Interaction of Mg+2 with beef heart mitochondrial ATPase (F1)

ATP binding and hydrolysis steps of the uni-site catalysis by the mitochondrial F(1)-ATPase are affected by inorganic phosphate

The effect of inorganic phosphate (P(i)) on uni-site ATP binding and hydrolysis by the nucleotide-depleted F(1)-ATPase from beef heart mitochondria (ndMF(1)) has been investigated. It is shown for the first time that P(i) decreases the apparent rate constant of uni-site ATP binding by ndMF(1) 3-fold with the K(d) of 0.38+/-0.14mM. During uni-site ATP hydrolysis, P(i) also shifts equilibrium between bound ATP and ADP+P(i) in the direction of ATP synthesis with the K(d) of 0.17+/-0.03mM. However, 10mM P(i) does not significantly affect ATP binding during multi-site catalysis.

Adenine nucleotide-binding sites on mitochondrial F1-ATPase: studies of the inactive complex formed upon binding ADP at a catalytic site

ADP-induced inhibition of mitochondrial F1-ATPase has been studied. It is shown that in the presence of magnesium and the absence of light, the photoaffinity ADP analog, 2-azido-ADP, induces a reversible inhibition of native F1 that is indistinguishable from that obtained with ADP. Photolysis of the inactive complex results in the predominant labeling of a catalytic-site peptide identified previously (Cross et al., 1987, Proc. Natl. Acad. Sci. USA 84, 5715-5719). Dissociation of the inactive complex formed between F1 and ADP is biphasic with a rapid azide-insensitive phase followed by a slow azide-sensitive phase (k approximately 3 x 10(-3) s-1). It is also shown that incubation of the ADP-inhibited enzyme with EDTA or phosphate does not result in release or migration of ADP from the catalytic site. However, it does convert the complex to a form that reactivates in the presence of 100 microM ATP at a rate too rapid to observe using manual mixing.

Interaction of Mg2+ with F0.F1 mitochondrial ATPase as related to its slow active/inactive transition

Bovine heart submitochondrial particles incubated with a low concentration of ADP in the presence of Mg2+ and passed through a Sephadex column equilibrated with EDTA exhibit sensitivity of their initial ATPase activity to preincubation with Mg2+. By using particles thus prepared, several characteristics of a Mg(2+)-specific inhibitory site on F0.F1 ATPase were studied. The inhibition was shown to be both time- and Mg(2+)-concentration-dependent, with an equilibrium constant (at infinite time) of 2 x 10(-6) M (25 degrees C, pH 7.5). The dependence of the pseudo-first-order rate constant for the inhibition process on Mg2+ concentration suggests the presence of a single Mg(2+)-binding site with K8 = 1.1 x 10(-4) M. The data obtained are consistent with a two-step mechanism of Mg(2+)-F0.F1 interaction which results in a loss of the ATPase activity; it includes rapid pH-dependent binding of Mg2+ at the site with K8 = 1.1 x 10(-4) M, followed by a slow interconversion of the Mg(2+)-F1 complex into inactive ATPase (kin. = 0.65 min-1, kact. = 0.01 min-1). The Mg(2+)-inhibited ATPase is very slowly (t1/2 approximately 90 min) re-activated in the presence of EDTA. The rate of EDTA-induced re-activation is pH-independent and can be dramatically increased by added ATP, Pi and sulphite. The dissociation constants for free ATP and P1 (5 x 10(-7) M and 1 x 10(-3) M respectively) and the maximal activation rates were determined by measuring the hyperbolic dependencies of the EDTA-induced re-activation of Mg(2+)-de-activated ATPase on the concentrations of the accelerating ligands. Taken together, the data obtained show two functionally detectable free nucleotide-specific binding sites, one site for Pi and one Mg(2+)-specific ATPase-inhibitory site on the F0.F1 mitochondrial ATP synthase complex.

The ADP that binds tightly to nucleotide-depleted mitochondrial F1-ATPase and inhibits catalysis is bound at a catalytic site

Previous studies have shown that the initial complex formed when ADP binds to nucleotide-depleted F1-ATPase is transformed with a half time of 2 to 3 min to form with a much lower rate of ADP release. The ADP binding results in a strong inhibition of ATPase activity. The present paper reports appraisal of where the inhibitory ADP binds by use of the photoreactive ADP analog, 2-N3-ADP. In presence of Mg2+ the 2-N3-ADP like ADP induces reversible inhibition of nucleotide-depleted F1 (ndF1) with a Kd of about 10 nM. Photoirradiation of the inactive 2-N3-[beta-32P]ADP-ndF1 complex results in labeling of only the beta-subunit. The major labeled peptide isolated from a trypic digest consists of residues from Ala-338 to Arg-356, with Tyr-345 as the site of labeling. This identifies the site of the inhibitory ADP binding as one of the catalytic sites of the enzyme.

