Irreversible step (s) in glycolysis is/are:
**Core Concept**
Glycolysis is a crucial metabolic pathway that converts glucose into pyruvate, generating energy for the cell. The process involves a series of enzyme-catalyzed reactions, some of which are reversible, while others are irreversible. These irreversible steps are critical in regulating the flow of glycolysis and ensuring that glucose is efficiently converted into energy.
**Why the Correct Answer is Right**
The irreversible steps in glycolysis are catalyzed by enzymes that are not easily reversible. **Option C**, the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate by glyceraldehyde-3-phosphate dehydrogenase, is an example of an irreversible step. This reaction involves the oxidation of glyceraldehyde-3-phosphate to form 1,3-bisphosphoglycerate, which is facilitated by the enzyme glyceraldehyde-3-phosphate dehydrogenase. This step is irreversible because the enzyme is covalently modified, making it difficult to revert to the original state. Another irreversible step is the conversion of phosphoenolpyruvate to pyruvate by pyruvate kinase, which is catalyzed by the enzyme pyruvate kinase.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the conversion of glucose to glucose-6-phosphate by hexokinase is a reversible step. Although hexokinase is a phosphorylating enzyme, the reaction is reversible, and glucose-6-phosphate can be converted back to glucose.
**Option B:** This option is incorrect because the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate by phosphofructokinase is a reversible step. Although phosphofructokinase is a phosphorylating enzyme, the reaction is reversible, and fructose-1,6-bisphosphate can be converted back to fructose-6-phosphate.
**Clinical Pearl / High-Yield Fact**
It is essential to remember that the irreversible steps in glycolysis are critical in regulating the flow of glycolysis and ensuring that glucose is efficiently converted into energy. The enzymes responsible for these irreversible steps, such as glyceraldehyde-3-phosphate dehydrogenase and pyruvate kinase, are also potential targets for various diseases, including cancer and diabetes.
**Correct Answer: C. The conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate by glyceraldehyde-3-phosphate dehydrogenase and the conversion of phosphoenolpyruvate to pyruvate by pyruvate kinase are the irreversible steps in glycolysis.