The Compartmentation of Acetyl-Coenzyme A Carboxylase in Plants

Abstract

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 CO, 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 11) of fatty acid synthase is found in plants. The synthase is composed of severa1 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:In plant cells, large amounts of malonyl-COA are needed in the plastids to sustain fatty acid synthesis, but malonyl-COA is also needed in the cytosol for the elongation of fatty acids exported from the plastids and for the synthesis of flavonoids and phytoalexins. As with fatty acid synthase, ACCase also occurs in prokaryotic and eukaryotic forms in nature. The prokaryotic form is composed of dissociable polypeptides, whereas the eukaryotic form is a homodimer of a multifunctional protein. But which form(s) of this enzyme occur(s) in plants? Both or only one? This biochemical mystery, which has been around since 1972, has finally been solved and the answer is intriguing, both from a ' Present address:

Seletividade do herbicida fluazifop-p-butil para cucurbitáceas

“…Espécies gramíneas apresentam células apenas com uma forma de ACCase, tanto no citoplasma quanto no estroma dos cloroplastos. Já nas dicotiledôneas a forma presente no citoplasma seria equivalente à das gramíneas, enquanto a presente nos cloroplastos seria insensível à ação desses herbicidas e responderia por toda síntese de lipídios quando a enzima citoplasmática estivesse inibida (Sasaki et al, 1995). Pela hipótese antiauxina, a seletividade se daria pela maior capacidade de repolarização da membrana das dicotiledôneas, quando na presença dos herbicidas (Holtum et al, 1991).…”

An Isoleucine Residue within the Carboxyl-Transferase Domain of Multidomain Acetyl-Coenzyme A Carboxylase Is a Major Determinant of Sensitivity to Aryloxyphenoxypropionate But Not to Cyclohexanedione Inhibitors

“…This reaction is a two-step process, consisting of the ATP-dependent carboxylation of the biotin group on the carboxyl carrier domain by the biotin-carboxylase activity, followed by the transfer of the carboxyl group from biotin to acetyl-CoA by the carboxyl-transferase (CT) activity. In plants, two ACCase isoforms are found in the cytosol and in the chloroplast, respectively (Sasaki et al, 1995;Konishi et al, 1996). The cytosolic ACCase isoform in all plants studied so far is a multidomain enzyme.…”

“…The cytosolic ACCase isoform in all plants studied so far is a multidomain enzyme. It provides malonyl-CoA for the synthesis of very long-chain fatty acids and flavonoids and for malonylation (Sasaki et al, 1995). The chloroplastic ACCase isoform catalyzes the first committed step in fatty acid biosynthesis.…”

Targeting of the Arabidopsis Homomeric Acetyl-Coenzyme A Carboxylase to Plastids of Rapeseeds

“…There are two ACCase structural types in plants (reviewed by Sasaki et al, 1995), and each type presents challenges to designing overexpression experiments. The heteromeric ACCase type, also referred to as the multisubunit or prokaryotic type, is present in plastids of dicotyledons and of non-Gramineae monocotyledons (Konishi et al, 1996).…”