Step of Gluconeogenesis is:
**Core Concept**
Gluconeogenesis is a metabolic pathway that generates glucose from non-carbohydrate sources, such as lactate, glycerol, and certain amino acids. This process is crucial for maintaining blood glucose levels during fasting, starvation, or low-carbohydrate diets. It involves a series of enzyme-catalyzed reactions that convert these precursors into glucose.
**Why the Correct Answer is Right**
The correct answer is **Pyruvate carboxylase**. This enzyme catalyzes the carboxylation of pyruvate to form oxaloacetate, which is a critical step in gluconeogenesis. Pyruvate carboxylase is a biotin-dependent enzyme that uses ATP and CO2 to convert pyruvate into oxaloacetate, bypassing the irreversible step of pyruvate dehydrogenase in glycolysis. This reaction is essential for gluconeogenesis because it allows the pathway to bypass the glycolytic step and generate glucose from non-carbohydrate sources.
**Why Each Wrong Option is Incorrect**
**Option A:** **Phosphoenolpyruvate carboxykinase (PEPCK)** is an enzyme involved in gluconeogenesis, but it catalyzes a different reaction, converting oxaloacetate to phosphoenolpyruvate.
**Option B:** **Glucose-6-phosphatase** is an enzyme involved in glucose metabolism, but it is primarily involved in gluconeogenesis and glycogenolysis, not in the initial steps of gluconeogenesis.
**Option C:** **Pyruvate kinase** is an enzyme involved in glycolysis, not gluconeogenesis, and catalyzes the conversion of phosphoenolpyruvate to pyruvate.
**Clinical Pearl / High-Yield Fact**
Pyruvate carboxylase is inhibited by malonyl-CoA, which is produced during fatty acid synthesis. This inhibition is a key regulatory mechanism that links gluconeogenesis to fatty acid metabolism, ensuring that glucose production is coordinated with energy availability.
**Correct Answer: C. Pyruvate carboxylase**