The interaction of calmodulin with regulatory peptides of phosphorylase kinase

The regulatory peptides Phk13 (301-327) and Phk5 (342-367) have been synthesized and their interaction with calmodulin studied. In the case of Phk13 modified forms were also synthesized in which a tryptophan group was placed at position 4 or 21, as well as a form with tryptophan at position 4 and nitrotyrosine at position 21. From tryptic digestion, circular dichroism, and radiationless energy transfer measurements, it appears that Phk13 forms an elongated complex with calmodulin in which the peptide is in a non-helical conformation, probably bent into a hairpin-shaped structure, the connecting strand of calmodulin is extended and exposed to the action of proteolytic enzymes, and the peptide makes contact with both the N- and C-terminal half-molecules of calmodulin. In contrast, the Phk5 peptide has an alpha-helical conformation in the complex, which is relatively compact in shape.