Which of the following is responsible for the inactive state of Phosphorylase b?
**Core Concept**
Phosphorylase b is an inactive form of glycogen phosphorylase, a key enzyme involved in glycogenolysis. The conversion between phosphorylase b and its active form, phosphorylase a, is a crucial regulatory mechanism in glucose metabolism.
**Why the Correct Answer is Right**
The inactive state of phosphorylase b is maintained by a covalent modification, specifically the phosphorylation of a serine residue (Ser14). This modification is catalyzed by a protein kinase, which adds a phosphate group to the enzyme, thereby inactivating it. The active form of phosphorylase, phosphorylase a, is obtained by the removal of this phosphate group through the action of a protein phosphatase.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the regulation of phosphorylase.
**Option B:** This option is incorrect because the conversion between phosphorylase a and b is a reversible reaction, and the inactive state of phosphorylase b is maintained by a covalent modification, not its structure.
**Option C:** This option is incorrect because the active form of phosphorylase is phosphorylase a, not phosphorylase b.
**Clinical Pearl / High-Yield Fact**
The regulation of glycogen phosphorylase is a classic example of a reversible covalent modification, which is a key mechanism for controlling enzyme activity in the body. This concept is essential for understanding glucose metabolism and its disorders, such as glycogen storage diseases.
**Correct Answer:** C. Phosphorylation of Ser14.