All of the following are true regarding glycolytic intermediates and their fates EXCEPT:

Explanation

## Glycolytic Intermediates: Fates and Checkpoints ### Correct Statements (Options 0, 1, 3) **High-Yield:** Glucose-6-phosphate is a critical branch point. It can enter glycolysis OR be shunted to the pentose phosphate pathway (oxidative phase) to generate NADPH for reductive biosynthesis (fatty acids, cholesterol, nucleotides) and antioxidant defense (glutathione reduction). **Key Point:** Aldolase cleaves fructose-1,6-bisphosphate into dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P) in equimolar amounts (1:1). DHAP is then isomerized to G3P, so both triose phosphates proceed through the lower glycolytic pathway. **Clinical Pearl:** Pyruvate is a metabolic hub. It can be: - **Transaminated** to alanine (glucose-alanine cycle during fasting) - **Carboxylated** to oxaloacetate (gluconeogenesis, TCA cycle entry) - **Oxidatively decarboxylated** to acetyl-CoA (fed state, lipogenesis) ### The Incorrect Statement (Option 2) **Warning:** Option 2 contains a critical error in the **sequence of ATP generation**. The first ATP-generating step in glycolysis is NOT at 3-phosphoglycerate. **Mnemonic:** **GAPDH Goes first** — **G**lyceraldehyde-**3**-**P**hosphate **D**ehydrogenase catalyzes the FIRST substrate-level phosphorylation, producing 1,3-bisphosphoglycerate, which then transfers its acyl phosphate to ADP via phosphoglycerate kinase to form ATP. ### Timeline of ATP Generation in Glycolysis | Step | Enzyme | Substrate | Product | ATP Generated? | |------|--------|-----------|---------|----------------| | 6 | Glyceraldehyde-3-phosphate dehydrogenase | G3P | 1,3-bisphosphoglycerate | **FIRST ATP** (coupled) | | 7 | Phosphoglycerate kinase | 1,3-bisphosphoglycerate | 3-phosphoglycerate | ATP ✓ | | 10 | Pyruvate kinase | Phosphoenolpyruvate | Pyruvate | ATP ✓ | **Key Point:** 3-Phosphoglycerate is the PRODUCT of the first ATP-generating step, not the site where ATP is generated. The high-energy acyl phosphate bond is in 1,3-bisphosphoglycerate (the immediate predecessor of 3-phosphoglycerate). [cite:Lehninger Principles of Biochemistry 8e Ch 14; Stryer Biochemistry 9e Ch 16]