Studies on Inhibition of Respiratory Cytochrome bc (1) Complex by the Fungicide Pyrimorph Suggest a Novel Inhibitory Mechanism

The respiratory chain cytochrome bc (1) complex (cyt bc (1)) is a major target of numerous antibiotics and fungicides. All cyt bc (1) inhibitors act on either the ubiquinol oxidation (Q(P)) or ubiquinone reduction (Q(N)) site. The primary cause of resistance to bc (1) inhibitors is target site mutations, creating a need for novel agents that act on alternative sites within the cyt bc (1) to overcome resistance. Pyrimorph, a synthetic fungicide, inhibits the growth of a broad range of plant pathogenic fungi, though little is known concerning its mechanism of action. In this study, using isolated mitochondria from pathogenic fungus Phytophthora capsici, we show that pyrimorph blocks mitochondrial electron transport by affecting the function of cyt bc (1). Indeed, pyrimorph inhibits the activities of both purified 11-subunit mitochondrial and 4-subunit bacterial bc (1) with IC(50) values of 85.0 μM and 69.2 μM, respectively, indicating that it targets the essential subunits of cyt bc (1) complexes. Using an array of biochemical and spectral methods, we show that pyrimorph acts on an area near the Q(P) site and falls into the category of a mixed-type, noncompetitive inhibitor with respect to the substrate ubiquinol. In silico molecular docking of pyrimorph to cyt b from mammalian and bacterial sources also suggests that pyrimorph binds in the vicinity of the quinol oxidation site.