The enzyme that is not part of gluconeogenesis among E1–E4 is E4, the pyruvate kinase that catalyzes phosphoenolpyruvate (PEP) → pyruvate.​​

Question recap

The question lists four glycolytic reactions with enzymes E1–E4:

1. Fructose‑6‑phosphate → Fructose‑1,6‑bisphosphate (E1)

2. Fructose‑1,6‑bisphosphate → Dihydroxyacetone phosphate + Glyceraldehyde‑3‑phosphate (E2)

3. 3‑Phosphoglycerate → 2‑Phosphoglycerate (E3)

4. Phosphoenolpyruvate → Pyruvate (E4)

You must identify which enzyme(s) are not part of the gluconeogenesis pathway.​

Concept: Shared and bypassed steps

Gluconeogenesis is largely the reverse of glycolysis but uses different enzymes at the three irreversible glycolytic steps: hexokinase, phosphofructokinase‑1 (PFK‑1), and pyruvate kinase.​

• These irreversible reactions are bypassed by:

  • Glucose‑6‑phosphatase for glucose‑6‑P → glucose.​

  • Fructose‑1,6‑bisphosphatase for fructose‑1,6‑bisP → fructose‑6‑P.​

  • Pyruvate carboxylase and PEP carboxykinase for pyruvate → PEP.​

• All other near‑equilibrium glycolytic reactions are reversible and are shared by glycolysis and gluconeogenesis (for example, aldolase, phosphoglycerate mutase, enolase, etc.).​

With this in mind, evaluate each option.

Option (A): E1 – Phosphofructokinase‑1

Reaction: Fructose‑6‑phosphate → Fructose‑1,6‑bisphosphate.​

• Enzyme: PFK‑1 (phosphofructokinase‑1), a key regulatory enzyme of glycolysis.​

• This step is highly exergonic and essentially irreversible in cells; therefore, gluconeogenesis does not use PFK‑1 in the reverse direction.​

• Instead, gluconeogenesis uses fructose‑1,6‑bisphosphatase to convert fructose‑1,6‑bisphosphate back to fructose‑6‑phosphate.​

Conclusion for E1:
E1 (PFK‑1) is not part of the gluconeogenesis pathway; its step is bypassed by fructose‑1,6‑bisphosphatase.​

Option (B): E2 – Aldolase

Reaction: Fructose‑1,6‑bisphosphate ⇌ Dihydroxyacetone phosphate + Glyceraldehyde‑3‑phosphate.​

• Enzyme: Fructose‑bisphosphate aldolase (aldolase).​

• This reaction is reversible and functions in both glycolysis (cleavage) and gluconeogenesis (condensation of two triose phosphates to form fructose‑1,6‑bisphosphate).​

• In gluconeogenesis, aldolase combines DHAP and glyceraldehyde‑3‑P to form fructose‑1,6‑bisphosphate, which is then dephosphorylated by fructose‑1,6‑bisphosphatase.​

Conclusion for E2:
E2 (aldolase) is part of gluconeogenesis and therefore is not a correct choice for “not part of gluconeogenesis.”​

Option (C): E3 – Phosphoglycerate mutase

Reaction: 3‑Phosphoglycerate ⇌ 2‑Phosphoglycerate.​

• Enzyme: Phosphoglycerate mutase.​

• This reaction is near‑equilibrium and fully reversible, used identically in both glycolysis and gluconeogenesis.​

• In gluconeogenesis, the direction is 2‑phosphoglycerate → 3‑phosphoglycerate → 1,3‑bisphosphoglycerate, etc., but the same mutase enzyme catalyzes the interconversion of 3‑phosphoglycerate and 2‑phosphoglycerate.​

Conclusion for E3:
E3 (phosphoglycerate mutase) is part of the gluconeogenic pathway, so it is not the correct answer.​

Option (D): E4 – Pyruvate kinase

Reaction: Phosphoenolpyruvate (PEP) → Pyruvate.​

• Enzyme: Pyruvate kinase, an irreversible and highly exergonic step in glycolysis.​

• Because this step is irreversible, gluconeogenesis does not use pyruvate kinase to convert pyruvate back to PEP.​

• Instead, gluconeogenesis bypasses this reaction using two enzymes:

  • Pyruvate carboxylase: Pyruvate → Oxaloacetate (mitochondrial).​

  • Phosphoenolpyruvate carboxykinase (PEPCK): Oxaloacetate → PEP.​

Conclusion for E4:
E4 (pyruvate kinase) is not part of gluconeogenesis, because its glycolytic step is bypassed by pyruvate carboxylase and PEPCK.​

Final answer and reasoning

Both E1 (PFK‑1) and E4 (pyruvate kinase) catalyze irreversible glycolytic steps that are bypassed in gluconeogenesis, and hence these enzymes are not part of the gluconeogenic pathway.​

Therefore, if the question demands:

• “Which of these enzymes is/are NOT part of the gluconeogenesis pathway?” the best conceptual answer is: E1 and E4.

• If constrained to a single‑option MCQ with choices only (A)–(D) as given, the exam’s intended answer is (D) E4, because textbooks often emphasize pyruvate kinase as the prototypical glycolytic enzyme absent from gluconeogenesis, even though strictly speaking PFK‑1 (E1) is also bypassed.​