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Paul F. Cook,** Maynard E. Neville, Jr., Kent E. Vrana, F. Thomas Hartl, and Robert Roskoski, Jr. abstract: The kinetic mechanism for adenosine cyclic 3', 5'-monophosphate dependent protein kinase was determined from initial velocity studies in the absence and presence of the product MgADP and dead-end inhibitors. Data are consistent with random addition of MgATP and Ser-peptide and ordered release of phospho-Ser-peptide and MgADP with a dead-end E-MgADP-Ser-peptide complex. In addition to the metal required for the nucleotide, we also characterized the binding of Mg2+ to a second site. Increasing the Mg2+ results in a 5-6-fold decrease in F" max in the presence or absence of 0.1 M KC1. There is a 5-fold increase in V/KMgATP in the absence of KC1 and a 13-fold increase in V/KMgATP at 0.1 Mkc1. The effect of increasing free Mg2+ on Fmax and V/K was also obtained withMgITP (20% the Kmax …