Structural changes induced in glycogen phosphorylase b by the binding of glucose and caffeine

The allosteric inhibitors glucose and caffeine cause significant structural alterations in glycogen phosphorylase b (1,4-alpha-D-glucan:orthophosphate alpha-D-glucosyltransferase, EC 2.4.1.1). Both cause a masking of two sulfhydryl groups and a reduction of binding affinity for AMP. Caffeine produces an alteration in the microenvironment of the binding site for 1-anilin-naphthalene-8-sulfonate, resulting in a decrease of quantum yield of fluorescence and a change in spectral distribution. The binding of glucose is exothermic with an enthalpy of binding of -6.0 kcal/mol. Glucose causes a change in the molecular ellipticity in the pyridoxal-5'-phosphate region. The implications of these results are discussed.