Question

Given below are four reactions of the glycolytic pathway catalyzed by the enzymes E1, E2, E3, and E4, as indicated. Which of these enzymes is/are NOT part of the gluconeogenesis pathway?
(i) Fructose-6-phosphate \(\xrightarrow{\text{E1}}\) Fructose-1,6-bisphosphate
(ii) Fructose-1,6-bisphosphate \(\xrightarrow{\text{E2}}\) DHAP + GAP
(iii) 3-Phosphoglycerate \(\xrightarrow{\text{E3}}\) 2-Phosphoglycerate
(iv) Phosphoenolpyruvate \(\xrightarrow{\text{E4}}\) Pyruvate)

• A. E1
• B. E2
• C. E3
• D. E4

Hint:

In gluconeogenesis, remember the three glycolytic {irreversible} steps are bypassed: hexokinase/GLUT (in liver, via G6Pase), PFK-1 (via F1,6BPase), and pyruvate kinase (via PC + PEPCK). Everything else runs in reverse.

Answer 1

The Correct Option is A, D

Step 1: Identify glycolytic enzymes and reversibility.
E1 is phosphofructokinase-1 (PFK-1): F6P \(\to\) F1,6BP (irreversible in glycolysis).
E2 is aldolase: F1,6BP \(\leftrightarrow\) DHAP + GAP (reversible).
E3 is phosphoglycerate mutase: 3PG \(\leftrightarrow\) 2PG (reversible).
E4 is pyruvate kinase: PEP \(\to\) pyruvate (irreversible).
Step 2: Gluconeogenic bypasses.
Gluconeogenesis bypasses the two irreversible glycolytic steps:
PFK-1 is replaced by fructose-1,6-bisphosphatase (F1,6BP \(\to\) F6P), and pyruvate kinase is replaced by the pyruvate carboxylase + PEP carboxykinase sequence (pyruvate \(\to\) OAA \(\to\) PEP).
Hence E1 and E4 are not enzymes of gluconeogenesis; E2 and E3 participate (reversible steps).