Catalytic and regulatory site composition of yeast AMP deaminase by comparative binding and rate studies. Resolution of the cooperative mechanism

Yeast AMP deaminase is allosterically activated by ATP and MgATP and inhibited by GTP and PO₄. The tetrameric enzyme binds 2 mol each of ATP, GTP, and PO₄/subunit with K(d) values of 8.4 ± 4.0, 4.1 ± 0.6, and 169 ± 12 μM, respectively. At 0.7 M KCl, ATP binds to the enzyme, but no longer activates. Titration with coformycin 5'-monophosphate, a slow, tight-binding inhibitor, indicates a single catalytic site/subunit. ATP and GTP bind at regulatory sites distinct from the catalytic site and their binding is mutually exclusive. Inorganic phosphate competes poorly with ATP for the ATP sites (K(d) = 20.1 ± 4.1 mM). However, near-saturating ATP reduces the moles of phosphate bound per subunit to 1 PO₄, which binds with a K(d) = 275 ± 22 μM. In the presence of ATP, PO₄ cannot effectively compete with ATP for the nucleotide triphosphate sites. The PO₄ which binds in the presence of ATP is competitive with AMP at the catalytic site since the K(d) equals the kinetic inhibition constant for PO₄. Initial reaction rate curves are a cooperative function of AMP concentration and activation by ATP is also cooperative. However, no cooperativity is observed in the binding of any of the regulator ligands and ATP binding and kinetic activation by ATP is independent of substrate analog concentration. Cooperativity in initial rate curves results, therefore, from altered rate constants for product formation from each (enzyme · substrate)(n) species and not from cooperative substrate binding. The traditional cooperative binding models of allosteric regulation do not apply to yeast AMP deaminase, which regulates catalytic activity by kinetic control of product formation. The data are used to estimate the rates of AMP hydrolysis under reported metabolite concentrations in yeast.