Structural characterization of the catalytic and regulatory subunits of phosphorylase kinase in the context of the hexadecameric enzyme complex

In the tightly regulated glycogenolysis cascade, the breakdown of glycogen to glucose-1-phosphate, phosphorylase kinase (PhK) plays a key role in regulating the activity of glycogen phosphorylase. PhK is a 1.3 MDa hexadecamer, with four copies each of four different subunits (, , and ), making the study of its structure challenging. Using hydrogen-deuterium exchange, we have analyzed the regulatory subunit and the catalytic subunit in the context of the intact non-activated PhK complex to study the structure of these subunits and identify regions of surface exposure. Our data suggest that within the non-activated complex the subunit assumes an activated conformation and are consistent with a previous docking model of the subunit within the cryoelectron microscopy envelope of PhK.