How is Malonyl-CoA primarily formed?

Malonyl-CoA is primarily formed through the carboxylation of Acetyl-CoA, a reaction catalyzed by the enzyme acetyl-CoA carboxylase. This process requires bicarbonate (HCO3) and ATP as cofactors. In this reaction, a carbon atom is added to Acetyl-CoA to form Malonyl-CoA, which plays a crucial role as a building block in fatty acid synthesis.

The importance of this formation lies in its contribution to fatty acid biosynthesis, where Malonyl-CoA provides the two-carbon units necessary for elongating the carbon chain of fatty acids. This conversion is a key regulatory step within lipid metabolism, making it significant for maintaining energy balance and mobilizing fats.

While other options mention various pathways, they do not accurately describe the primary method of Malonyl-CoA synthesis. Thus, this reaction shows how Acetyl-CoA interacts with bicarbonate to yield Malonyl-CoA, confirming the correctness of the selected answer.