THE EVOLUTION OF AN ALLOSTERIC SITE IN PHOSPHORYLASE

Background: Glycogen phosphorylases consist of a conserved catalytic c ore onto which different regulatory sites are added. By comparing the structures of isozymes, we hope to understand the structural principle s of allosteric regulation in this family of enzymes. Here, we focus o n the differences in the glucose 6-phosphate (Glc-6-P) binding sites o f two isozymes. Results: We have refined the structure of Glc-6-P inhi bited yeast phosphorylase b to 2.6 Angstrom and compared it with known structures of muscle phosphorylase. Glc-6-P binds in a novel way, int eracting with a distinct set of secondary elements, Structural links c onnecting the Glc-6-P binding sites and catalytic sites are conserved, although the specific contacts are not. Conclusions: Our comparison r eveals that the Glc-6-P binding site was modified over the course of e volution from yeast to vertebrates to become a bi-functional switch, T he additional ability of muscle phosphorylase to be activated by AMP r equired the recruitment of structural elements into the binding site a nd sequence changes to create a binding subsite for adenine, whilst ma intaining links to the catalytic site.