The cytochrome bc1 complex inhibitor Ametoctradin has an unusual binding mode

Ametoctradin is an agricultural fungicide that selectively inhibits the cytochrome bc₁ complex of oomycetes. Previous spectrophotometric studies using the purified cytochrome bc₁ complex from Pythium sp. showed that Ametoctradin binds to the Q_o-site of the enzyme. However, as modeling studies suggested a binding mode like that of the substrate ubiquinol, the possibility for a dual Q_o- and Qi-site binding mode was left open.In this work, binding studies and enzyme assays with mitochondrial membrane preparations from Pythium sp. and an S. cerevisiae strain with a modified Q_i-site were used to investigate further the binding mode of Ametoctradin. The results obtained argue that the compound could bind to both the Q_o- and Qi-sites of the cytochrome bc₁ complex and that its position or binding pose in the Q_i-site differs from that of Cyazofamid and Amisulbrom, the two Q_i-site-targeting, anti-oomycetes compounds. Furthermore, the data support the argument that Ametoctradin prefers binding to the reduced cytochrome bc₁ complex. Thus, Ametoctradin has an unusual binding mode and further studies with this compound may offer the opportunity to better understand the catalytic cycle of the cytochrome bc₁ complex.