Part V. A Closer Look at Glycogen Phosphorylase

The activation of the glycogenolysis pathway via the cascade of phosphorylation reactions was illustrated in the preceeding pages. Glycogen phosphorylase is the ultimate enzyme in the cascade which catalyzes the degradation of glycogen to glucose-1-phosphate. Glycogen phosphorylase is also called phosphorylase and in it's functionally active form is composed of a dimer of two identical monomers each with molecular weight of 97,444 daltons. Phosphorylation of phosphorylase occurs on serine-14 and converts the inactive phosphorylase b to the active phosphorylase a.

Not only is phosphorylase regulated by phosphorylation, it is allosterically regulated by small molecule activators and inhibitors.

Crystallographic structural studies on rabbit muscle phosphorylase have yielded a wealth of information contributing to the understanding of the mechanism of catalytic activation due to phosphorylation. The structural studies were done in the laboratories of Louise Johnson and co-workers and Robert Fletterick and co-workers.