Glycogen is the principal storage form of carbohydrate in mammals and consists of chains of 11-14 (~-cl -P 4)-linked o-glucose residues interconnected by a-0 --) 6)-linked branch pointsl. Glycogen phosphorylase, the major enzyme found bound to glycogen in vivo, cleaves the a-0 + 4) glucosidic linkages at the nonreducing termini producing a-D-glucopyranosyl phosphate, which is then metabolised. Each glycogen phosphorylase monomer has two glycogen-binding sites, the catalytic site and a higher-affinity glycogen "storage" site. This latter site is involved in enzyme regulation and in binding of the enzyme to the glycogen particles. Crystallographic studies using maltoheptaose bound at the glycogen-storage site have been used to model the phosphorylase-glycogen complex 2_ However, little has been learned of the local dynamics of glycogen in its complex with the enzyme or of the number of enzyme molecules associated with a single glycogen particle.
The synthesis of a glycogen derivative in which all the nonreducing terminal sugars are 4-deoxy-4-fluoro-glucosyl residues (here termed "4-F-glycogen")3 has allowed a further look at the phosphorylase-glycogen interaction in solution. This glycogen analogue has been shown to bind to phosphorylase, but not to act as a substrate in the direction of glycogen synthesis (i.e., in the presence of a-D-glucopyranosyl phosphate)3. This is expected, since the hydroxyl group of the glycogen which acts as the nucleophile in the reaction has been replaced by fluorine. Interestingly however, 4-F-glycogen binds to phosphorylase some lOO-fold more tightly than does normal glycogen3, although the basis of this improved binding is not yet understood. The presence of an NMR-active nucleus (19F) in the molecule should make it possible to monitor the binding interaction between phosphorylase and glycogen, allowing measurements of the stoichiometry of interaction and possibly providing insights into the mode of binding. This technique, which may prove useful for other carbohydrate-binding proteins, is investigated in this work,