Final answer:
Protein Kinase A (PKA) is responsible for the activation of Glycogen Phosphorylase Kinase (GPK) via phosphorylation, following a cascade initiated by hormone binding that results in cAMP production and PKA activation.
Step-by-step explanation:
The enzyme that activates Glycogen Phosphorylase Kinase (GPK) via phosphorylation is Protein Kinase A (PKA). Following the binding of hormones like epinephrine or glucagon to their receptors, the enzyme adenylyl cyclase is activated, which converts ATP to cyclic AMP (cAMP). Subsequently, cAMP activates PKA, which then phosphorylates specific substrates including GPK, leading to its activation.
This activation forms part of a phosphorylation cascade that is essential for the rapid mobilization of glucose reserves from glycogen, especially during the fight-or-flight response. The active GPK then catalyzes the phosphorylation of glycogen phosphorylase, converting it to its active form, which hydrolyzes glycogen to glucose-1-phosphate. This is crucial for the retrieval of free glucose from liver cells into the circulation, providing energy for immediate physiological needs.