The minimum substrate of cyclic AMP-stimulated protein kinase, as studied by synthetic peptides representing the phosphorylatable site of pyruvate kinase (type L) of rat liver.

Synthetic peptides, representing part of the phosphorylatable site of rat liver pyruvate kinase, were phosphorylated by (32P)ATP and the catalytic subunit of cyclic AMP-stimulated protein kinase. The shortest peptide which could be significantly phosphorylated was Arg-Arg-Ala-Ser-Val, with an apparent Km of 0.08 mM. The apparent Km for Arg-Arg-Ala-Ser-Val-Ala was 0.02 mM and that for Leu-Arg-Arg-Ala-Ser-Val-Ala was less than 0.01 mM. Peptides in which threonine was substituted for serine, or leucine for the one or the other arginine of the pentapeptide were not detectably phosphorylated. Substitution of phenylalanine for valine increased, and substitution of lysine or glycine for valine considerably decreased the rate of phosphorylation.

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