The phosphorylation of a peptide substrate by the catalytic subunit of cAMP-dependent protein kinase was monitored over short time periods (2-1000 ms) using a rapid quench flow mixing device and a radioactive assay. The production of phosphokemptide [LRRAS(P)LG] as a function of time is characterized by a rapid "burst" phase (250 s-1) followed by a slower, linear phase (L/[E]t = 21 s-1) at 100 microM Kemptide. The amplitude of this "burst" phase varies linearly with the enzyme concentration and represents approximately 100% of the total enzyme concentration, indicating that the "burst" phase is not due to product inhibition. The observed rate constants for the "burst" and linear phases and the "burst" amplitude vary hyperbolically with the substrate concentration. From these dependencies, a maximum "burst" rate constant of 500 +/- 60 s-1 and a Km and Kd for Kemptide of 4.9 +/- 1.4 and 200 +/- 60 microM were determined. The kcat and Km data extracted from the linear portion of the rapid quench flow transients are indistinguishable from those obtained by standard steady-state kinetic analyses using low catalytic subunit concentrations and a spectrophotometric, coupled enzyme assay. Both rate constants for the "burst" and linear phases decreased in the presence of Mn2+. The data imply that the phosphorylation of Kemptide by the catalytic subunit occurs by a mechanism in which the substrate is loosely bound, is rapidly phosphorylated at the active site, and is released at a steady-state rate that is likely controlled by the dissociation rate constant for ADP. The combined pre-steady-state kinetic data establish a comprehensive, kinetic mechanism that predicts all the steady-state kinetic and viscosometric data. This study represents the first chemical observation and characterization of phosphoryl transfer at the active site of a protein kinase and will be useful for further structure-function studies on this and other protein kinases.

When the catalytic (rC) subunit of cAMP-dependent protein kinase (cAPK) is expressed in Escherichia coli, it is autophosphorylated at four sites, Ser10, Ser139, Ser338 and Thr197 (49). Three of these sites, Ser10, Ser338 and Thr197, are also found in the mammalian enzyme. To understand the functional importance of these phosphorylation sites, each was replaced with Ala, Glu or Asp. The expression, solubility and phosphorylation state of each mutant protein was characterized by immunoprecipitation following in vivo labeling with 32Pi. When possible, isoforms were resolved and kinetic properties were measured. The two stable phosphorylation sites in the mammalian enzyme, Ser338 and Thr197, were shown to play different roles. Ser338, which stabilizes a turn near the C-terminus, is important for stability. Both rC(S338A) and rC(S338E) were very labile; however, the kinetic properties of rC(S338E) were similar to the wild-type catalytic subunit (C-subunit). Ser338 most likely helps to anchor the C-terminus to the surface of the small lobe. Thr197 is in the activation loop near the cleft interface. Mutagenesis of T197 caused a significant loss of catalytic activity with increases in Kms for both peptide and MgATP, as well as a small decrease in k(cat) indicating that this phosphate is important for the correct orientation of catalytic residues at the active site. Replacement of Ser139, positioned at the beginning of the E-helix, with Ala had no effect on the kinetic parameters, stability or phosphorylation at the remaining sites. In contrast, mutation of Ser10, located at the beginning of the A-helix, produced mostly insoluble, inactive, unphosphorylated protein, suggesting that this region, though far removed from the active site, is structurally important at least for the expression of soluble phosphoprotein in E.coli. Since the mutation of active site residues as well as deletion mutants generate underphosphorylated proteins, these phosphorylations in E.coli all result from autophosphorylation.

The conserved glycines in the glycine-rich loop (Leu-Gly 50 -Thr-Gly 52 -Ser-Phe-Gly 55 -ArgVal) of the catalytic (C) subunit of cAMP-dependent protein kinase were each mutated to Ser (G50S, G52S, and G55S). The effects of these mutations were assessed here using both steady-state and presteady-state kinetic methods. While G50S and G52S reduced the apparent affinity for ATP by approximately 10-fold, substitution at Gly55 had no effect on nucleotide binding. In contrast to ATP, only mutation at position 50 interfered with ADP binding. These three mutations lowered the rate of phosphoryl transfer by 7-300-fold. The combined data indicate that G50 and G52 are the most critical residues in the loop for catalysis, with replacement at position 52 being the most extreme owing to a larger decrease in the rate of phosphoryl transfer (29 vs 1.6 s -1 in contrast to 500 s -1 for wild-type C). Surprisingly, all three mutations lowered the affinity for Kemptide by approximately 10-fold, although none of the loop glycines makes direct contact with the substrate. The inability to correlate the rate constant for net product release with the dissociation constant for ADP implies that other steps may limit the decomposition of the ternary product complex. The observations that G52S (a) selectively affects ATP binding and (b) significantly lowers the rate of phosphoryl transfer without making direct contact with either the nucleotide or the peptide imply that this residue serves a structural role in the loop, most likely by positioning the backbone amide of Ser53 for contacting the γ-phosphate of ATP. Energyminimized models of the mutant proteins are consistent with the observed kinetic consequences of each mutation. The models predict that only mutation of Gly52 will interfere with the observed hydrogen bonding between the backbone and ATP.Many nucleotide binding enzymes utilize glycine-rich loops that help position nucleotides for catalysis (1-3). Crystal structures of dinucleotide and mononucleotide binding enzymes define several glycine-rich loops that are distinct both in their amino acid sequence and in their orientation relative to the nucleotide. The glycine-rich loops found in dinucleotide binding enzymes [e.g., pyruvate dehydrogenase (4) and dihydrofolate reductase (5)] are typically Gly-X-Gly-X-X-Gly and provide space for the dinucleotide pyrophosphate moiety. The glycine-rich loops, or P-loops, found in mononucleotide binding enzymes [e.g., p21ras (6), adenylate kinase (7), Rec A (8), and elongation factor Tu (9)] have the consensus sequence Gly-X-X-X-X-Gly-Lys-Ser/Thr and interact with the γ-phosphate of ATP (10). In contrast, the protein kinase family of enzymes contain a glycine-rich loop with a consensus sequence Gly-X-Gly-X-X-Gly-X-Val. On the basis of numerous crystal structures, this loop is part of a -hairpin that connects -strands 1 and 2 and serves as a nucleotide-positioning loop (NPL) 1 that interacts with all three phosphates of ATP (11)(12)(13)(14)(15). The glycines in the protein kinase NPL are ...