Arrange the following 4 enzymes of gluconeogenesis in sequence: A. Pyruvate carboxylase B. Glucose – 6 – phosphatase C. Phosphoenol pyruvate carboxy kinase D. Fructose 1,6 Bisphosphatase
**Core Concept**
Gluconeogenesis is the metabolic pathway that synthesizes glucose from non-carbohydrate precursors. The sequence of enzymes in this pathway must follow a specific order to ensure efficient conversion of intermediates, with each step catalyzed by a distinct enzyme that operates in the reverse direction of glycolysis.
**Why the Correct Answer is Right**
The correct sequence is A-C-D-B:
- **Pyruvate carboxylase (A)** converts pyruvate to oxaloacetate in the mitochondria — the first committed step.
- **Phosphoenolpyruvate carboxykinase (C)** converts oxaloacetate to phosphoenolpyruvate (PEP), continuing in the mitochondria.
- **Fructose-1,6-bisphosphatase (D)** hydrolyzes fructose-1,6-bisphosphate to fructose-6-phosphate in the cytoplasm.
- **Glucose-6-phosphatase (B)** dephosphorylates glucose-6-phosphate to glucose, the final step.
This sequence reflects compartmentalization (mitochondria → cytoplasm) and reversal of glycolytic steps.
**Why Each Wrong Option is Incorrect**
Option A: D-C-A-B — Incorrect because fructose-1,6-bisphosphatase (D) acts after PEP formation, and glucose-6-phosphatase (B) must be last; placing A before C violates mitochondrial pathway logic.
Option C: B-A-D-C — Incorrect because glucose-6-phosphatase (B) cannot act before pyruvate carboxylase (A); glucose-6-phosphate is not a precursor in early gluconeogenesis.
Option D: C-D-B-A — Incorrect because it places PEP carboxykinase (C) before pyruvate carboxylase (A), which would imply oxaloacetate comes before pyruvate, violating metabolic logic.
**Clinical Pearl / High-Yield Fact**
Remember the mitochondrial-to-cytoplasmic transition: Pyruvate carboxylase and PEP carboxykinase act in mitochondria, while fructose-1,6-bisphosphatase and glucose-6-phosphatase act in the cytoplasm. This compartmentalization is a key exam trap — always sequence by location and metabolic direction.
✓ Correct Answer: B. A-C-D-B