Computational and Experimental Insight into the Molecular Mechanism of Carboxamide Inhibitors of Succinate-Ubquinone Oxidoreductase

Succinate-ubiquinone oxidoreductase (SQR, EC 1.3.5.1), also known as mitochondrial respiratory complex II or succinate dehydrogenase (SDH), catalyzes the oxidation of succinate to fumarate as part of the tricarboxylic acid cycle. SQR has been identified as a novel target of a large family of agricultural fungicides. However, the detailed mechanism of action between the fungicides and SQR is still unclear, and the bioactive conformation of fungicides in the SQR binding pocket has not been identified. In this study, the kinetics of porcine SQR inhibition by ten commercial carboxamide fungicides were measured, and noncompetitive inhibition was observed with respect to succinate, DCIP, and cytochrome c, while competitive inhibition was observed with respect to ubiquinone. With the aim to uncover the binding conformation of these fungicides, molecular docking, molecular dynamics simulation, and molecular mechanics/Poisson-Boltzmann surface area (MM/PBSA) calculations were then performed. The excellent correlation (r(2) = 0.94) between the calculated (Delta G(cal)) and experimental (Delta G(exp)) binding free energies indicates that the obtained docking conformation could be the bioactive conformation. The acid moiety of carboxamide fungicides inserts into the ubiquinone binding site (Q-site) of SQR, forming van der Waals (vdW) interactions with C_R46, C_S42, B_I218, and B_P169, while the amine moiety extends to the mouth of the Q-site, forming vdW interactions with C_W35, C_I43, and C_I30. The carbonyl oxygen atom of the carboxamide forms hydrogen bonds with B_W173 and D_Y91. These findings provide valuable information for the design of more potent and specific inhibitors of SQR.