title: Heterotropic interactions of ligands with phosphorylase b.
abstract: 1. The interaction of rabbit muscle glycogen phosphorylase b with pairs of ligands has been examined. 2. The electron spin resonance spectrum of a spin label, covalently attached to the protein, provided information about dissociation constants, formation of ternary complexes and both negative and positive interactions between different ligand pairs. 3. AMP competes with a series of nucleotides (ADP, ATP, CMP aand cytosine) but with adenosine a ternary enzyme - AMP - adenosine complex can be formed. 4. ADP binding is tight and ADP inhibits the AMP activation of phosphorylase b in a physiologically important concentration range. 5. The substrates glucose 1-phosphate and glycogen tighten AMP binding in the ternary complex as does the competitive inhibitor UDPG. Inorganic phosphate is different in this respect. Gluconolactone, a transition state analogue, competes with glucose 1-phosphate (but not with glycogen) but does not prevent completely the binding of the sugar phosphate. 6. The effect of glucose b-phosphate on phosphorylase is rather complex as it 'formally competes' with both AMP and UDPG probably mediated by a conformational changes and not by 'direct' interactions with these two ligands. Glycerol 2-phosphate, a commonly used buffer for phosphorylase, also shows complex interactions.
mesh_terms: D000249:Adenosine Monophosphate; D001665:Binding Sites; D001667:Binding, Competitive; D004578:Electron Spin Resonance Spectroscopy; D007700:Kinetics; D008024:Ligands; D046911:Macromolecular Substances; D008433:Mathematics; D006005:Phosphorylases / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D012265:Ribonucleotides; D013113:Spin Labels
chemical_list: D008024:Ligands; D046911:Macromolecular Substances; D012265:Ribonucleotides; D013113:Spin Labels; D000249:Adenosine Monophosphate; D006005:Phosphorylases