The pentose phosphate pathway is divided into two phases. Which of the following correctly describes the product and function of the oxidative phase?

## The Oxidative Phase of the Pentose Phosphate Pathway ### Two-Phase Structure **Key Point:** The pentose phosphate pathway consists of two distinct phases: 1. **Oxidative phase** — produces NADPH and ribose-5-phosphate 2. **Non-oxidative (reductive) phase** — interconverts sugars and regenerates glucose-6-phosphate ### The Oxidative Phase: Reactions and Products **High-Yield:** The oxidative phase comprises three enzymatic steps: | Step | Enzyme | Substrate → Product | Key Feature | |------|--------|---------------------|-------------| | 1 | Glucose-6-phosphate dehydrogenase | Glucose-6-phosphate → 6-phosphogluconolactone | **Produces NADPH** (1st) | | 2 | 6-Phosphogluconolactone hydrolase | 6-phosphogluconolactone → 6-phosphogluconate | Hydrolysis | | 3 | 6-Phosphogluconate dehydrogenase | 6-phosphogluconate → Ribulose-5-phosphate | **Produces NADPH** (2nd) | **Net reaction of oxidative phase:** $$\text{Glucose-6-phosphate} + 2\text{NADP}^+ \rightarrow \text{Ribulose-5-phosphate} + 2\text{NADPH} + 2\text{H}^+ + \text{CO}_2$$ ### Functions of NADPH **Clinical Pearl:** NADPH is the reducing power currency of the cell, essential for: - **Reductive biosynthesis:** Fatty acid synthesis, cholesterol synthesis, nucleotide synthesis - **Antioxidant defense:** Reduction of oxidized glutathione (GSSG → GSH) via glutathione reductase, protecting cells from oxidative damage - **Detoxification:** Cytochrome P450 monooxygenase reactions in the smooth endoplasmic reticulum ### The Non-Oxidative Phase The ribose-5-phosphate produced in the oxidative phase can be: - Used directly for nucleotide synthesis (DNA/RNA) - Converted back to glucose-6-phosphate via the non-oxidative phase if NADPH demand is high but ribose demand is low **Mnemonic:** **NADPH Builds (Biosynthesis) and Protects (Antioxidant)** — The two major roles of NADPH produced in the oxidative phase.