Glycogen Phosphorylase b

This chapter provides an account of the current understanding of the catalytic and control mechanisms of muscle glycogen phosphorylase b with special emphasis on the results from the crystal structure determination of the rabbit muscle enzyme. Glycogen phosphorylase catalyzes the first step in the intracellular degradation of glycogen. The importance of skeletal muscle glycogen phosphorylase is demonstrated by patients with McArdles syndrome, an inborn error of metabolism caused by lack of glycogen phosphorylase in muscle. Glycogen itself can be considered as a further effector promoting the dimeric state and contributing to activation. Mammalian phosphorylases show a high degree of conservation. The glycogen storage site is situated on the surface of the molecule well-removed from other sites and the subunit-subunit interface. Rabbit muscle phosphorylase can utilize either glycogen or oligosaccharide with similar catalytic rates at saturating substrate concentrations, but the relative affinity for glycogen is much higher than with linear oligosaccharides.