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.