All of the following are gluconeogenic substrates EXCEPT:

Explanation

## Gluconeogenic Substrates Overview **Key Point:** Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors. The main substrates are lactate, amino acids, and glycerol. Acetyl-CoA, despite being a central metabolic hub, is NOT a gluconeogenic substrate. ## Why Acetyl-CoA Cannot Be Gluconeogenic ### The Pyruvate Dehydrogenase Irreversibility Problem Acetyl-CoA is generated from pyruvate via the pyruvate dehydrogenase complex (PDH): $$\text{Pyruvate} + \text{CoA} + \text{NAD}^+ \rightarrow \text{Acetyl-CoA} + \text{CO}_2 + \text{NADH}$$ This reaction is **irreversible** — there is no enzyme in mammalian cells that can regenerate pyruvate from acetyl-CoA. The CO₂ is lost permanently. ### Why This Matters Even though acetyl-CoA enters the TCA cycle and generates oxaloacetate (a gluconeogenic intermediate), the carbon atoms of acetyl-CoA are eventually lost as CO₂ during subsequent TCA cycle turns. The net result is **zero net glucose synthesis** from acetyl-CoA. ## Valid Gluconeogenic Substrates | Substrate | Enzyme(s) | Key Feature | |-----------|-----------|-------------| | **Lactate** | Lactate dehydrogenase → pyruvate; pyruvate carboxylase → oxaloacetate | Cori cycle; major substrate during exercise | | **Glycerol** | Glycerol kinase → G3P; enters lower gluconeogenesis | Released from adipose tissue lipolysis | | **Amino acids** | Transamination/deamination → pyruvate or TCA intermediates | Alanine cycle; major source during fasting | | **Odd-chain fatty acids** | β-oxidation → propionyl-CoA → succinyl-CoA | Minor contributor; even-chain FAs yield only acetyl-CoA | **High-Yield:** Acetyl-CoA is lipogenic (drives fatty acid synthesis) but not gluconeogenic. This is a frequent NEET PG trap. **Clinical Pearl:** During prolonged fasting, the body cannot use fat oxidation (acetyl-CoA) to maintain blood glucose — it must rely on amino acid catabolism and glycerol. This is why protein wasting occurs in starvation. ## Mnemonic: "LAG" for Gluconeogenic Substrates - **L**actate - **A**mino acids - **G**lycerol (NOT fatty acids or acetyl-CoA)