The biosynthesis of saturated fatty acids requires a primer molecule, usually acetic acid in the form of its Coenzyme A ester, and a chain extender, malonyl-CoA. The latter is formed from acetyl CoA by the activity of the enzyme acetyl-CoA carboxylase in which biotin is the prosthetic group (and thus can be inhibited by avidin). In the first step of the reaction, carbon dioxide is linked to the biotin moiety, and this is subsequently transferred to acetyl-CoA to form malonyl-CoA. In microorganisms such as Escherichia coli, the enzyme complex comprises three dissociable proteins, but in plants and animals, the enzyme is a single multifunctional complex that exists in two main isoforms. Malonyl-CoA is also involved in the regulation of fatty acid oxidation by inhibiting carnitine palmitoyl-CoA transferase-1.