This is a family of oligosaccharide phosphorylases. It includes yeast and mammalian glycogen phosphorylases, plant starch/glucan phosphorylase, as well as the maltodextrin phosphorylases of bacteria. The members of this family catalyze the breakdown of oligosaccharides into glucose-1-phosphate units. They are important allosteric enzymes in carbohydrate metabolism. The allosteric control mechanisms of yeast and mammalian members of this family are different from that of bacterial members. The members of this family belong to the GT-B structural superfamily of glycoslytransferases, which have characteristic N- and C-terminal domains each containing a typical Rossmann fold. The two domains have high structural homology despite minimal sequence homology. The large cleft that separates the two domains includes the catalytic center and permits a high degree of flexibility.