Question

All of the following statements about acetyl-CoA carboxylase are correct except A. it catalyzes the rate-limiting step of fatty acid synthesis. B. it requires biotin. C. it is inhibited by cAMP-mediated phosphorylation. D. it is activated by palmitoyl CoA. E. its content in a cell responds to changes in fat content in the diet.

Short answer

Answer: D. Acetyl-CoA carboxylase is activated by palmitoyl CoA.

Step-by-step solution

Statement A

Acetyl-CoA carboxylase catalyzes the rate-limiting step of fatty acid synthesis. This statement is correct. Acetyl-CoA carboxylase converts acetyl-CoA to malonyl-CoA, which is the rate-limiting step in fatty acid synthesis.

Statement B

Acetyl-CoA carboxylase requires biotin. This statement is also correct. Biotin is a coenzyme that is essential for the carboxylation reaction performed by acetyl-CoA carboxylase.

Statement C

Acetyl-CoA carboxylase is inhibited by cAMP-mediated phosphorylation. This statement is correct as well. The enzyme is regulated by both allosteric factors and covalent modification through phosphorylation, which is mediated by the cAMP-activated protein kinase A.

Statement D

Acetyl-CoA carboxylase is activated by palmitoyl CoA. This statement is incorrect. In fact, the enzyme is inhibited by palmitoyl CoA, a long-chain fatty acyl-CoA, which reflects that when the products of the fatty acid synthesis pathway are abundant, the enzyme is downregulated to prevent excessive synthesis.

Statement E

Acetyl-CoA carboxylase content in a cell responds to changes in fat content in the diet. This statement is correct. The enzyme is regulated at both the transcriptional and post-translational levels, and the content of the enzyme in a cell is influenced by nutritional factors and hormones. Based on the analysis of each statement, the incorrect statement about acetyl-CoA carboxylase is: D. Acetyl-CoA carboxylase is activated by palmitoyl CoA.