Question

The pyruvate-phosphate dikinase (PPDK) is a key enzyme of photosynthesis and catalyses a reaction similar to the one catalyzed by the pyruvate kinase (PK)-a glycolytic enzyme. Which of the following statements about these two enzymes is true?

• A. PPDK catalyses an irreversible reaction.
• B. PPDK consumes one molecule of ATP for each molecule of pyruvate converted into PEP.
• C. PK catalyses a reversible reaction.
• D. PK consumes one molecule of ATP for each molecule of PEP converted into pyruvate.

Hint:

Although PPDK consumes only one ATP molecule, the subsequent hydrolysis of pyrophosphate ($\text{PP}_i$) means that converting pyruvate to PEP energetically costs the cell the equivalent of two high-energy phosphate bonds (ATP to AMP).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Pyruvate Kinase (PK) and Pyruvate-Phosphate Dikinase (PPDK) are enzymes that catalyze interconversions between pyruvate and phosphoenolpyruvate (PEP).
PK operates in glycolysis, while PPDK plays a key role in the $\text{C}_4$ and CAM photosynthetic pathways.

Step 2: Detailed Explanation:
Let us analyze the biochemical reactions catalyzed by both enzymes to evaluate the statements:
1. Pyruvate Kinase (PK): This glycolytic enzyme catalyzes the transfer of a high-energy phosphoryl group from PEP to ADP:
\[ \text{PEP} + \text{ADP} \rightarrow \text{Pyruvate} + \text{ATP} \]
This reaction is highly exergonic and physiologically irreversible in the cell.
It produces one molecule of ATP rather than consuming it.
Therefore, statements (C) and (D) are incorrect.
2. Pyruvate-Phosphate Dikinase (PPDK): This enzyme catalyzes the reversible conversion of pyruvate, ATP, and inorganic phosphate ($\text{P}_i$) into PEP, AMP, and pyrophosphate ($\text{PP}_i$):
\[ \text{Pyruvate} + \text{ATP} + \text{P}_i \rightleftharpoons \text{PEP} + \text{AMP} + \text{PP}_i \]
This reaction is reversible in vitro. In vivo, it is driven forward toward PEP formation by the rapid hydrolysis of the pyrophosphate product by pyrophosphatase.
Therefore, statement (A) is incorrect.
The reaction consumes exactly one molecule of ATP for each molecule of pyruvate converted into PEP, although the ATP is cleaved into AMP and $\text{PP}_i$.
Therefore, statement (B) is correct.

Step 3: Final Answer:
The true statement is that PPDK consumes one molecule of ATP for each molecule of pyruvate converted into PEP, corresponding to option (B).