In mammals, fatty acid utilization is initiated by activation of fatty acid, catalyzed by acyl-CoA synthetase(ACS, EC6.2.1.3). This enzyme reaction is essential in fatty acid metabolism, since mammalian fatty acid synthetase contains a specific thioesterase to produce fatty acid as th $varepsilon$ final reaction product. Acyl-CoA, the product of ACS, is utilized in various metabolic pathways including membrane biogenesis, energy production and fat deposition. (omitted)