Sulphate activates phosphorylase b by binding to the Ser (P) site

The notion, in a recent crystallographic study on the R state phosphorylase b (Barford, D. and Johnson, L.N. (1989) Nature 340, 609-616), that sulphate ions activate the enzyme by interacting within 2 A of the phosphorylatable Ser-14, is directly supported by kinetic studies on phosphorylase b', a proteolytic species which lacks the N-terminal 16 residues. The results show that this form of the enzyme is no longer activated by ammonium sulphate.