Thumbnail: Fatty acid synthase. The construction of fatty acids is a stepwise process, adding carbon atoms two at a time to the growing chain. It takes four separate chemical reactions at each step, first adding the carbon atoms and then reducing them to the proper hydrocarbon form. It also takes several additional enzymes to start the process, and a few more to clip off the finished fatty acid at the end. Fatty acid synthase is a multi-enzyme protein that catalyzes fatty acid synthesis. It is not a single enzyme but a whole enzymatic system composed of two identical 272 kDa multifunctional polypeptides, in which substrates are handed from one functional domain to the next.