KINETIC STUDIES ON THE REACTION CATALYZED BY PHOSPHOGLYCERATE KINASE. I. THE EFFECT OF MG2+ AND ADENOSINE 5'-TRIPHOSPHATE

Antagonistic binding of substrates to 3-phosphoglycerate kinase monitored by the fluorescent analogue 2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate

The analogue of ATP, 2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate (TNP-ATP), binds tightly to pig muscle 3-phosphoglycerate kinase. A dissociation constant Kd of 0.0095 +/- 0.0015 mM was determined by fluorimetric titration on the basis of 1:1 stoichiometry. TNP-ATP is a strong competitive inhibitor towards MgATP and MgADP with a Ki of 0.008 +/- 0.001 mM for both substrates. It is also a mixed-type inhibitor towards 3-phosphoglycerate with similar inhibition constants. Binding of TNP-ATP to 3-phosphoglycerate kinase is accompanied by a tenfold intensity increase and a blue shift of about 20 nm in its fluorescence emission spectrum and a shift of the pK of its trinitrophenyl group towards a more acidic pH. These findings suggest that the negatively charged trinitrophenyl group of TNP-ATP significantly contributes to the binding of the analogue. By stepwise replacement of the fluorescent TNP-ATP, the dissociation constants (Kd) for ADP and MgADP binding were determined and found to be 0.78 +/- 0.08 and 0.048 +/- 0.006 mM respectively, which are consistent with the values previously determined by equilibrium dialysis [Molnár and Vas (1993) Biochem J. 293, 595-599]. In similar competitive-titration experiments, ATP and MgATP did not completely substitute for TNP-ATP. For the fraction of the analogue that could be substituted, the dissociation constants for MgATP and ATP were estimated to be 0.27 +/- 0.09 and 0.33 +/- 0.15 mM respectively, close to the values determined by equilibrium dialysis. Using the same method, a significant weakening of binding of both (Mg)ADP and (Mg)ATP could be detected in the presence of 3-phosphoglycerate: their respective Kd values became 0.34 +/- 0.04 and 0.51 +/- 0.22 mM. The reciprocal effect, i.e. weakening of 3-phosphoglycerate binding in the presence of the nucleotide substrates, has been observed previously [Vas and Batke (1984) Eur. J. Biochem. 139, 115-123]. Similarly, a much weaker binding of (Mg)ATP could be observed in the presence of 1,3-bisphosphoglycerate (Kd = 2.30 +/- 0.68 mM). The possible reason for the mutual weakening of substrate binding is discussed in the light of the available structural data.

Mg2+ affects the binding of ADP but not ATP to 3-phosphoglycerate kinase. Correlation between equilibrium dialysis binding and enzyme kinetic data

The role of Mg2+ in the binding of ADP and ATP to pig muscle and yeast 3-phosphoglycerate kinases has been studied by equilibrium dialysis. Whereas the Kd of ATP binding varies between 0.17 and 0.23 mM (S.E.M. 0.03 mM) for both enzymes, independently of the presence of Mg2+, the Kd values for ADP and MgADP binding are in the range 0.18-0.27 mM (S.E.M. 0.04 mM) and 0.05-0.06 mM (S.E.M. 0.01 mM) respectively. Thus Mg2+ exclusively tightens the interaction of ADP, but not of ATP, with the protein molecule. Although the equilibrium dialysis data are consistent with a model possessing a single site for nucleotides, the existence of a much weaker secondary site (with a Kd value at least two orders of magnitude larger) cannot be excluded. The binding of AMP and adenosine to pig muscle 3-phosphoglycerate kinase is weaker than binding of MgATP; the respective Kd values are 0.36 +/- 0.05 mM and 0.65 +/- 0.05 mM. Thus, in addition to the interaction of the alpha-phosphate that is detectable by crystallography [Banks, Blake, Evans, Haser, Rice, Hardy, Merrett and Phillips (1979) Nature (London) 279, 773-777], the beta- and/or gamma-phosphate(s) of MgATP may also interact with the enzyme molecule. The fact that MgADP binds more tightly than ADP is consistent with its stronger inhibition of the reaction catalysed by the enzyme between 3-phosphoglycerate and MgATP. MgADP is a product of this reaction, and inhibits it competitively with both substrates; as an inhibitor its KI is comparable with the Kd found in binding studies. At the same time, the Km value for MgADP in the reverse reaction (0.18 +/- 0.05 mM; mean +/- S.E.M.) is higher than these constants; this may be due either to a different kinetic mechanism in this direction of the enzymic reaction, or to different binding modes of MgADP as inhibitor and as substrate. The reason why inhibition by MgADP is competitive with 3-phosphoglycerate may be that its binding prevents the specific change in conformation that the enzyme undergoes [Harlos, Vas and Blake (1992) Proteins 12, 133-144] when it binds 3-phosphoglycerate.