Adenosine cyclic 3',5'-monophosphate dependent protein kinase: interaction of the catalytic subunit and holoenzyme with lin-benzoadenine nucleotides.

The interaction of lin-benzoadenosine di- and triphosphates with the catalytic subunit and type II holoenzymes of adenosine cyclic 3',5'-monophosphate (cAMP) dependent protein kinase has been investigated by steady-state kinetics and fluorescence spectroscopy. lin-Benzo-ADP is a competitive inhibitor of the catalytic subunit with respect to ATP with a Ki (8.0 microM) similar to the Ki for ADP (9.0 microM). This value agrees well with the Kd (9.0 microM) determined by fluorescence polarization titration. Type II holoenzymes from bovine brain and skeletal muscle have Kd values for lin-benzo-ADP of 3.4 microM and 3.5 microM, respectively, and each binds approximately 2 mol/mol of R2C2 tetramer. Furthermore, fluorescence polarization studies indicate that both the catalytic subunit and type II holoenzyme bind lin-benzo-ADP rigidly, so that there is little or no rotation of the lin-benzoadenine portion of the molecule within the nucleotide binding site. lin-Benzo-ATP is a substrate for the phosphotransferase activities of protein kinase with peptides, water, or type II regulatory subunit as phosphoryl acceptors. With Leu-Arg-Arg-Ala-Ser-Leu-Gly as phosphoryl acceptor, the Km for lin-benzo-ATP is 11.3 microM, and that for ATP is 11.9 microM. The Vmax with lin-benzo-ATP is 20% of the Vmax with ATP as the substrate [24.9 +/- 1.8 mumol/(min . mg) vs. 5.0 +/- 1.2 mumol/(min . mg)]. Thus lin-benzo-ATP is the best nucleotide substrate (besides ATP) for the catalytic subunit reported. 1,N6-Etheno-ATP (epsilon ATP), on the other hand, is a poor substrate for the catalytic subunit with a Km of 1.8 mM and a Vmax that is 4% of the Vmax for ATP, making it unsuitable as a fluorescence probe for cAMP-dependent protein kinase.