Plant acyl carrier protein

Ohlrogge, J.B. (1987) Biochemistry of plant acyl carrier proteins, in The Biochemistry of Plants, Vol. 9, Lipids Structure and Function, ed. P.K. Stumpf, Academic Press, Orlando, FL, pp. 137-157. 

Guerra, D.J., J.B. Ohlrogge and M. Frentzen. 1987. A possible differential role for plant acyl carrier protein isoforms in higher plants. In Stumpf, P.K., B. Mudd and D. Nes, eds.. The Metabolism, Structure and Function of Plant Lipids. Plenum Publishing Co. 

A Possible Differential Role for Plant Acyl Carrier Protein... 

Ohlrogge, J. B., The Biochemistry of Plant Acyl Carrier Proteins. 

Sephadex chromatography (6). Plant acyl carrier proteins were purified as described (10), except that the heat step was omitted. Assays for the thloesterase and the acyl transferase were as described (8). 

In bacteria and plants, the individual enzymes of the fatty acid synthase system are separate, and the acyl radicals are found in combination with a protein called the acyl carrier protein (ACP). However, in yeast, mammals, and birds, the synthase system is a multienzyme polypeptide complex that incorporates ACP, which takes over the role of CoA. It contains the vitamin pantothenic acid in the form of 4 -phosphopan-tetheine (Figure 45-18). The use of one multienzyme functional unit has the advantages of achieving the effect of compartmentalization of the process within the cell without the erection of permeability barriers, and synthesis of all enzymes in the complex is coordinated since it is encoded by a single gene. 

In green plants a soluble A9 stearoyl-acyl carrier protein desaturase uses 02 and NADH or NADPH to introduce a double bond into fatty acids. The structure of this protein (Fig. 16-20B,C) is related to those of methane oxygenase and ribonucleotide reductase.333347 Tire desaturase mechanism is discussed in Chapter 21. 

Both bacteria and plants have separate enzymes that catalyze the individual steps in the biosynthetic sequence (Fig. 17-12). The fatty acyl group grows while attached to the small acyl carrier protein (ACP).54 58 Control of the process is provided, in part, by the existence of isoenzyme forms. For example, in E. coli there are three different P-oxoacyl-ACP synthases. They carry out the transfer of any acyl primer from ACP to the enzyme, decarboxylate malonyl-ACP, and carry out the Claisen condensation (steps b, e, and/in Eq. 17-12)58a e One of the isoenzymes is specialized for the initial elongation of acetyl-ACP and also provides feedback regulation.58c The other two function specifically in synthesis of unsaturated fatty acids. 

Figure 21-3 Major pathways of synthesis of fatty acids and glycerolipids in the green plant Arabidopsis. The major site of fatty acid synthesis is chloroplasts. Most is exported to the cytosol as oleic acid (18 1). After conversion to its coenzyme A derivative it is converted to phosphatidic acid (PA), diacylglycerol (DAG), and the phospholipids phosphatidylcholine (PC), phosphatidylinositol (PI), phosphatidylglycerol (PG), and phosphatidylethanolamine (PE). Desaturation also occurs, and some linoleic and linolenic acids are returned to the chloroplasts. See text also. From Sommerville and Browse.106 See also Figs. 21-4 and 21-5. Other abbreviations monogalactosyldiacylglycerol (MGD), digalactosyldiacylglycerol (DGD), sulfolipid (SL), glycerol 3-phosphate (G3P), lysophosphatidic acid (LPA), acyl carrier protein (ACP), cytidine diphosphate-DAG (CDP-DAG).

Suh, M.C., Schultz, D.J. and Ohlrogge, J.B. (1 999) Isoforms of acyl carrier protein involved in seed specific fatty acid synthesis. Plant Journal 1 7(6), 679-688. 

Reipa V, Shanklin J, Vilker V. Substrate binding and the presence of ferredoxin affect the redox properties of the soluble plant Delta9-18 0-acyl carrier protein desaturase. Chem. Coimnun. (Camb). 2004 21 2406-2407. 

