Alternative binding modes for maltopentaose in the activation site of glycogen phosphorylase a

The structure of the complex between glucose-inhibited glycogen phosphorylase a and the substrate analogue maltopentaose has been solved at 2.4 A resolution. The structure of the oligosaccharide in this complex is compared with the structure of the maltopentaose bound in the presence of substrate phosphate (activated GPa) and of maltoheptaose bound to the glucose-inhibited form. Two chains of oligosaccharide bind in a total of eleven glucan subsites in both the maltopentaose-activated GPa complex and the maltoheptaose-glucose inhibited complex. In contrast, the complex between maltopentaose and the glucose inhibited enzyme contains only a single oligosaccharide chain. The conformational changes induced in the protein by binding of oligosaccharides in the presence or absence of substrate phosphate are highly correlated with one another.