Quantitative Structure–Activity Relationships in the Protein Kinase C Reaction with Synthetic Peptides Derived from Myelin Basic Protein

A set of peptides, Lys-Arg-Pro-Ser-X-Arg-Ala-Lys-Ala, where X stands for Ala, Val, Leu, Ile, Phe, Pro, Lys, Arg, Asp, Glu, Asn, Gln, and His, was synthesized and the kinetics of their phosphorylation by protein kinase C was studied. All compounds, except the peptide with Pro at the position X, were effectively phosphorylated by this enzyme, and for these substrates the kinetic constantsKm, maximal velocity constantsV, and second-order rate constantskIIwere determined. The data were analyzed by means of quantitative structure–activity relationships, taking into account hydrophobicity of the variable amino acids, bulkiness of their side groups quantified by molecular refractivity constants MR, and ionic status of these substituents by using an independent variable +1 for cationic, −1 for anionic, and 0 for nonionic substituents. These structural factors influenced theKmvalues, while the maximal velocity of phosphorylation depended mostly on the ionic status of the variable amino acid. The latter effect seems to characterize electrostatic interaction between the substrate molecule and some negative charge located in the enzyme active center.