Fatty-acyl-CoA Synthase, or more commonly known as yeast fatty acid synthase (and not to be confused with Long Chain fatty acyl-CoA synthetase), is an enzyme complex responsible for fatty acid biosynthesis, and is of Type I Fatty Acid Synthesis (FAS). Yeast fatty acid synthase plays a pivotal role in fatty acid synthesis. It is a 2.6 MDa barrel shaped complex and is composed of two, unique multi-functional subunits: alpha and beta. Together, the alpha and beta units are arranged in an α6β6 structure. The catalytic activities of this enzyme complex involves a coordination system of enzymatic reactions between the alpha and beta subunits. The enzyme complex therefore consists of six functional centers for fatty acid synthesis. Reaction The enzyme catalyzes the reaction: Acetyl-CoA + n malonyl-CoA + 4n NADPH + 4n H+ long-chain-acyl-CoA + n CoA + n CO2 + 4n NADP+ The 4 substrates of this enzyme are acetyl-CoA, malonyl-CoA, NADPH, and H+, whereas its 4 products are Acyl-CoA, CoA, CO2, and NADP+. More specifically, the FAS catalysis mechanism consumes an acetyl-coenzyme A (acetyl-CoA) and seven malonyl-CoA molecules to produce a Palmitoyl-CoA. Background Synthesis of fatty acids is generally performed by fatty acid synthase (FAS). Though the syntheses of fatty acids are very similar across all organisms, the enzymes and subsequent enzymatic mechanisms involved in fatty acid synthesis vary between eukaryotes and prokaryotes. There are two types of fatty acid synthesis (FAS) mechanisms: type I FAS and type II FAS. Type I FAS exists in eukaryotes, including mammalian cells and fungi. Type II FAS are found in prokaryotes. The type I FAS system utilizes a multi-enzyme complex, which are highly integrated, while the type II FAS system utilizes individual, separate enzymes to catalyze the reactions involved in fatty acid synthesis. Yeast fatty acyl synthase belongs to the Type I FAS and was the first of Type I FAS to be studied. Structure Yeast fatty acyl synthase, of Type I FAS, is composed of a α6β6 complex in which an αβ unit forms one functional center for fatty acid synthesis. Yeast fatty acyl synthase therefore has six reaction units for its fatty acid synthesis, in which each of these units function independently from one another. Each α and β subunit, in turn, has four functional domains, and together, the eight functional domains catalyze all the reactions of fatty acid synthesis in yeast, which includes: activation, priming, elongation, and termination. Consequently,