F1 ATPase from the thermophilic bacterium PS3 (TF1) shows ATP modulation of oxygen exchange

The ATPase from the ATP synthase of the thermophilic bacterium PS3 (TF1), unlike F1 ATPase from other sources, does not retain bound ATP, ADP, and Pi at a catalytic site under conditions for single-site catalysis [Yohda, M., & Yoshida, M. (1987) J. Biochem. 102, 875-883]. This raised a question as to whether catalysis by TF1 involved alternating participation of catalytic sites. The possibility remained, however, that there might be transient but catalytically significant retention of bound reactants at catalytic sites when the medium ATP concentration was relatively low. To test for this, the extent of water oxygen incorporation into Pi formed by ATP hydrolysis was measured at various ATP concentrations. During ATP hydrolysis at both 45 and 60 degrees C, the extent of water oxygen incorporation into the Pi formed increased markedly as the ATP concentration was lowered to the micromolar range, with greater modulation observed at 60 degrees C. Most of the product Pi formed arose by a single catalytic pathway, but measurable amounts of Pi were formed by a pathway with high oxygen exchange. This may result from the presence of some poorly active enzyme. The results are consistent with sequential participation of three catalytic sites on the TF1 as predicted by the binding change mechanism.

The effect of inorganic pyrophosphate on the activity and Pi-binding properties of mitochondrial F1-ATPase

Interaction of F1-ATPase from beef heart mitochondria with PPi has been investigated. The presence of PPi in the ATPase assay medium does not affect the initial rate of ATP hydrolysis by F1-ATPase, but slows down the decrease of enzyme activity in the course of ATP hydrolysis and increases the steady-state rate of ATP hydrolysis. Being present in the ATPase assay medium, PPi accelerates the ATP-dependent reactivation of an inactive complex formed by F1-ATPase and ADP. This inactive complex is also reactivated after preincubation with PPi. F1-ATPase, preincubated with PPi, is inactivated by azide much more slowly than is the non-preincubated enzyme. PPi stimulates the binding of Pi to F1-ATPase by decreasing mainly the Kd for Pi and only slightly raising the stoichiometry of high-affinity Pi binding. It follows from the results obtained that PPi interacts with the non-catalytic site(s) of F1-ATPase.

Interaction of azide with beef heart mitochondrial ATPase

This study examined the inhibition of azide as a probe of the magnesium regulation of beef heart mitochondrial ATPase (F1) catalysis. Azide elicited a slow hysteretic effect on both ATP and ITP hydrolysis of F1. This hysteretic effect was shown to be due to the consecutive binding of magnesium and azide, and to be independent of catalytic turnover. The azide binding site was also shown to be separate from the anion binding HCO3- site on F1. The results presented indicate that metal binding is important in the inhibition of the hydrolytic activity and regulation of F1. A model is presented which is consistent with the hysteretic inhibition of F1 by azide, in which there is a slow equilibration between free enzyme and the enzyme-magnesium-azide complex.

Pre-steady-state kinetics of beef heart mitochondrial ATPase

The pre-steady-state kinetics of beef heart mitochondrial ATPase (F1) were examined. F1 was found to exhibit hysteretic behavior when hydrolyzing ATP. The hysteretic property was expressed as an activation process which occurred when the enzyme was mixed with its substrate, MgATP. Many catalytic turnovers were required before the activation was complete. The lag in hydrolysis increased hyperbolically as the concentration of enzyme increased. Passage of F1 through Sephadex G25 eliminated the activation process. Several kinetically distinct possibilities for explaining these data, including multiple nucleotide dissociations, enzyme conformational changes, and regulatory site interactions, are discussed. The enzyme was apparently able to recognize nucleotide in a noncatalytic manner, as evidenced by the fact that F1 preincubated with ADP in the absence of substrate achieved partial activation (smaller lag times) before being introduced to substrate. ADP is also a time-dependent inhibitor, exhibiting a slow hysteretic inhibition in addition to immediate competitive inhibition.

Kinetic mechanism of mitochondrial adenosine triphosphatase. ADP-specific inhibition as revealed by the steady-state kinetics

1. A substantial increase of the initial rate of ATP hydrolysis was observed after preincubation of bovine heart submitochondrial particles with phosphoenolpyruvate and pyruvate kinase. 2. The activation was accompanied by an increase of Vmax, without change of Km for ATP. 3. The activated particles catalysed the biphasic hydrolysis of ATP in the presence of an ATP-regenerating system; the initial rapid phase was followed by a second, slower, phase in a time-dependent fashion. 4. The higher the ATP concentration used as a substrate, the higher is the rate of transition between these two phases. 5. The particles catalysed the hydrolysis of ITP with a lag phase; after preincubation with phosphoenolpyruvate and pyruvate kinase, ITP was hydrolysed at a constant rate. 6. Qualitatively the same phenomena were observed when soluble mitochondrial ATPase (F1-ATPase) prepared by the conventional method in the presence of ATP was used as nucleotide triphosphatase. 7. A kinetic scheme is proposed, in which the intermediate active enzyme-product complex (E.ADP) formed during ATP hydrolysis is in slow equilibrium with the inactive E*.ADP complex forming as a result of dislocation of ADP from the active site of ATPase to the other site, which is not in rapid equilibrium with the surrounding medium.