Biosynthesis of bacterial glycogen. The nature of the binding of substrates and effectors to ADP-glucose synthase.
Kinetic studies with ADP-glucose synthase show that 1,6-hexanediol bisphosphate (1,6-hexanediol-P2) is an effective activator that causes the enzyme to have a higher apparent affinity for ATP- and ADP-glucose than when fructose-1,6-P2 is the activator. Furthermore, in the presence of 1,6-hexanediol-P2, substrate saturation curves are hyperbolic shaped rather than sigmoidal shaped. CrATP behaves like a nonreactive analogue of ATP. Kinetic studies show that it is competitive with ATP. CrATP is not a competitive inhibitor of ADP-glucose. However, the combined addition of CrATP and glucose-1-P inhibits the enzyme competitively when ADP-glucose is the substrate. In binding experiments, CrATP, ATP, and fructose-P2 appear to bind to only half of the expected sites in the tetrameric enzyme, while ADP-glucose, the activators, pyridoxal-P and 1,6-hexanediol-P2, and the inhibitor, AMP, bind to four sites/tetrameric enzyme. Fructose-P2 inhibits 1,6-hexanediol-P2 binding, suggesting competition for the same sites. Glucose-1-P does not bind to the enzyme unless MgCl2 and CrATP are present and binds to four sites/tetrameric enzyme. Alternatively, CrATP in the presence of glucose-1-P binds to four sites/tetrameric enzyme. Thus, there are binding sites for the substrates, activators, and inhibitor located on each subunit and the binding sites can interact homotropically and heterotropically. ATP and fructose-P2 binding is synergistic showing heterotropic cooperativity. ATP and fructose-P2 must also be present together to effectively inhibit AMP binding. A mechanism is proposed which explains some of the kinetic and binding properties in terms of an asymmetry in the distribution of the conformational states of the four identical subunits.
D000245:Adenosine Diphosphate Glucose; D004789:Enzyme Activation; D004926:Escherichia coli / Q000201:enzymology*; D006003:Glycogen / Q000096:biosynthesis*; D006018:Glycols; D007700:Kinetics; D009713:Nucleotidyltransferases / Q000378:metabolism*; D009943:Organophosphorus Compounds; D011485:Protein Binding; D013379:Substrate Specificity
D006018:Glycols; D009943:Organophosphorus Compounds; D000245:Adenosine Diphosphate Glucose; C018313:1,6-hexanediol bisphosphate; D006003:Glycogen; D009713:Nucleotidyltransferases