How many ATP are required per 2 carbon units added during fatty acid synthesis in eukaryotes?

In eukaryotic fatty acid synthesis, the process of elongating the fatty acid chain involves the addition of 2-carbon units derived from acetyl CoA. For each 2-carbon unit that is incorporated into the growing fatty acid chain, 3 ATP equivalents are consumed.

This consumption of ATP occurs during the conversion of acetyl CoA to malonyl CoA, which is the first committed step in fatty acid synthesis. The enzyme acetyl-CoA carboxylase catalyzes this reaction and requires ATP for the carboxylation of acetyl CoA to form malonyl CoA. Specifically, the energy from ATP is used to convert ATP to ADP and inorganic phosphate, allowing this crucial carboxylation to occur.

Additionally, during the subsequent steps of fatty acid synthesis, each elongation step involves the reduction of the carbon chain using NADPH, but the direct ATP cost associated with the addition of 2-carbon units remains focused on the initial carboxylation step. Thus, when looking at the overall energetic cost associated with adding 2-carbon units during fatty acid synthesis in eukaryotes, 3 ATP equivalents are needed for each 2-carbon unit added, confirming that the right answer is