The rate-limiting step in glycolysis is catalyzed by
**Core Concept**
The rate-limiting step in glycolysis is a crucial regulatory point that controls the flow of glucose through the glycolytic pathway. This step is essential for maintaining energy homeostasis in cells, particularly in high-energy-demanding tissues such as the brain and muscles.
**Why the Correct Answer is Right**
The rate-limiting step in glycolysis is catalyzed by the enzyme hexokinase (or glucokinase in the liver). Hexokinase phosphorylates glucose to form glucose-6-phosphate (G6P), which is a key regulatory step in glycolysis. This step commits glucose to the glycolytic pathway and prevents its re-entry into the bloodstream. The phosphorylation of glucose by hexokinase is an irreversible reaction, making it the primary regulatory point in glycolysis.
**Why Each Wrong Option is Incorrect**
* **Option A:** Phosphofructokinase-1 (PFK-1) is a key enzyme in glycolysis, but it is not the rate-limiting step. PFK-1 catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, but its activity is regulated by allosteric effectors such as ATP and citrate.
* **Option B:** Pyruvate kinase is an enzyme that catalyzes the final step of glycolysis, converting phosphoenolpyruvate (PEP) to pyruvate. While pyruvate kinase is an important regulatory enzyme, it is not the rate-limiting step in glycolysis.
* **Option C:** Glucose-6-phosphatase is an enzyme that catalyzes the conversion of glucose-6-phosphate to glucose, which is an important step in gluconeogenesis. However, it is not involved in glycolysis and is not the rate-limiting step in this pathway.
**Clinical Pearl / High-Yield Fact**
In the liver, glucokinase (a variant of hexokinase) is the primary enzyme responsible for phosphorylating glucose to form glucose-6-phosphate. This is because glucokinase has a higher Km for glucose than hexokinase, allowing it to phosphorylate glucose efficiently at high concentrations.
**Correct Answer:** C. Pyruvate kinase is an enzyme that catalyzes the final step of glycolysis, converting phosphoenolpyruvate (PEP) to pyruvate. While pyruvate kinase is an important regulatory enzyme, it is not the rate-limiting step in glycolysis.