Kinetic analysis of the separate phosphorylation events in the phosphorylase kinase reaction

Glycogen phosphorylase, a dimer of identical subunits, is activated by phosphorylase kinase-catalyzed phosphorylation of one serine residue in each subunit. In this paper, the effect of the phosphorylation of one subunit on the phosphorylation of the other subunit was examined. The three forms of phosphorylase, phosphorylase b (nonphosphorylated), phosphorylase ab (one subunit phosphorylated), and phosphorylase a (both subunits phosphorylated), were separated by anion-exchange high-performance liquid chromatography (HPLC). Purified phosphorylase ab was found to be stable under the conditions of the phosphorylase kinase assay. Initial rate kinetics showed that phosphorylase kinase had a lower KM for phosphorylase ab (3.9 +/- 0.24 microM) than for phosphorylase b (14.9 +/- 2.6 microM). Using the HPLC separation as a simultaneous assay for the three forms of phosphorylase during the phosphorylase kinase reaction, it was found that the pseudo-first-order rate constant for the second phosphorylation step (k2) was 3.7 times greater than that for the first step (k1). The activator AMP reduced the ratio k2/k1 from 3.7 without AMP to 1.4. When the monomeric gamma delta complex of phosphorylase kinase subunits was used as the enzyme, the ratio k2/k1 was 2.1, compared to 3.7 with the multimeric holophosphorylase kinase. One explanation for these data is that phosphorylation of one subunit of phosphorylase b causes conformational changes that make the other subunit a better substrate for the kinase. In this context, the effect of AMP is to reduce the conformational differences between phosphorylases b and ab, and the gamma delta complex is less sensitive to the conformational differences between the two forms of phosphorylase.