Allosteric stimulator of glycogen phosphorylase
**Core Concept**
The question is testing the understanding of allosteric modulation of glycogen phosphorylase, a key enzyme involved in glycogenolysis. Glycogen phosphorylase is a crucial enzyme in the regulation of blood glucose levels, and its allosteric stimulation has significant clinical implications.
**Why the Correct Answer is Right**
The allosteric stimulator of glycogen phosphorylase is **Epinephrine**. Epinephrine binds to the R-state of glycogen phosphorylase, causing a conformational change that increases the enzyme's activity. This results in the breakdown of glycogen to glucose-1-phosphate, which can then be converted to glucose-6-phosphate and released into the bloodstream. The binding of epinephrine to glycogen phosphorylase also promotes the formation of the active T-state of the enzyme.
**Why Each Wrong Option is Incorrect**
**Option A:** Glucose is incorrect because it is actually an inhibitor of glycogen phosphorylase, not a stimulator. Glucose binds to the T-state of glycogen phosphorylase, promoting the formation of the inactive R-state.
**Option B:** Calcium is incorrect because it is not a direct allosteric stimulator of glycogen phosphorylase. While calcium is involved in the regulation of glycogenolysis, it acts through other mechanisms, such as the activation of protein kinase A.
**Option C:** Glucagon is incorrect because it is actually a hormone that stimulates glycogenolysis indirectly by activating protein kinase A, which in turn phosphorylates and activates glycogen phosphorylase. However, glucagon is not an allosteric stimulator of glycogen phosphorylase itself.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the allosteric regulation of glycogen phosphorylase is a critical mechanism for the rapid mobilization of glucose stores in response to stress or low blood glucose levels.
**Correct Answer:** A. Epinephrine.