The evolution of an allosteric site in phosphorylase

Background: Glycogen phosphorylases consist of a conserved catalytic core onto which different regulatory sites are added. By comparing the structures of isozymes, we hope to understand the structural principles of allosteric regulation in this family of enzymes. Here, we focus on the differences in the glucose 6-phosphate (Glc-6-P) binding sites of two isozymes. Results: We have refined the structure of Glc-6-P inhibited yeast phosphorylase b to 2.6 Angstrom and compared it with known structures of muscle phosphorylase. Glc-6-P binds in a novel way, interacting with a distinct set of secondary elements, Structural links connecting the Glc-6-P binding sites and catalytic sites are conserved, although the specific contacts are not. Conclusions: Our comparison reveals that the Glc-6-P binding site was modified over the course of evolution from yeast to vertebrates to become a bi-functional switch, The additional ability of muscle phosphorylase to be activated by AMP required the recruitment of structural elements into the binding site and sequence changes to create a binding subsite for adenine, whilst maintaining links to the catalytic site.