The role of magnesium in muscle phosphorylase kinase activity

Phosphorylase kinase (ATP:phosphorylase phosphotransferase, EC 2.7.1.38), the enzyme which regulates glycogen breakdown by converting α-glucan phosphorylase (α-1,4-glucan:orthophosphate glucosyltransferase, EC 2.4.1.1) into its active form, requires ATP and divalent metal ions for its activity. We have shown that: 1. 1.MgATP^2-2 chelate is the substrate for the reaction with an apparent K_m of 0.07 mM ± 0.02 (S.D.). 2. 2.Free Mg²⁺ in mM concentration is also required for activity. The apparent K_a for free Mg²⁺ is 0.6 mM ± 0.2 (S.D.). 3. 3.Free ATP^4- is not inhibitory. 4. 4.Mn and Ca at mM concentrations are inhibitors of the enzyme even though μM concentrations of Ca are required for activity^1,2. With both metals the inhibition is not competitive with respect to MgATP^2- and Mg²⁺. Mn as well as Al, Ba and Co, in the absence of Mg, will allow the phosphorylase kinase reaction to proceed but only at approx. 5% of the rate observed in the presence of Mg.