Detection of conformational changes along the kinetic pathway of protein kinase A using a catalytic trapping technique.

The dissociation rate constants for the two products of the reaction catalyzed by protein kinase A, ADP and phosphopeptide, were measured using a catalytic trapping technique to determine the role of product release in enzyme turnover. The enzyme was preequilibrated with ADP, and the reaction was initiated with a peptide substrate, LRRASLG, and ATP in a rapid quench flow instrument. At high, free magnesium concentrations (>2 mM), the large 'burst' in phosphopeptide production disappears, and, at low concentrations of free magnesium (0.5-1 mM), the kinetic transients become sigmoidal prior to the linear turnover phase. Increasing the concentrations of ATP or ADP did not influence the shape of the kinetic transients in the first 20 ms. ADP preequilibration protects the enzyme from inhibition by the covalent inactivator p-fluorosulfonylbenzoyl 5'-adenosine at 0.5 mM free magnesium, indicating that a competent E. ADP complex forms at low metal concentrations and the sigmoidal behavior in the catalytic trapping experiment is not due to free enzyme at high ATP concentrations. Simulations of the data indicate that ADP release is rate-limiting for turnover at high magnesium concentrations, but, at lower physiological levels of 0.5 and 1 mM, the off rate of ADP is 3- and 2-fold higher than k(cat), respectively. In contrast, the initial portions of the kinetic transients at 0.5 mM free magnesium were unaffected by phosphopeptide preequilibration, indicating that the release rate of this product is significantly larger than turnover. The transient kinetic data, coupled with a previous report [Shaffer and Adams (1999) Biochemistry 38, 5572-5581], support a phosphorylation mechanism under physiological magnesium concentrations that incorporates two partially rate-determining conformational changes, one prior to and one after the phosphoryl transfer step. We propose that the initial step activates the enzyme through key positioning of one or more active-site residues and the second step relaxes this conformation, a prerequisite for a subsequent catalytic cycle.