Characterization of the catalytic and noncatalytic ADP binding sites of the F1-ATPase from the thermophilic bacterium, PS3.

M Yoshida,W S Allison

doi:10.1016/s0021-9258(17)38441-7

Abstract

Two classes of ADP binding sites at 20 degrees C have been characterized in the F1-ATPase from the thermophilic bacterium, PS3 (TF1). One class is comprised of three sites which saturate with [3H]ADP in less than 10 s with a Kd of 10 microM which, once filled, exchange rapidly with medium ADP. The binding of ADP to these sites is dependent on Mg2+. [3H]ADP bound to these sites is removed by repeated gel filtrations on centrifuge columns equilibrated with ADP free medium. The other class is comprised of a single site which saturates with [3H]ADP in 30 min with a Kd of 30 microM. [3H]ADP bound to this site does not exchange with medium ADP nor does it dissociate on gel filtration through centrifuge columns equilibrated with ADP free medium. Binding of [3H]ADP to this site is weaker in the presence of Mg2+ where the Kd for ADP is about 100 microM. [3H]ADP dissociated from this site when ATP plus Mg2+ was added to the complex while it remained bound in the presence of ATP alone or in the presence of ADP, Pi, or ADP plus Pi with or without added Mg2+. Significant amounts of ADP in the 1:1 TF1.ADP complex were converted to ATP in the presence of Pi, Mg2+, and 50% dimethyl sulfoxide. Enzyme-bound ATP synthesis was abolished by chemical modification of a specific glutamic acid residue by dicyclohexylcarbodiimide, but not by modification of a specific tyrosine residue with 7-chloro-4-nitrobenzofurazan. Difference circular dichroism spectra revealed that the three Mg2+ -dependent, high affinity ADP binding sites that were not stable to gel filtration were on the alpha subunits and that the single ADP binding site that was stable to gel filtration was on one of the three beta subunits. It has also been demonstrated that enzyme-bound ATP is formed when the TF0.F1 complex containing bound ADP was incubated with Pi, Mg2+, and 50% dimethyl sulfoxide.

Highlights

(E),it is generally accepted that here are six adenine nucleotide binding sites inthe Fl-ATPases (16)
Enzyme-bound ATP synthesis was which are exchangeable (17,18).The former are noncatalytic abolished by chemical modification of a specific glu- sites, the function of which is uncertain, and the lattersites tamic acid residue by dicyclohexylcarbodiimide, but are catalytic
To test thepossibility that theother class of ADP binding sites which are dependent on M g + might form enzymebound ATP in thepresence of 50% dimethyl sulfoxide, these sites were filled at low ADP concentration (1.12 pM) in the presence of Mg2+ where the single, stable site remained unoccupied

Summary

EXPERIMENTAL PROCEDURES

The reaction mixture was incubated for 30 min a t 23 "C at which time 150pl of 0.5M dithiothreitol were added to quench residual reagent. Solid ammonium sulfate (15g) was dissolved in the solution and the resulting protein precipitate was collected by centrifugation and was dissolved in a minimal volume of water. After 50-min incubation a t 23 "C, 3 g of solid ammonium sulfate were added to precipitate the protein. Mg complex was prepared by incubating the TF1.ADP complex with 1 mM MgSO, in 50 mM triethanolamine sulfate, pH 7.3, for 10min a t 20 "C.The mixture was subjected to centrifugeelution on a 1-ml syringe column equilibrated with 50 mM triethanolamine sulfate, pH 7.3, containing 10 p~ CDTA. The molecular weight of TF, was taken tobe 380,000 (7)

RESULTS

ADPI mol TFI

After treatment

ATP ADP