Irreversible steps of Glycolysis are catalyzed by:
**Core Concept**
Glycolysis is the first step in cellular respiration, where glucose is converted into pyruvate, generating a small amount of ATP and NADH. The irreversible steps of glycolysis are crucial in committing glucose to the glycolytic pathway.
**Why the Correct Answer is Right**
The irreversible steps of glycolysis are catalyzed by two key enzymes: hexokinase (or glucokinase in the liver) and phosphofructokinase-1 (PFK-1). Hexokinase phosphorylates glucose to form glucose-6-phosphate (G6P), while PFK-1 phosphorylates fructose-6-phosphate to form fructose-1,6-bisphosphate (F1,6BP). These reactions are irreversible due to the high energy phosphate bonds formed, making them committed steps in the glycolytic pathway.
**Why Each Wrong Option is Incorrect**
**Option A:** Glucose-6-phosphate isomerase is an enzyme involved in the reversible step of glycolysis, converting glucose-6-phosphate to fructose-6-phosphate.
**Option B:** Pyruvate kinase is an enzyme involved in the final step of glycolysis, converting phosphoenolpyruvate to pyruvate, but it is not one of the irreversible steps.
**Option C:** Phosphoglycerate kinase is an enzyme involved in a reversible step of glycolysis, generating ATP from 1,3-bisphosphoglycerate.
**Clinical Pearl / High-Yield Fact**
In diabetes mellitus, the enzyme glucokinase is impaired, leading to reduced glucose phosphorylation and decreased glycolysis in the liver.
**Correct Answer:** A. Hexokinase.