Question

Acetyl-CoA carboxylase is an enzyme in fatty acid synthesis. In response to increasing fatty acid levels, ACC activity

increases and the number of R form subunits increase, decreases and the number of T form subunits increase, increases and the number of R form subunits decrease, or decreases and the number of T form subunits decrease.

Answer 1

Acetyl CoA carboxylase is an enzyme that carboxylates acetyl CoA to malonyl CoA. This conversion requires the biotin carboxyl carrier protein (BCCP), which is covalently bound by biotin. It is primarily involved in converting citrate to long chain fatty acids.

Citrate is known to cause ACC octamers to polymerize and form active filaments. When there are free fatty acids present in medium, they will be used to form palmitoyl CoA. Palmitoyl CoA causes the ACC filaments to disassemble and form inactive octamers. This, effect of citrate is inhibited by palmitoyl CoA on ACC. Hence, acetyl CoA will no longer convert acetyl CoA to malonyl CoA. Palmitoyl CoA also inhibits the translocase protein that transports citrate to cytosol from the mitochondria for action of ACC. ACC is a cytoplasmic enzyme. Thus, free fatty acids will decrease ACC action. R (relaxed) form of the enzyme is the ACC filaments and T (tense) form is the inactive octamer. As free fatty acid promote inactive octamer formation, the T form subunits increases.

Right choice: decreases and the number of T form subunits increase.