BIOCHEMICALAND MOLECULARBIOLOGICALCHARACTERIZATIONOFACETYL-CoA CARBOXYLASES

Acetyl-CoA carboxylase (ACCase) catalyzes the ATP-dependent carboxylation of acetyl-CoA to form malonyl-CoA. Classically this reaction is considered to be the first, rate-limiting reaction of de novo fatty acid biosynthesis. In contrast to all other organisms, which synthesize fatty acids in the cytosol, plants are unique in that de novo fatty acid biosynthe sis occurs almost exclusively in plastids . In addition , plants utilize malonyl-CoA for the synthesis of a number of secondary metabolites, including flavonoids, very long chain fatty acids, stilbenoids, and many malonyl derivatives; these reactions are thought to occur in the cytosol of plant cells (1,2, introduction of reference #3). To characterize how malonyl-CoA is generated from acetyl­ CoA, in at least two compartments, we have been characterizing ACCase via a comprehensive study of biotinylated proteins . Our studies, and those from other labs, indicate that plants contain at least two structurally distinct ACCases. One form of ACCase has a biotin-containing subunit of approximately 240 kDa, and a second form has a biotin-containing polypeptide of about 60 kDa.