Structure of glycogen phosphorylase a at 3.0 A resolution and its ligand binding sites at 6 A.

A model of the polypeptide backbone of the dimer of glycogen phosphorylase a (EC 2.4.1.1) was built from a 3 A resolution electron density map derived from x-ray diffraction analysis of native tetragonal crystals and two heavy atom isomorphous replacement derivatives. Each identical subunit of the dimer has a compact shape with overall dimensions of 85 X 75 X 55 A and is tightly associated with its 2-fold symmetry related subunit. There are three major excursions of the polypeptide chain of one monomer across the 2-fold axis to make extensive contacts with the other subunit. The active site, of which there are two per dimer, is shared between the two subunits at their interface and comprises a pocket-like region within a "V"-shaped framework of two alpha helices. Within this region are found the binding sites for the substrates, glucose-1-P and arsenate, a competitive inhibitor, UDP-glucose, and the allosteric effector, AMP. The site of metabolic control, Ser-14 phosphate, is hydrogen-bonded to a side chain on the outside of one of the alpha helices forming the active site and is 15 A from the AMP binding site. Maltoheptaose, a glycogen analogue and substrate for these enzymatically active crystals, binds in a second region of interest. Even at concentrations above its Km, when binding is sufficiently tight that all seven glucose moieties may be discerned, the closest of these is 25 A from the glucose-1-P binding site. We suggest that this polysaccharide binding site may represent a storage site whereby phosphorylase is bound to the glycogen particle in the muscle cell. The polypeptide chain in a third region has the same topological structure as has been observed for the nucleotide binding domains in the dehydrogenases. Adenine or adenosine (but not AMP) bind here in a position similar to the adenine ring of NAD in the dehydrogenases while glucose binds 17 A away in an interior crevice near the center of the monomer.