Properties of the gamma subunit of phosphorylase kinase

Enzymatic properties of the isolated, active gamma subunit of phosphorylase kinase were characterized. Kinetic parameters indicated that the gamma subunit binds the substrates MgATP and phosphorylase b as well as the holoenzyme with a Km (MgATP) of 98 microM and a Km (phosphorylase b) of 80 microM at pH 8.2, but maximal velocities are significantly lower than the holoenzyme's. Unlike the gamma-calmodulin complex, the gamma subunit activity is dependent on pH in the range of pH 6.2-9.0, with a ratio of activity at pH 6.8 to activity at pH 8.2 of 0.5-0.6. Calmodulin activates the gamma subunit more at low pH than at high pH. ADP inhibits the gamma subunit in a competitive manner with a Ki of 60 microM. Free Mg2+ stimulates gamma subunit activity 3.5-fold at both pH 6.8 and 8.2. MnATP is equivalent to MgATP as a substrate for the enzyme, but free Mn2+ inhibits gamma subunit activity. Several protein substrates of holophosphorylase kinase were found also to be phosphorylated by the gamma subunit. These included kappa-casein, myelin basic protein, the troponin complex, and troponin T alone. In the troponin complex, the proportion of 32P incorporated by the gamma subunit into troponin I compared with troponin T was not Ca2+ dependent, but with the holoenzyme, this proportion was changed greatly by Ca2+ concentration.