Phosphorylase kinase is a protein kinase involved in a specific cell signalling pathway in the body, in particular it is used when a lot of energy is needed in a short amount of time e.g. in a emergency situation where epinephrine is produced causing the pathway to be triggered.
Along the pathway another protein called protein kinase A is activated to form active protein kinase A which then activates phosphorylase kinase through phosphorylation. The activation of this enzyme means it can catalyse the phosphorylation of phosphorylase b to phosphorylase a, which is an enzyme that is necessary for the release of glucose-1-phosphate from glycogen. This glucose can then go through the process of glycolysis allowing for large quantites of energy to be available for the individual to use in the emergency situation.