The Compartmentation of Acetyl-Coenzyme A Carboxylase in Plants

Although the biochemical pathways for fatty acid synthesis are more or less similar in plants and animals (Harwood, 1988), there is a major cell biological difference between these two groups of eukaryotes. In plants, the major site of fatty acid synthesis is the plastid, an organelle absent from the animal cell. Many aspects of plastid biology, including fatty acid synthesis, reflect the organelle's origins as a prokaryotic symbiont. The synthesis of fatty acids, such as palmitic acid, the prototype 16-carbon fatty acid, requires one molecule of acetyl-CoA and seven molecules of malonyl-CoA, which are added sequentially with the addition of two carbons to the growing fatty acid chain and the release of CO2 at each step. These reactions are catalyzed by fatty acid synthase, an enzyme complex known to exist in a prokaryotic and a eukaryotic form (Wakil et al., 1983; Harwood, 1988). The prokaryotic form (type II) of fatty acid synthase is found in plants. The synthase is composed of several dissociable proteins, whereas the eukaryotic form (type I) found in animals and yeasts is composed of one or two large multifunctional, nondissociable proteins. For either form, the synthesis requires malonyl-CoA, which is supplied by ACCase in the following reaction: