ATP sulfurylase from Penicillium chrysogenum. Molecular basis of the sigmoidal velocity curves induced by sulfhydryl group modification.

ATP sulfurylase from Penicillium chrysogenum is a noncooperative homooligomer containing three free sulfhydryl groups per subunit. Under nondenaturing conditions, one SH group per subunit was modified by 5,5'-dithiobis-(2-nitrobenzoate), or N-ethylmaleimide. Modification had only a small effect on kcat, but markedly increased the [S]0.5 values for the substrates, MgATP and SO4(2-). MgATP and adenosine-5'-phosphosulfate protected against modification. The SH-modified enzyme displayed sigmoidal velocity curves for both substrates with Hill coefficients (nH) of 2. Fluorosulfonate (FSO3-) and other dead-end inhibitors competitive with SO4(2-) activated the SH-modified enzyme at low SO4(2-) concentration. In order to determine whether the sigmoidicity resulted from true cooperative binding (as opposed to a kinetically based mechanism), the shapes of the binding curves were established from the degree of protection provided by a ligand against phenylglyoxal-dependent irreversible inactivation under noncatalytic conditions. Under standard conditions (0.05 M Na-N-(2-hydroxyethyl)piperazine-N'-3-propanesulfonic acid buffer, pH 8, 30 degrees C, and 3mM phenylglyoxal) the native enzyme was inactivated with a k of 2.67 +/- 0.25 X 10-3 s-1, whereas k for the SH-modified enzyme was 5.44 +/- 0.27 X 10-3 s-1. The increased sensitivity of the modified enzyme resulted from increased reactivity of ligand-protectable groups. Both the native and the SH-modified enzyme displayed hyperbolic plots of delta k (i.e. protection) versus [MgATP], or [FSO3-], or [S2O3(2-]) in the absence of coligand (nH = 0.98 +/- 0.06). The plots of delta k versus [ligand] for the native enzyme were also hyperbolic in the presence of a fixed concentration of coligand. However, in the presence of a fixed [FSO3-] or [S2O3(2-]), the delta k versus [MgATP] plot for the SH-modified enzyme was sigmoidal, as was the plot of delta k versus [FSO3-] or [S2O3(2-]) in the presence of a fixed [MgATP]. The nH values were 1.92 +/- 0.09. The results indicate that substrates (or analogs) bind hyperbolically to unoccupied SH-modified subunits, but in a subunit-cooperative fashion to form a ternary complex.