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Received July 30, 2009; Revision received September 18, 2009
Binding of MgADP and MgATP to Escherichia coli F1-ATPase (EcF1) has been assessed by their effects on extent of the enzyme inhibition by 7-chloro-4-nitrobenz-2-oxa-1,3-diazole (NBD-Cl). MgADP at low concentrations (Kd 1.3 µM) promotes the inhibition, whereas at higher concentrations (Kd 0.7 mM) EcF1 is protected from inhibition. The mutant βY331W-EcF1 requires much higher MgADP, Kd of about 10 mM, for protection. Such MgADP binding was not revealed by fluorescence quenching measurements. MgATP partially protects EcF1 from inactivation by NBD-Cl, but the enzyme remains sensitive to NBD-Cl in the presence of MgATP at concentrations as high as 10 mM. The activating anion selenite in the absence of MgATP partially protects EcF1 from inhibition by NBD-Cl. A complete protection of EcF1 from inhibition by NBD-Cl has been observed in the presence of both MgATP and selenite. The results support a bi-site catalytic mechanism for MgATP hydrolysis by F1-ATPases and suggest that stimulation of the enzyme activity by activating anions is due to the anion binding to a catalytic site that remains unoccupied at saturating substrate concentration.
KEY WORDS: anion activation, ATP synthase, catalytic cooperativity, bi-site catalysis, nucleotide binding, tri-site catalysis