Effects of the inhibitors azide, dicyclohexylcarbodiimide, and aurovertin on nucleotide binding to the three F₁-ATPase catalytic sites measured using specific tryptophan probes

Equilibrium nucleotide binding to the three catalytic sites of Escherichia coli F₁-ATPase was measured in the presence of the inhibitors azide, dicyclohexylcarbodiimide, and aurovertin to elucidate mechanisms of inhibition. Fluorescence signals of β-Trp-331 and β-Trp-148 substituted in catalytic sites were used to determine nucleotide binding parameters. Azide brought about small decreases in K(d)(MgATP) and K(d)(MgADP). Notably, under MgATP hydrolysis conditions, it caused all enzyme molecules to assume a state with three catalytic site-bound MgATP and zero bound MgADP. These results rule out the idea that azide inhibits by 'trapping' MgADP. Rather, azide blocks the step at which signal transmission between catalytic sites promotes multisite hydrolysis. Aurovertin bound with stoichiometry of 1.8 (mol/mol of F₁) and allowed significant residual turnover. Cycling of the aurovertin- free β-subunit catalytic site through three normal conformations was indicated by MgATP binding data. Aurovertin did not change the normal ratio of I bound MgATP/2 bound MgADP in catalytic sites. The results indicate that it acts to slow the switch of catalytic site affinities ('binding change step') subsequent to MgATP hydrolysis. Dicyclohexylcarbodiimide shifted the ratio of catalytic site-bound MgATP/MgADP from 1:2 to 1.6:1.4, without affecting K(d)(MgATP) values. Like azide, it also appears to affect activity at the step after MgATP binding, in which signal transmission between catalytic sites promotes MgATP hydrolysis.