Acyl- higher plants

In higher plants, animals, protozoa, and fungi, saturated fatty acids are acted upon by desaturases to introduce double bonds, usually of the cis (Z) configuration. The substrates may be fatty acyl-ACP, fatty acyl-CoA molecules, membrane phospholipids,97 or glycolipids.98 The A9 desaturase, isolated from liver or from yeast, converts stearoyl-CoA to oleoyl-CoA (Eq. 21-3).99-102 This membrane-associated enzyme system... 

Siegenthaler PA, Tremolieres A. Role of Acyl Lipids in the Fnnc-tion of Photosynthetic Membranes in Higher Plants. In Lipids in Photosynthesis Structure, Function and Genetics. Siegenthaler PA and Murata N, eds. 1998. Klnwer Academic Publishers, Dordrecht, Holland, pp. 145-173... 

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... 

Particulate preparations from higher plants will esterify plant sterols diacyl glycerols are the most effective acyl donors. However in Phycomyces blakesleamis the donor is phosphatidylcholine [5]. 

Phytotoxic diloroacetamides provided a linear relationship between severe inhibition of growth and inhibition of the incorporation of [ CJoleic acid into VLCFAs in Scenedesmus acutus [7]. In higher plants, the incorporation of [ C] stearic acid or malonyl-CoA into VLCFAs was inhibited by diloroacetamides while the formation of fatty acids up to Cig was not influenced [8]. Acyl elongation with 20 0-CoA and 18 0-CoA primer substrates was inhibited by the active (S)-enantiomer of metolachlor but not by the (R)-isomer [1, 9). Inhibition of VLCFA formation was also observed in metazachlor-resistance mutant (Mz-1) cells of S. acutus [9]. Thus, the phytotoxic action of chloroacetamide herbicides is most likely by the inhibition of VLCFA synthesis. 

These are glycosides of N-acyl long-chain bases (ceramides). Galactose and glucose are the monosaccharides commonly found. The structures of two representative molecules - one from mammalian brain and the other from a higher plant - are shown below. 

Chloroplasts from photosynthetic tissues of higher plants and algae contain -3-hexadecenoate (8) (Fig. 2.13), an unusual palmitate-derived fatty acid. Desaturation of the 3-position requires light and may occur after the palmitic acid precursor is attached to the acyl moiety. This acid is found almost entirely at the 2-position of phosphatidyl glycerol (9) (Fig. 2.2). 

Eichenberger, W., Steryl glycosides and acylated steryl glycosides, in Lipids and Lipid Polymers in Higher Plants (M. Tevini and H. K. Lichtenthaler eds.), 169-182, Springer-Verlag, Berlin, 1977. 

In addition, limited information is available concerning the lipid content of flowers (Thompson and Hedin, 1965 Thompson et al., 1968) and pollen (Opute, 1975). Many of the acyl lipids and fatty acids found in such large amounts in higher plants are also present in algae, mosses, ferns, and yeast, though in general they have been less studied (but cf. Wassef, 1977 Levring et al., 1969 Weete, 1974). 

Acyl lipids are not evenly distributed within the cell. Indeed, if the rapidly accumulating evidence from microorganisms and mammals holds good for higher plants, membranes probably also have a distinct sidedness with relation to lipids. With the rapid improvements in techniques of subcellular fractionation over recent years, information has quickly built up with regard to the lipids of cellular membranes. A recent review by Mazliak (1977) is a good source of information. The main components of different subcellular organelles and membranes are listed in Table XIV. 

There is increasing evidence that in plant tissues the synthesis of palmitic acid, the most abundant and important saturated fatty acid in higher plants, involves at/e novo system, so called because it utilizes acetyl-CoA, malonyl-CoA, acyl carrier protein (ACP), and a battery of soluble, nonassociated enzymes (palmitoyl-ACP synthetase) to form, as its terminal product, palmi-toyl-ACP. Palmitoyl-ACP is then elongated by another set of enzymes called... 

The most important saturated fatty acid in higher plants is palmitic acid. Stearic acid, in contrast, occurs in low amounts as an acyl component of complex lipids. While the individual enzymes for the synthesis of palmitic acid have not been examined, there is no evidence at present that they are associated. There is now increasing evidence that the initial product of de novo fatty acid synthesis in plants is palmitoyl-ACP. The components of systems for the synthesis of palmitoyl-ACP include ACP, NADPH, NADH, acetyl-CoA, and malonyl CoA. In support of this, the following observations may be cited ... 

Fig. 11. Various hypotheses proposed by which higher plants may attain high levels of unsaturated fatty acids in their chloroplast membrane galactolipids. (a) Phosphatidylcholine acts as a carrier molecule involved in the desaturation, (b) Desaturation of fatty acids occurs after formation of the galactolipid molecule, (c) Desaturation occurs before formation of the galactolipid molecule. In the first hypothesis, all the desaturases involved are confined in the chloroplast in the second hypothesis, the conversion of 18 1 to 18 2 is maximal in microsomes," whereas desaturation of 18 2 to 18 3 is highest in chloroplast membranes, (d) Deacylation-reacylation mechanism in which X can be a CoA-thioester, a polar lipid, etc. D, Desaturases T, acyl-ACP thioesterase e.r., endoplasmic reticulum.

Two major groups of phenols exist that are formed by the sequential addition of C2 units to a growing chain. Acetyl-CoA is generally used as the primer in fungal systems, whereas a shikimic acid-derived acyl-CoA is required for flavonoid synthesis in higher plants. Flavonoids are multiringed phenols found most commonly in flowers and fruit, where they are responsible for the color. 

A number of further questions related to phenol synthesis may be asked, and most remain to be answered satisfactorily. What initiates development of the synthetase responsible for the appropriate acyl transfer and condensation reactions What controls the site of reductase and dehydratase activity when these are expressed And what brings about the release of single- or multiringed products Similar points may also be raised with regard to the formation of flavonoids and other phenolic products in higher plants. 

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