Regarding the pentose phosphate pathway (PPP), all of the following statements are correct EXCEPT:

Question

Accepted Answer

A. Glucose-6-phosphatase is the first enzyme of the oxidative phase and catalyzes the conversion of glucose-6-phosphate to ribulose-5-phosphate. ## Pentose Phosphate Pathway — Enzyme & Phase Clarification

### The Oxidative Phase

**Key Point:** The oxidative phase begins with **glucose-6-phosphate dehydrogenase (G6PD)**, NOT glucose-6-phosphatase.

- **Glucose-6-phosphate dehydrogenase (G6PD)** catalyzes the first committed step: glucose-6-phosphate → 6-phosphogluconolactone
- This is a **rate-limiting, irreversible reaction** that generates the first NADPH
- **Glucose-6-phosphatase** is a different enzyme entirely — it catalyzes the final step of gluconeogenesis and glycogenolysis (glucose-6-phosphate → free glucose), and is found primarily in liver and kidney, not in the PPP

### Why Option 1 is Incorrect

The stem states that "glucose-6-phosphatase is the first enzyme of the oxidative phase." This is **factually wrong**:
- Glucose-6-phosphatase does NOT participate in the PPP
- It is a gluconeogenic enzyme involved in glucose release from the liver
- The first enzyme of the oxidative PPP is **G6PD**

### Verification of Other Options

| Statement | Correct? | Reason |
|-----------|----------|--------|
| Oxidative phase generates NADPH | ✓ Yes | NADPH is the primary product; used in fatty acid synthesis, cholesterol synthesis, and glutathione reduction |
| Non-oxidative phase is reversible | ✓ Yes | Transketolase and transaldolase catalyze reversible reactions; carbons return to glycolysis |
| Pathway upregulated in biosynthetic tissues | ✓ Yes | Liver, adipose, RBCs, adrenal cortex all have high PPP flux due to NADPH demand |

**High-Yield:** G6PD deficiency causes hemolytic anemia under oxidative stress because RBCs cannot regenerate reduced glutathione (GSH) without NADPH. This is the most clinically tested PPP concept in NEET PG.

**Mnemonic — PPP Enzymes:** **"G6PD Transports Carbons"** = Glucose-6-phosphate Dehydrogenase (oxidative phase) → Transketolase & Transaldolase (non-oxidative phase).

Answer

B. The oxidative phase generates NADPH, which is essential for reductive biosynthesis and antioxidant defense

Answer

C. The non-oxidative phase is reversible and can feed carbons back into glycolysis as fructose-6-phosphate and glyceraldehyde-3-phosphate

Answer

D. The pathway is upregulated in tissues with high biosynthetic activity, such as liver, adipose tissue, and red blood cells

Regarding the pentose phosphate pathway (PPP), all of the following statements are correct EXCEPT:

Explanation

Pentose Phosphate Pathway — Enzyme & Phase Clarification

The Oxidative Phase

Why Option 1 is Incorrect

Verification of Other Options

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Statement	Correct?	Reason
Oxidative phase generates NADPH	✓ Yes	NADPH is the primary product; used in fatty acid synthesis, cholesterol synthesis, and glutathione reduction
Non-oxidative phase is reversible	✓ Yes	Transketolase and transaldolase catalyze reversible reactions; carbons return to glycolysis
Pathway upregulated in biosynthetic tissues	✓ Yes	Liver, adipose, RBCs, adrenal cortex all have high PPP flux due to NADPH demand