Gluconeogenesis steps: which of the following does NOT occur as a single enzymatic reaction (it requires multiple intermediate steps) during conversion of lower-glycolytic intermediates back to hexose phosphates?

Introduction / Context:
Gluconeogenesis reverses glycolysis with strategic bypasses. This question targets which transformation is not a single enzymatic step but a pathway segment requiring several enzymes and intermediates.

Given Data / Assumptions:

• Physiological context: hepatic/renal gluconeogenesis.
• Candidate steps include pyruvate↔oxaloacetate↔PEP and upstream reactions toward hexose phosphates.
• We are asked to spot a multi-step conversion disguised as if it were a single reaction.

Concept / Approach:
Some gluconeogenic changes are single enzymes (e.g., pyruvate carboxylase makes oxaloacetate; PEP carboxykinase makes PEP from OAA with decarboxylation). By contrast, PEP does not become fructose 1,6-bisphosphate in one step; it proceeds through several reversible glycolytic reactions.

Step-by-Step Solution:
1) Pyruvate → oxaloacetate: a single carboxylation by pyruvate carboxylase (biotin-dependent).2) Oxaloacetate → PEP: a single decarboxylation/phosphorylation by PEP carboxykinase.3) PEP → F1,6BP: requires multiple enzymes (enolase, phosphoglycerate mutase, phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase, triose phosphate isomerase, aldolase) to reach fructose 1,6-bisphosphate.4) Therefore, the statement that PEP is converted to fructose 1,6-bisphosphate is not a single reaction step and is the correct choice.

Verification / Alternative check:
Pathway diagrams show distinct nodes: PEP → 2-phosphoglycerate → 3-phosphoglycerate → 1,3-bisphosphoglycerate → glyceraldehyde-3-phosphate/dihydroxyacetone phosphate → fructose 1,6-bisphosphate via aldolase, confirming multi-step nature.

Why Other Options Are Wrong:
Pyruvate carboxylation: one step catalyzed by pyruvate carboxylase.Pyruvate → PEP: effectively accomplished via two defined, single-enzyme reactions (pyruvate carboxylase, then PEPCK), each discrete.Oxaloacetate decarboxylation to PEP: a single PEPCK reaction.

Common Pitfalls:
Assuming pathway segments are single reactions or overlooking the aldolase step that forms F1,6BP from triose phosphates.

Final Answer:
The conversion of PEP to fructose 1,6-bisphosphate