## 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).
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