Phosphorylase kinase (ATP:phosphorylase phosphotransferase, EC 126.96.36.199), the enzyme which regulates glycogen breakdown by converting α-glucan phosphorylase (α-1,4-glucan:orthophosphate glucosyltransferase, EC 188.8.131.52) into its active form, requires ATP and divalent metal ions for its activity. We have shown that: 1. 1.MgATP2-2 chelate is the substrate for the reaction with an apparent Km of 0.07 mM ± 0.02 (S.D.). 2. 2.Free Mg2+ in mM concentration is also required for activity. The apparent Ka for free Mg2+ is 0.6 mM ± 0.2 (S.D.). 3. 3.Free ATP4- 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 activity1,2. With both metals the inhibition is not competitive with respect to MgATP2- 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.
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