A model for activation of the hexadecameric phosphorylase kinase complex deduced from zero-length oxidative crosslinking

The regulation of glycogenolysis in the brain

The key regulatory enzymes of glycogenolysis are phosphorylase kinase, a hetero-oligomer with four different types of subunits, and glycogen phosphorylase, a homodimer. Both enzymes are activated by phosphorylation and small ligands, and both enzymes have distinct isoforms that are predominantly expressed in muscle, liver, or brain; however, whole-transcriptome high-throughput sequencing analyses show that in brain both of these enzymes are likely composed of subunit isoforms representing all three tissues. This Minireview examines the regulatory properties of the isoforms of these two enzymes expressed in the three tissues, focusing on their potential regulatory similarities and differences. Additionally, the activity, structure, and regulation of the remaining enzyme necessary for glycogenolysis, glycogen-debranching enzyme, are also reviewed.

Structural characterization of the catalytic γ and regulatory β 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 (α, β, γ and δ), making the study of its structure challenging. Using hydrogen-deuterium exchange, we have analyzed the regulatory β subunit and the catalytic γ 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 γ subunit assumes an activated conformation and are consistent with a previous docking model of the β subunit within the cryoelectron microscopy envelope of PhK.

Activation of Phosphorylase Kinase by Physiological Temperature

In the six decades since its discovery, phosphorylase kinase (PhK) from rabbit skeletal muscle has usually been studied at 30 °C; in fact, not a single study has examined functions of PhK at a rabbit's body temperature, which is nearly 10 °C greater. Thus, we have examined aspects of the activity, regulation, and structure of PhK at temperatures between 0 and 40 °C. Between 0 and 30 °C, the activity at pH 6.8 of nonphosphorylated PhK predictably increased; however, between 30 and 40 °C, there was a dramatic jump in its activity, resulting in the nonactivated enzyme having a far greater activity at body temperature than was previously realized. This anomalous change in properties between 30 and 40 °C was observed for multiple functions, and both stimulation (by ADP and phosphorylation) and inhibition (by orthophosphate) were considerably less pronounced at 40 °C than at 30 °C. In general, the allosteric control of PhK's activity is definitely more subtle at body temperature. Changes in behavior related to activity at 40 °C and its control can be explained by the near disappearance of hysteresis at physiological temperature. In important ways, the picture of PhK that has emerged from six decades of study at temperatures of ≤30 °C does not coincide with that of the enzyme studied at physiological temperature. The probable underlying mechanism for the dramatic increase in PhK's activity between 30 and 40 °C is an abrupt change in the conformations of the regulatory β and catalytic γ subunits between these two temperatures.