SUBSTRATE-SPECIFICITY OF PROTEIN-KINASE-C STUDIED WITH PEPTIDES CONTAINING D-AMINO-ACID RESIDUES

A set of stereoisomeric nonapeptides KRPSQRAKY with one, two, or all L-amino acid residues replaced by the corresponding D-amino acids, and two analogs with L- and D-threonine instead of serine, were synthesized and tested as substrates for protein kinase C. All of the peptides were phosphorylated by the enzyme. The maximal rate of the reaction with the all-D peptide was more than one order of magnitude lower than that for the all-L peptide with serine. The same applied to the peptides with D-Ser or with D-Arg in position + 2 with respect to Ser. The K(m) values for the peptides containing one D-amino acid were close to that for the prototype peptide (53 muM). On the other hand, when two or more D-amino acids were present, the K(m) value increased considerably. Replacement of serine by threonine also reduced the phosphorylation rate and increased the K(m) values. One can conclude that the stereospecificity of protein kinase C is much less pronounced than that of protein kinase A, which is in agreement with the less clearly pronounced substrate specificity of the former enzyme.