Which of the following enzymes is stimulated by glucagon?
**Core Concept**
Glucagon is a hormone secreted by the pancreas that plays a crucial role in regulating blood glucose levels. It stimulates the breakdown of glycogen to glucose and promotes gluconeogenesis, thus increasing blood glucose levels. The enzyme in question is involved in the process of glycogenolysis, the breakdown of glycogen to glucose.
**Why the Correct Answer is Right**
The correct enzyme is phosphorylase kinase, which is activated by glucagon. The signaling cascade initiated by glucagon involves the activation of adenylate cyclase, resulting in an increase in cyclic AMP (cAMP) levels. cAMP, in turn, activates protein kinase A (PKA), which phosphorylates and activates phosphorylase kinase. Phosphorylase kinase then phosphorylates and activates glycogen phosphorylase, the enzyme responsible for the breakdown of glycogen to glucose. This process is essential for maintaining blood glucose levels during fasting or starvation.
**Why Each Wrong Option is Incorrect**
* **Option A:** Glucokinase is an enzyme involved in glucose phosphorylation, not glycogen breakdown. It is stimulated by high glucose levels, not glucagon.
* **Option B:** Phosphofructokinase-1 is an enzyme involved in glycolysis, the breakdown of glucose to pyruvate. It is inhibited by glucagon, not stimulated.
* **Option D:** Pyruvate kinase is an enzyme involved in glycolysis, the breakdown of glucose to pyruvate. It is also inhibited by glucagon, not stimulated.
**Clinical Pearl / High-Yield Fact**
The glucagon-cAMP-PKA pathway is a classic example of a signaling cascade that regulates metabolism. It is essential for students to understand the molecular mechanisms underlying hormonal regulation of metabolism, as they are frequently tested on these topics in exams.
**Correct Answer:** C. Phosphorylase kinase.