## **Core Concept**
Glycolysis is a metabolic pathway that converts glucose into pyruvate, releasing energy. The first committed step in glycolysis is a crucial regulatory point. This step is characterized by an irreversible reaction that commits glucose to being broken down to pyruvate.

## **Why the Correct Answer is Right**
The correct answer, **Phosphofructokinase-1 (PFK-1)**, catalyzes the conversion of fructose-6-phosphate into fructose-1,6-bisphosphate. This step is considered the first committed step of glycolysis because it is irreversible under physiological conditions and commits the glucose molecule to being metabolized through the glycolytic pathway. PFK-1 is a key regulatory enzyme in glycolysis, influenced by various factors including ATP, citrate, and AMP levels.

## **Why Each Wrong Option is Incorrect**
* **Option A:** Hexokinase catalyzes the conversion of glucose to glucose-6-phosphate, which is the first step in glycolysis but not the first committed step, as glucose-6-phosphate can still be used in other pathways like glycogen synthesis or the pentose phosphate pathway.
* **Option B:** Glucose-6-phosphatase is involved in gluconeogenesis and glycogenolysis, catalyzing the conversion of glucose-6-phosphate to glucose, which is the opposite of what occurs in glycolysis.
* **Option D:** Pyruvate kinase catalyzes the final step of glycolysis, converting phosphoenolpyruvate (PEP) into pyruvate, which is not the first committed step.

## **Clinical Pearl / High-Yield Fact**
A key clinical correlation is that **PFK-1 deficiency** leads to a rare genetic disorder known as Glycogen Storage Disease Type VII, or Tarui's disease, characterized by an inability to break down glycogen to glucose for energy. This highlights the importance of PFK-1 in energy metabolism.

## **Correct Answer:** . Phosphofructokinase-1 (PFK-1)