NUCLEOTIDE BINDING TO GLYCOGEN PHOSPHORYLASE-B IN THE CRYSTAL

The binding of the allosteric activator, AMP, and the inhibitor, ATP, to glycogen phosphorylase b has been studied in the crystal at 3 Å resolution. The nucleotides bind to two sites on the enzyme which are identified as site N, the allosteric effector site which is close to the subunit-subunit interface, and site I, a nucleoside inhibitor site which blocks the entrance to the active site crevasse. AMP when bound at the allosteric effector site makes several defined interactions with the enzyme in agreement with the results of solution studies. The contacts involve the N-10 position of the base, the 2′ hydroxyl of the ribose and the phosphate. IMP, analysed at 4 Å resolution, appears to bind in an identical conformation to AMP. At 3 Å resolution no well defined conformational changes are observed on binding AMP, although there are indications of a disturbance of the crystal lattice. It is concluded that the forces which stabilise the crystal lattice prevent the allosteric response of the enzyme in the crystal. © 1979.