Rate Limiting step of Pathway (Figure) is catalyzed by
**Core Concept**
The question pertains to the glycolytic pathway, a crucial metabolic process that converts glucose into pyruvate, generating energy for the cell. The glycolytic pathway involves a series of enzyme-catalyzed reactions, with one specific step acting as the rate-limiting step, dictating the overall flux through the pathway.
**Why the Correct Answer is Right**
The rate-limiting step in the glycolytic pathway is catalyzed by the enzyme phosphofructokinase-1 (PFK-1). PFK-1 is a key regulatory enzyme that catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. This step is crucial because it commits glucose to the glycolytic pathway, making it the primary regulatory point. The activity of PFK-1 is influenced by various factors, including ATP, ADP, and citrate levels, allowing the cell to adjust glycolysis according to energy demands.
**Why Each Wrong Option is Incorrect**
**Option A:** Glucose-6-phosphatase is incorrect because it is an enzyme involved in gluconeogenesis, not glycolysis, and is not the rate-limiting step in the glycolytic pathway.
**Option B:** Hexokinase is incorrect because, although it is an enzyme involved in the glycolytic pathway, it is not the rate-limiting step. Hexokinase catalyzes the conversion of glucose to glucose-6-phosphate, but its activity is not as tightly regulated as PFK-1.
**Option C:** Pyruvate kinase is incorrect because it is an enzyme involved in the final step of glycolysis, catalyzing the conversion of phosphoenolpyruvate to pyruvate. While it is an important regulatory enzyme, it is not the primary rate-limiting step in the glycolytic pathway.
**Clinical Pearl / High-Yield Fact**
The glycolytic pathway is essential for energy production in cells, particularly in situations where oxygen is limited, such as during intense exercise or in tumor cells. Understanding the regulation of glycolysis, particularly the role of PFK-1, is crucial for appreciating how cells adapt to changing energy demands.
**Correct Answer: C. Pyruvate kinase is incorrect because it is an enzyme involved in the final step of glycolysis, catalyzing the conversion of phosphoenolpyruvate to pyruvate.**