Allosteric nucleotide specificity of phosphorylase kinase: correlation of binding, conformational transitions, and activation. Utilization of lin-benzo-ADP to measure the binding of other nucleoside diphosphates, including the phosphorothioates of ADP.

Recent work has shown that ADP is an allosteric activator of nonphosphorylated phosphorylase kinase from rabbit skeletal muscle (Cheng, A., Fitzgerald, T. J., and Carlson, G. M. (1985) J. Biol. Chem. 260, 2535-2542). The specificity of the allosteric site for nucleoside diphosphates is further investigated in this study. Only purine nucleoside diphosphates are capable of causing allosteric activation, and an amino group at position 2 or 6 of the purine ring is required. Comparisons are made of the abilities of 5'-diphosphate analogs of ADP, including phosphorothioates, to activate, to bind, and to induce in the enzyme's beta subunits conformational changes associated with activation. Binding is measured by competition titrations utilizing fluorescence polarization of lin-benzo-ADP, itself an allosteric activator; and conformational changes are measured by partial proteolysis and chemical cross-linking. When measured at an identical percentage of saturation at the allosteric site, the abilities of ADP analogs to induce conformational changes in the beta subunits parallel their abilities to activate the holoenzyme. An unmodified beta-phosphate of ADP, although not necessary for binding at the allosteric site, is needed to fully drive the activating conformational transition. The activating nucleoside diphosphate appears to be the free species, as opposed to its Mg2+ complex.