Rate limiting step in HMP shunt ?
**Core Concept**
The Hexose Monophosphate (HMP) shunt, also known as the Pentose Phosphate Pathway (PPP), is a metabolic pathway that generates NADPH and pentoses from glucose-6-phosphate. This pathway is crucial for various cellular processes, including fatty acid synthesis, cholesterol synthesis, and antioxidant defenses.
**Why the Correct Answer is Right**
The rate-limiting step in the HMP shunt is the conversion of glucose-6-phosphate to 6-phosphogluconolactone, catalyzed by the enzyme glucose-6-phosphate dehydrogenase (G6PD). This step is critical because it commits glucose-6-phosphate to the HMP shunt, bypassing glycolysis. G6PD is a NADP+-dependent enzyme that maintains the reducing power of the cell by generating NADPH and reducing oxidative stress.
**Why Each Wrong Option is Incorrect**
* **Option A:** Glucose-6-phosphate isosmerase is not the rate-limiting step in the HMP shunt. This enzyme is involved in the conversion of glucose-6-phosphate to fructose-6-phosphate in glycolysis, which is in parallel to the HMP shunt.
* **Option B:** The conversion of ribose-5-phosphate to xylulose-5-phosphate is not the rate-limiting step. This step is catalyzed by ribose-5-phosphate isomerase and is part of the HMP shunt, but it is not the critical regulatory step.
* **Option D:** The conversion of 6-phosphogluconate to ribulose-5-phosphate is not the rate-limiting step. This step is catalyzed by 6-phosphogluconate dehydrogenase and is part of the HMP shunt, but it is not the key regulatory step.
**Clinical Pearl / High-Yield Fact**
G6PD deficiency is a common genetic disorder that affects the HMP shunt, leading to oxidative stress and hemolytic anemia. This condition is often triggered by exposure to certain medications, foods, or infections.
**Correct Answer:** B. Glucose-6-phosphate dehydrogenase (G6PD) is the correct answer.