Fatty acids serve a multitude of roles inside cells that have a huge impact on cellular energy metabolism and signaling. Upon entering the cell, fatty acids must first be activated to acyl-CoAs prior to their metabolism. The Mashek Laboratory has worked on numerous acyl-CoA synthetases that catalyze this reaction. However, recently we have become interested in studying Acyl-CoA thioesterases (ACOTs), the family of enzymes that catalyze the reverse reaction, the generation of free fatty acids from acyl-CoA. We are particularly interested in ACOT1, a cytosolic ACOT that is higly induced by fasting. Since fatty acis are produced from so many sources (uptake, synthesis, hydrolysis, etc.), the question is why does this enzyme even exist? Why would we need another enzyme to produce fatty acids that seemingly just antagonize fatty acid metabolism? Ongoing studies are dissecting the role of this enzyme in whole-body and hepatic energy metabolism and in lipid signaling to influence these processes.