Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : The addition of fluoride to fermenting yeast extracts causes the buildup of 3-phosphoglycerate and 2-phosphoglycerate.
Reason (R) : The fluoride strongly inhibits the enolase enzyme in the presence of inorganic phosphate ($\text{P}_i$) by forming a tightly bound complex with the $\text{Mg}^{2+}$ at the enzymes's active site.
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

This inhibitory mechanism is why sodium fluoride ($\text{NaF}$) is added to blood collection tubes used for glucose testing. It blocks glycolysis, preventing red blood cells from consuming glucose and ensuring accurate blood sugar measurements over time.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Glycolysis is the metabolic pathway that converts glucose into pyruvate.
Specific chemical inhibitors can target key enzymes in this pathway, blocking glycolysis and causing preceding metabolic intermediates to accumulate.

Step 2: Detailed Explanation:
Let us evaluate both the Assertion and the Reason:
- Reason (R): Enolase is the ninth enzyme of glycolysis, catalyzing the dehydration of 2-phosphoglycerate (2-PG) to phosphoenolpyruvate (PEP).
This enzyme requires magnesium ions ($\text{Mg}^{2+}$) as cofactors to coordinate with the substrate at the active site.
In the presence of inorganic phosphate ($\text{P}_i$), fluoride ions ($\text{F}^-$) bind with $\text{Mg}^{2+}$ and phosphate to form a highly stable magnesium fluorophosphate complex at the active site, competitively and strongly inhibiting enolase.
Thus, Reason (R) is true.
- Assertion (A): Inhibiting enolase prevents the conversion of 2-phosphoglycerate (2-PG) to phosphoenolpyruvate (PEP).
As a result, 2-PG accumulates in the cell.
Because the preceding step in glycolysis—catalyzed by phosphoglycerate mutase—is freely reversible, 2-phosphoglycerate is in rapid equilibrium with 3-phosphoglycerate (3-PG):
\[ \text{3-phosphoglycerate} \rightleftharpoons \text{2-phosphoglycerate} \]
As 2-PG accumulates, the equilibrium shifts, causing a corresponding buildup of 3-phosphoglycerate.
Therefore, adding fluoride causes both 2-PG and 3-PG to accumulate in fermenting extracts.
Thus, Assertion (A) is true, and the mechanism described in Reason (R) directly explains this accumulation.

Step 3: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation of (A), corresponding to option (A).