Final answer:
Protein kinase A (PKA) exerts direct control over the glycogen synthesis and breakdown pathways when epinephrine stimulates glycogen conversion to glucose in liver cells.
Step-by-step explanation:
The protein that exerts direct control over the glycogen synthesis and breakdown pathways when epinephrine stimulates glycogen conversion to glucose in liver cells is protein kinase A (PKA).
Epinephrine, also known as adrenaline, activates β-adrenergic receptors in liver cells and triggers a signaling pathway that leads to an increase in cyclic AMP (cAMP). This cAMP activates PKA, which phosphorylates two enzymes involved in glycogen metabolism. One of these enzymes is glycogen synthase (GS), which is inhibited by phosphorylation, preventing the synthesis of glycogen from glucose. The other enzyme is intermediate glycogen phosphorylase kinase (GPK), which is activated by phosphorylation and promotes glycogen breakdown into glucose.
Therefore, when epinephrine stimulates glycogen conversion to glucose in liver cells, PKA plays a crucial role in regulating the glycogen synthesis and breakdown pathways.