobenzoate type act as photosystem IIherbicides: A functional and structural study

The Cl- channel blocker NPPB (5-nitro-2-(3-phenylpropylamino) benzoic acid) inhibited photosynthetic oxygen evolution of isolated thylakoid membranes in a pH-dependent manner with a K-i of about 2 muM at pH 6. Applying differ ent electron accepters, taking electrons either directly from photosystem I I (PS II) or photosystem I (PS I), the site of inhibition was localized wit hin PS IT. Measurements of fluorescence induction kinetics and thermolumine scence suggest that the binding of NPPB to the Q(B) binding site of PS II i s similar to the herbicide DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea). The effects of different arylaminobenzoate derivatives and other Cl- channe l inhibitors on photosynthetic electron transport were investigated. The st ructure-activity relationship of the inhibitory effect on PS TI shows inter esting parallels to the one observed for the arylaminobenzoate block of mam malian Cl- channels. A molecular modeling approach was used to fit NPPB int o the Q(B) binding site and to identify possible molecular interactions bet ween NPPB and the amino acid residues of the binding site in PS II. Taken t ogether, these data give a detailed molecular picture of the mechanism of N PPB binding.