Glycogen phosphorylase b - the molecular anatomy of a large regulatory enzyme

The crystal structure of glycogen phosphorylase b, the key control enzyme of glycogen degradation, has been solved at 2.25 A resolution. Refinement is in progress. This paper describes the molecular anatomy of this large protein. It is shown that the phosphorylase has a greater solvent-accessible surface area than a data base of small proteins and that, surprisingly, some polar residues (Thr, Ser, Lys, Gln) are buried more than predicted from the data base. These results are discussed with reference to the overall stabilisation of the large protein and their possible significance for allostery. The large molecule can be divided into two domains and each of these further divided into subdomains. The structure of the domains is discussed with reference to the structure of smaller proteins and in terms of domain-domain interactions through secondary structural elements. The secondary structure composition is α-helix 47%; β-sheet 18%; coil 35%. The topological relationships and possible connecting pathways between the catalytic site, the allosteric effector site, the glycogen storage site and the nucleoside inhibitor site are described. A brief comparison of the mammalian, plant and bacterial phosphorylases shows how the lack of regulatory properties in the non-mammalian phosphorylases can be explained in terms of the available sequence data and the known structure of phosphorylase b. © 1985.