Question

Which of the following statements about the enzyme acetyl $\text{CoA}$ carboxylase (ACCase), involved in the biosynthesis of fatty acids, is incorrect?}

• A. It catalyses ATP-dependent conversion of acetyl CoA into malonyl CoA
• B. It is a member of a family of biotin-dependent carboxylases
• C. Reaction catalysed by ACCase is a single-step reaction
• D. The ACCase in plants could be multifunctional or multisubunit type

Hint:

ACCase is a multi-functional or multi-subunit enzyme that carries out carboxylation through two distinct catalytic steps:
1. Carboxylation of the biotin carrier protein.
2. Transfer of the carboxyl group to acetyl-CoA.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Acetyl-CoA carboxylase (ACCase) is a key regulatory enzyme in lipid metabolism.
It catalyzes the irreversible carboxylation of acetyl-CoA to produce malonyl-CoA, which provides the building blocks for fatty acid elongation.

Step 2: Detailed Explanation:
Let us evaluate each of the statements concerning ACCase:
1. Statement A: ACCase catalyzes the ATP-dependent conversion of acetyl-CoA and bicarbonate into malonyl-CoA, which is the committed step of de novo fatty acid synthesis. This statement is correct.
2. Statement B: ACCase belongs to the family of biotin-dependent carboxylases and uses a covalently bound biotin prosthetic group to transfer carboxyl groups. This statement is correct.
3. Statement C: The reaction catalyzed by ACCase is a two-step reaction, not a single-step reaction.
In the first step, biotin carboxylase (BC) catalyzes the carboxylation of the biotin cofactor on the biotin carboxyl carrier protein (BCCP), utilizing ATP and bicarbonate:
\[ \text{BCCP-biotin} + \text{HCO}_3^- + \text{ATP} \rightarrow \text{BCCP-biotin-CO}_2 + \text{ADP} + \text{P}_i \]
In the second step, carboxyltransferase (CT) transfers the activated carboxyl group from biotin to acetyl-CoA to form malonyl-CoA:
\[ \text{BCCP-biotin-CO}_2 + \text{acetyl-CoA} \rightarrow \text{BCCP-biotin} + \text{malonyl-CoA} \]
Therefore, Statement C is incorrect.
4. Statement D: In plants, ACCase exists in two forms: a multi-subunit prokaryotic-type in plastic stroma (composed of distinct BC, BCCP, and CT proteins) and a single-chain, multi-functional eukaryotic-type in the cytosol. This statement is correct.

Step 3: Final Answer:
The incorrect statement is that the reaction catalyzed by ACCase is a single-step reaction, which corresponds to Option (C).