Activity of which of the following enzyme is NOT affected by insulin:
**Core Concept**
Insulin is a hormone that plays a crucial role in glucose metabolism by regulating the activity of various enzymes involved in glycolysis and glycogen synthesis. Insulin promotes the storage of glucose in the form of glycogen by activating key enzymes such as glycogen synthase and inhibiting enzymes like phosphofructokinase-1 (PFK-1) and pyruvate kinase.
**Why the Correct Answer is Right**
The correct answer is **C. Pyruvate kinase**. Pyruvate kinase is an enzyme that catalyzes the final step of glycolysis, converting phosphoenolpyruvate (PEP) into pyruvate. Unlike other enzymes involved in glycolysis, pyruvate kinase is not directly affected by insulin. This is because insulin's primary action is to promote the storage of glucose in the form of glycogen, rather than to directly influence glycolysis.
**Why Each Wrong Option is Incorrect**
**Option A:** Glycogen synthase is an enzyme that is activated by insulin, which promotes glycogen synthesis. Insulin stimulates the activity of glycogen synthase by activating the enzyme and increasing its substrate availability.
**Option B:** Phosphofructokinase-1 (PFK-1) is an enzyme that is inhibited by insulin. Insulin's action on PFK-1 is part of its broader strategy to reduce glucose breakdown and promote storage.
**Option D:** Hexokinase is an enzyme that phosphorylates glucose to form glucose-6-phosphate, an early step in glycolysis. Insulin's action on hexokinase is indirect, but it does affect the enzyme's activity by promoting the storage of glucose in the form of glycogen.
**Clinical Pearl / High-Yield Fact**
Insulin's effects on glucose metabolism are complex and multifaceted. Understanding the specific actions of insulin on various enzymes is crucial for managing diabetes and other metabolic disorders.
**Correct Answer: C. Pyruvate kinase**