Irreversible steps of Glycolysis are catalysed by:
**Core Concept**
Glycolysis is a metabolic pathway that converts glucose into pyruvate, generating energy in the form of ATP and NADH. The irreversible steps of glycolysis are critical regulatory points that commit the pathway to completion, ensuring the efficient utilization of glucose.
**Why the Correct Answer is Right**
The irreversible steps of glycolysis are catalysed by enzymes that have low Km values for their substrates, ensuring that they remain active even at low substrate concentrations. **Hexokinase** (Option C) is the first enzyme in glycolysis, catalysing the phosphorylation of glucose to form glucose-6-phosphate (G6P). This step is irreversible due to the high energy phosphate bond formed, committing the glucose molecule to the glycolytic pathway. **Phosphofructokinase-1 (PFK-1)** (Option D) is another key regulatory enzyme, catalysing the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate (F1,6BP). This step is also irreversible, as the energy invested in forming the bisphosphate bond cannot be easily reversed.
**Why Each Wrong Option is Incorrect**
**Option A:** Glyceraldehyde-3-phosphate dehydrogenase is a reversible enzyme that catalyses the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate. It is not an irreversible step in glycolysis.
**Option B:** Pyruvate kinase is indeed an enzyme involved in glycolysis, but it catalyses the conversion of phosphoenolpyruvate (PEP) to pyruvate, which is a reversible step.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the irreversible steps of glycolysis are critical regulatory points, and their activity is tightly controlled by allosteric modulators and feedback inhibition to prevent excessive glucose utilization and maintain energy homeostasis.
**Correct Answer: C, D. Hexokinase and Phosphofructokinase-1.**