Fatty acid synthase (FAS) is the enzyme responsible for de novo synthesis of fatty acids from acetyl-CoA, malonyl-CoA and NADPH. FAS (550 kDa) is a homodimer of two multifunctional polypeptides, each with seven distinct catalytic activities and a site for the prosthetic group, 4’- phosphopantetheine, acyl carrier protein (ACP). These domains are organized from the N- to the C-terminus as follows: keto acylsynthase, acetyl/malonyl transacetylase, dehydratase, the interdomain, enoyl reductase, ketoreductase, ACP and thioesterase. The two polypeptides are held together through the interdomain and oriented in an anti-parallel manner, each contributing complementary half sites and giving rise to two independently active centers for palmitate synthesis. Interest in FAS arises from its involvement in human disorders, such as obesity, hyperlipidemia and carcinogenesis.

Human FAS purified from a breast cancer cell line, ZR75-1, was vitrified at 50-70 μg/ml on holey grids in the presence of NADPH and acetyl-CoA and kept at -166°C in a Gatan 626 cryo-holder.