Partial Activation of Muscle Phosphorylase by Replacement of Serine 14 with Acidic Residues at the Site of Regulatory Phosphorylation

: Phosphorylation of glycogen phosphorylase at residue Ser14 triggers a conformational transition that activates the enzyme. The N-terminus of the protein, in response to phosphorylation, folds into a 310 helix and moves from its location near a cluster of acidic residues on the protein surface to a site at the dimer interface where a pair of arginine residues form charged hydrogen bonds with the phosphoserine. Site-directed mutagenesis was used to replace Ser14 with Asp and Glu residues, analogs of the phosphoserine, that might be expected to participate in ionic interactions with the arginine side chains at the dimer interface. Kinetic analysis of the mutants indicates that substitution of an acidic residue in place of Ser14 at the site of regulatory phosphorylation partially activates the enzyme. The S14D mutant shows a 1.6-fold increase in Vmax, a 10-fold decrease in the apparent dissociation constant for AMP, and a 3-fold decrease in the S0.5 for glucose 1-phosphate. The S14E mutant behaves similarly, showing a 2.2-fold increase in Vmax, a 6-fold decrease in the apparent dissociation constant for AMP, and a 2-fold decrease in the S0.5 for glucose 1-phosphate. The ability of the mutations to enhance binding of AMP and glucose 1-phosphate and to raise catalytic activity suggests that the introduction of a carboxylate side chain at position 14 promotes docking of the N-terminus at the subunit interface and concomitant stabilization of the activated conformation of the enzyme. Like the native enzyme, both mutants show significant activity only in the presence of the activator, AMP. Full activation, analogous to that provided by covalent phosphorylation of the enzyme, likely is not achieved because of differences in the charge and the geometry of ionic interactions at the phosphorylation site.