In muscle, Phosphorylase a is inactivated by
**Core Concept**
Phosphorylase a is a key enzyme in glycogenolysis, responsible for breaking down glycogen into glucose-1-phosphate in muscle cells. Its activation and inactivation are crucial for regulating glycogen metabolism and energy production in muscle tissue.
**Why the Correct Answer is Right**
Phosphorylase a is inactivated by dephosphorylation, which is mediated by the enzyme phosphoprotein phosphatase 1 (PP1). When PP1 dephosphorylates phosphorylase a, it removes the phosphate group that activates the enzyme, thereby inactivating it. This process is an important feedback mechanism to regulate glycogen breakdown and prevent excessive energy consumption.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because phosphorylase a is not inactivated by phosphorylation, which actually activates the enzyme.
**Option B:** Incorrect because dephosphoenolpyruvate carboxykinase is an enzyme involved in gluconeogenesis, not glycogenolysis.
**Option C:** Incorrect because glycogen synthase is an enzyme involved in glycogenesis, the process of glycogen synthesis, not glycogen breakdown.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the regulation of glycogen metabolism is a critical aspect of muscle physiology, particularly in the context of exercise and energy production.
**Correct Answer: D. Dephosphorylation by phosphoprotein phosphatase 1 (PP1)**