Steady-state kinetic mechanism of PDK1

PDK1 catalyzes phosphorylation of Thr in the conserved activation loop region of a number of its downstream AGC kinase family members. In addition to the consensus sequence at the site of phosphorylation, a number of PDK1 substrates contain a PIF sequence ( PDK1-interacting fragment), which binds and activates the kinase domain of PDK1 (PDK1(Delta PH)). To gain further insight to PIF-dependent catalysis, steady-state kinetic and inhibition studies were performed for His6-PDK1(Delta PH)-catalyzed phosphorylation of PDK1-Tide ( Tide), which contains an extended "PIF" sequence C-terminal to the consensus sequence for PDK1 phosphorylation. In two-substrate kinetics, a large degree of negative binding synergism was observed to occur on formation of the active ternary complex (alpha K-d(ATP) = 40 mu M and alpha K-d(Tide) = 80 mu M) from individual transitory binary complexes (K-d(ATP) = 0.6 mu M and K-d(Tide) = 1 mu M). On varying ATP concentrations, the ADP product and the (T/E)-PDK1-Tide product analog (p' Tide) behaved as competitive and noncompetitive inhibitors, respectively; on varying Tide concentrations, ADP and p' Tide behaved as noncompetitive and competitive inhibitors, respectively. Also, negative binding synergism was associated with formation of dead-end inhibited ternary complexes. Time progress curves in pre-steady-state studies under "saturating" or k(cat) conditions showed (i) no burst or lag phenomena, ( ii) no change in reaction velocity when adenosine 5'-O(thiotriphosphate) was used as a phosphate donor, and (iii) no change in reaction velocity on increasing relative microviscosity ( 0 <= eta/eta(0) 0 <= 3). Taken together, PDK1-catalyzed trans-phosphorylation of PDK1-Tide approximates a Rapid Equilibrium Random Bi Bi system, where motions in the central ternary complex are largely rate-determining.