Interaction of high-affinity nucleotide binding sites in mitochondrial ATP synthesis and hydrolysis

Novel approaches towards characterization of the high-affinity nucleotide binding sites on mitochondrial F1-ATPase by the fluorescence probes 3'-O-(1-naphthoyl)adenosine di- and triphosphate

The fluorescence properties of 3'-O-(1-naphthoyl)adenosine di- and triphosphates (termed N-ADP and N-ATP, respectively) were investigated in detail. Of special importance for the use of these analogues as environmental probes is their high quantum yield (0.58 in water) and the polarity dependence of shape and wavelength position of the emission spectrum. Upon binding of N-ADP and N-ATP to mitochondrial F1-ATPase, the fluorescence intensity is markedly decreased, due to polarity changes and 'ground-state' quenching. Using this signal for equilibrium binding studies, three (at least a priori) equivalent nucleotide-binding sites were detected on the enzyme. The perspective intrinsic dissociation constants are as follows: N-ADP/Mg2+ 120 nM; N-ATP/Mg2+ 160 nM; N-ADP/EDTA 560 nM; N-ATP/EDTA 3500 nM. For bound ligand the environment was found to be rather unipolar; the rotational mobility of the fluorophore is restricted, its accessibility for iodide anions (as a quencher) is hindered. These facts show a location of the binding sites quite deeply embedded in the protein. The conformation of the binding domains is strongly dependent on the absence or presence of Mg2+, as can be seen from the relative efficiencies of the singlet-singlet energy transfer from tyrosine residues in the protein to bound naphthoyl moieties. Investigation of the binding kinetics revealed this process as biphasic (in presence of Mg2+). After the first fast step (kon greater than 1 X 10(6) M-1 X s-1), in which the analogue is bound to the enzyme, a slow local conformational rearrangement occurs.

Kinetics of ATP hydrolysis catalyzed by isolated TF1 and reconstituted TF0F1 ATPase

The rate of ATP hydrolysis catalyzed by isolated TF1 and reconstituted TF0F1 was measured as a function of the ATP concentration in the presence of inhibitors [ADP, Pi and 3'-O-(1-naphthoyl)ATP]. ATP hydrolysis can be described by Michaelis-Menten kinetics with Km(TF1) = 390 microM and Km (TF0F1) = 180 microM. The inhibition constants are for ADP Ki(TF1) = 20 microM and Ki(TF0F1) = 100 microM, for 3'-O-(1-naphthoyl)ATP Ki(TF1) = 150 microM and Ki(TF0F1) = 3 microM, and for Pi Ki(TF1) = 60 mM. From these results it is concluded that upon binding of TF0 to TF1 the mechanism of ATP hydrolysis catalyzed by TF1 is not changed qualitatively; however, the kinetic constants differ quantitatively.

Azidonaphthoyl-ADP: a specific photolabel for the high-affinity nucleotide-binding sites of F1-ATPase

3'-O-[5-azidonaphthoyl]-ADP has been synthesized as a photoreactive analog to 3'-O-naphthoyl(1)-ADP which is known to bind to the high-affinity nucleotide sites of mitochondrial F1-ATPase, considered to be the catalytic sites. The photolabel in the dark acts as a ligand to F1-ATPase and as a competitive inhibitor with Ki = 11 microM. Binding to the enzyme is accompanied by a quench of endogenous protein fluorescence leveling off at an occupancy of 1 mol/mol F1, whereas the total number of reversible sites accessible to the analog is 3 mol/mol F1 as measured by isotope studies. Covalent insertion by near ultraviolet activation of the probe yields labeling of both alpha and beta polypeptides of F1; it is accompanied by corresponding removal of reversible high-affinity sites for ADP or naphthoyl-ADP and by an inhibition of the enzyme; total inactivation occurs at a covalent occupancy of 2 mol/mol F1. This is the maximum number of sites accessible to covalent modification by the label; one reversible site is still available in the totally inactivated enzyme. This observation is discussed in terms of a stochastic model requiring a minimum of two interacting catalytic domains out of three in order to commence catalysis.