Catalytic subunit of cAMP-dependent protein kinase: electrostatic features and peptide recognition.

The electrostatic field was calculated for the mammalian cAMP-dependent protein kinase (PKA) catalytic subunit (C-subunit) complexed with a 20-residue peptide from a heat stable protein kinase inhibitor (PKI: 5-24). The electrostatic field was also calculated for the C-subunit complexed with a modeled heptapeptide substrate that has been used extensively in structure/function studies for the C-subunit. Perturbations in the electrostatic free energy were calculated when single ionizable active site residues were mutated to alanine. These perturbations in electrostatic free energy were correlated to changes in the binding energy measured in a charge-to-alanine scan of the homologous yeast C-subunit by M. J. Zoller and C. S. Gibbs [(1991) Journal of Biological Chemistry, Vol. 266, pp. 8923-8931; C. S. Gibbs and M. J. Zoller (1991) Biochemistry, Vol. 30, p. 22]. This analysis indicated that the substrate binding parameters primarily depend on electrostatic interactions between a substrate or inhibitor and the C-subunit. Amino acid replacements that led to large perturbations in the electrostatic field are listed in the text. pKa shifts were also calculated for the substrate's phosphate accepting atom, the serine hydroxyl oxygen, when the active site ionizable residues were changed to structurally similar uncharged amino acids. The theoretical mutation of three active site residues caused large shifts in this parameter: E91Q, D166N, and D184N. The calculated pKa shifts for these mutants indicate that the rate of phosphotransfer should be markedly reduced in these cases. This prediction has been experimentally confirmed for the D166N mutant. The correlation between calculated electrostatic free energy changes and measured binding energy, and pKa shifts with phosphotransfer for C-subunit mutants were within experimental error of the measurements. The calculations of electrostatic energy and delta pKa have identified previously unconsidered active site residues in the mammalian C-subunit that contribute to binding energy and phosphotransfer.