Uncompetitive substrate inhibition and noncompetitive inhibition by 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole (UHDBT) and 2-n-nonyl-4-hydroxyquinoline-N-oxide (NQNO) is observed for the cytochrome bo3 complex: implications for a Q(H2)-loop proton translocation mechanism.