Investigations on the Mechanism of Allosteric Activtion of Rabbit Muscle Glycogen Phosphorylase b by AMP

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Bigley, Andrew N.
Allostery, Regulation, Glycogen Phosphorylase, AMP
book description
Much work has been carried out on glycogen phosphorylase over the last seventy years. Interest has persisted due not only to the usefulness of phosphorylase as a model system of allostery, but also due to the connection to the disease state in type II diabetes. The bulk of research consists of structural studies utilizing the wild-type enzyme from rabbit muscle. In this study we have employed linkage analysis in combination with structural perturbations via site-directed mutagenesis to test kinetic models of activation of phosphorylase b by AMP, and to examine the roles of the N-terminus, the acidic patch, ?-helix 1 and the 280?s loop in activation by AMP. Experiments have been carried out on purified glycogen phosphorylase b variants to determine the effects of perturbations in vitro. The kinetic models of activation by AMP are found to be a relatively accurate description of kinetic behavior of wild-type phosphorylase b, but are found to be technically incorrect with respect to the absolute requirements of two equivalents of AMP to be bound prior to catalysis. Phosphorylase b demonstrates activity in the absence of AMP, though only at high concentrations of phosphate, and a hybrid phosphorylase b with only a single functional AMP binding sight shows slight activation. The truncate ?2-17 shows weakened binding to AMP and phosphate in the apo enzyme, but maintains activation by AMP to an affinity similar to that of wild-type, indicating that the N-terminus is not required for activation by AMP, but has a role in establishing the affinity for both AMP and phosphate in the apo enzyme. Perturbations of the acidic patch indicate that interactions between the acidic patch and the N-terminus enhance the affinities in the apo enzyme, suggesting that the structures of the N-terminus at the acidic patch may represent an active form of the enzyme. ?-helix 1 is found to have a role in homotropic cooperativity in phosphorylase b, but not in heterotropic activation by AMP, while the 280?s loop is confirmed to have a role in the heterotropic coupling between AMP and phosphate. Based on the findings in this study an alternate structural model of activation by AMP involving ?-helix 8 is proposed.