In plant systems, de novo synthesis occurs in the plastid and results mainly in the conversion of acetate to palmitate. All 16 carbon atoms in palmitic acid are derived from acetate— half from the methyl carbon and half from the acyl carbon. Two of the carbon atoms (C-15 and C-16) come directly from acetate, and the other 14 come from acetate via the more reactive malonate. Production of malonate requires the incorporation of an additional carbon atom into the acetyl group. This is supplied as bicarbonate, and this same carbon atom is subsequently lost as carbon dioxide. The acyl groups are attached to co-enzyme A (CoASH) during part of the cycle and to acyl carrier protein (ACPSH) during another part. The abbreviated symbols used for these co-enzymes emphasize the thiol groups (SH) to which the acyl chains are attached. 

In mammals Fatty Acid Synthase (FAS) catalyzes fatty acid synthesis on a homodimeric enzyme, each monomer of which has seven catalytic activities, and eight sites (In bacteria such as E. colt there are seven separate enzymes plus an acyl-carrier protein. Plants also have individual proteins for the various activities which are associated in a quaternary complex. In eukaryotes other than plants the FAS are complexes of multifunctional proteins. The enzyme weighs approximately 500,000 Daltons. 

Pantothenic acid is of ubiquitous occurrence in nature, where it is synthesized by most microorganisms and plants fi-om pantoic acid (D-2,4-dihydroxy-3,3-dimethylbutyric acid) derived from L-vafine, and p-alanine derived from L-aspartate. Addition of cysteamine at the C-terminal end and phosphorylation at C4 of pantoic acid forms 4 -phosphopantetheine, which serves as a covalently attached prosthetic group of acyl carrier proteins, and, when attached... 

The type III plant and bacterial synthases feature the least complex architecture among the three PKS types, occurring as comparatively small homodimers possessing subunits between 40-45 kDa in size. As in the case for type II enzymes, type III PKSs catalyze iterative decarboxylative condensation reactions typically using malonyl-CoA extender units, however in contrast to type II synthases, the subsequent cyclization and aromatization of the nascent polyketide chains occurs within the same enzyme active site (25). Also unique to this family of PKSs, free CoA thioesters are used directly as substrates (both starter an extender units) without the involvement of acyl carrier proteins. 

The soluble stearoyl-acyl carrier protein A desaturase from higher plants such as castor, cucumber, spinach, turnip, rape, and avocado catalyzes an important step in plant lipid desaturation (27, 28, 45). The reaction is important for regulation of membrane fluidity and is also an important factor in nutrition. 

A-ACPs in higher plants are homodimeric soluble proteins of about 70 kDa. They catalyze the first and most important desaturation of stearoyl-acyl carrier protein, resulting in an oleoyl-acyl carrier protein, which is a major precursor in fatty acid biosynthesis in plants. A-ACP is involved in controlling the ratio of saturated to unsaturated... 

According to its solubility these enzymes can be classified into two non evolutively related groups the soluble acyl carrier protein (AGP) desaturases and the membrane-bound desaturases, which includes the acyl-lipid desaturases and the acyl-CoA desaturases. The soluble AGP desaturases introduce double bonds into fatty acids esterified to AGP, and are found in the stroma of plant plastids (Shanklin and Gaboon, 1998) and some bacteria, as Mycobacterium and Streptomyces (Phetsuksiri et al., 2003). The acyl-lipid desaturases, that... 

Early studies by Overath and Stumpf (P. Overath, 1964) established not only that the constituents of the avocado fatty acid synthesis system could be dissociated and reconstituted, but also that the heat stable fraction from E. coli known as acyl carrier protein (ACP) could replace the corresponding fraction from avocado. Plant ACPs share both extensive sequence homology and significant elements of three-dimensional structure with their bacterial counterparts. In plants, this small, acidic protein not only holds the growing acyl chain during fatty acid synthesis, but also is required for synthesis of monounsaturated fatty acids and plastidial glycerolipids. 

Jones, A., Davies, H.M., Voelker, T.A. 1995. Palmitoyl-acyl carrier protein (ACP) thioesterase and the evolutionary origin of plant acyl-ACP thioesterases. Plant Cell 7 359-371. 

---