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|Title:||Ligand Binding and Catalysis by Phosphoribosyl Pyrophosphate Synthetase|
|Author(s):||Gibson, Katharine Janet|
|Department / Program:||Biochemistry|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||The binding of the substrates, ATP and ribose-5-phosphate (R5P), and the inhibitor ADP to phosphoribosylpyrophosphate (PRPP) synthetase from Salmonella typhimurium has been studied by equilibrium dialysis of these compounds labeled with ('32)P. The simplest model that fits the data is the suggestion that the enzyme possesses three distinct binding sites for these compounds. The active site appears to contain separate sites for ATP and R5P. ADP can bind either to the active site in place of ATP, or to an ADP-specific allosteric site. Binding of ATP, (alpha),(beta)-methylene ATP (mATP), and ADP to the active site was weakly cooperative and reached half-saturation at 50 to 90 (mu)M. In the presence of R5P, mATP or ADP binding at the active site became tighter (K(,d) = 3 to 6 (mu)M at 10 mM R5P) and lost its cooperativity. ADP binding at the allosteric site site was apparent only in the presence of R5P; half-saturation was reached at 150 to 300 (mu)M ADP, and Hill coefficients ranged from 3 to 4. Binding of R5P was not detectable in the absence of nucleotides, but a K(,d) of 50 (mu)M was observed in the presence of 2 mM mATP or ADP. These data account very satisfactorily for the properties of both mATP and ADP inhibition of the enzyme. A paper (Gibson, K. J., Schubert, K. R., and Switzer, R. L.) taken from these studies has been accepted for publication in the Journal of Biological Chemistry early in 1982.
A separate set of experiments (Gibson, K. J., and Switzer, R. L. (1980) J. Biol. Chem. 255 694-696) was conducted with the isomers of adenosine 5'-thiotriphosphate (ATP (alpha)S) and adenosine 5'-thiomonophosphate (AMPS). In the presence of Mg('2+), PRPP synthetase was absolutely specific for ATP(alpha) S(A); Cd('2+) allowed both isomers to react at 1/5 rate for ATP. With Mg('2+), AMPS was a weak inhibitor of reaction with ATP, but inhibition became 34 times stronger when Cd('2+) replaced Mg('2+). These studies led to the suggestion that a divalent cation bridge forms between the enzyme and the (alpha)-phosphate of nucleotides during catalysis by PRPP synthetase. Attempts to confirm this suggestion by nuclear magnetic resonance (Granot et. al., (1980) J. Biol. Chem. 255 10931-10937) were unsuccessful.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1982.
|Date Available in IDEALS:||2014-12-15|