Fatty acids synthesis in plants

The paradigm for fatty acid synthesis in plants has been the avocado, which has one of the highest fatty acid contents in the plant kingdom. Early animal studies centered primarily on... 

Fatty acid synthesis, in plants, 13 295 Fatty alcohols... 

FATTY ACID SYNTHESIS IN PLANTS Because of less intensive research efforts and several technical problems, plant fatty acid synthesis is less well... 

Metabolic control. The regulation of fatty acid synthesis in plants is poorly understood. It remains unclear whether the reaction catalyzed by acetyl-CoA carboxylase is a rate-limiting step in plants, because malonyl-CoA is used in several other biosynthetic pathways (e.g., bioflavonoid synthesis). (Recall that this reaction is rate-limiting in animal cells. See p. 390.)... 

How does fatty acid synthesis in plants differ from fatty acid synthesis in animals ... 

Fatty acid synthesis in plants differs from that in animals in the following ways location (plant fatty acid synthesis occurs mainly in the chloroplasts, whereas in animals fatty acid biosynthesis occurs in the cytoplasm), metabolic control (in animals the rate-limiting step is catalyzed by acetyl-CoA carboxylase, whereas in plants, this does not appear to be the case), enzyme structure (the structures of plant acetyl-CoA carboxylase and fatty acid synthetase are more closely related to similar enzymes in E. coli than to those in animals). 

These are chemical compounds that inhibit the acetyl-CoA carboxylase enzyme, which is responsible for catalyzing an early step in lipid and fatty acid synthesis in plants. Lipids are essential components of cell membranes, and without them,... 

Rawsthorne, S. (2002). Carbon flux and fatty acid synthesis in plants. Progress in Lipid Research, Vol.41, No.2, (March 2002), pp. 182-1%, ISSN 0163-7827... 

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 ... 

Roughan, G. Post-Beittenmiller, D. Ohlrogge, J. and Browse, J. (1993) Is acetylcarnitine a substrate for fatty acid synthesis in plants Plant Physiol, 101, 1 1 57-62. 

Yanada M., Kato M., Nishida I., Kawano K., Kawaguchi A. and Ehara T., 1987. Modulation of fatty acid synthesis in plants by thiolactomycin. In The metabolism, structure and function of plant lipids, Stumpf P.K., Mud3. B. and Nes W.D. ed.. Plenum Press, New York, pp. 447-454. 

Modulation of Fatty Acid Synthesis in Plants by Thiolactomycin M. Yamada, M. Kato, I. Nishida, K. Kawano, A. Kawaguchi and T. Ehara... 

MODULATION OF FATTY ACID SYNTHESIS IN PLANTS BY THIOLACTOMYCIN... 

De novo fatty acid synthesis in plant tissues appears to be catalysed by the operation of a high molecular weight (multifunctional protein) acetyl-CoA carboxylase and a Type II fatty acid synthetase. Elongation of the product of the synthetase, or of endogenous acids, is catalysed by Type III fatty acid synthetases with malonyl-CoA as the source of the additional carbons. 

Fatty acid synthesis in plants is carried out by a type I dissociable fatty acid synthase (FAS). There are three p-ketoacyl-ACP synthases (KAS) associated with FAS in plants. The short-chain condensing enzyme (KAS III) catalyses the initial condensation of acetyl-CoA with malonyl-ACP [1,2] to form acetoacetyl-ACP (4C) and may catalyse further rounds of condensation in vivo [3]. Further rounds of condensation are carried out by KAS I which catalyses the condensation of malonyl-ACP with intermediate length acyl-ACPs from acetoacetyl-ACP to myristoyl-ACP (14C) to give palmitoyl-ACP (16C). KAS II elongates palmItoyl-ACP (16C) to stearoyl-ACP (18C) [4J. 

Figure 3.13 Major pathways for polyunsaturated fatty acid synthesis in plants and algae.

The enzymes of fatty acid synthesis in animals are joined in a single polypeptide chain called fatty acid synthase. In contrast, the degradative enzymes do not seem to be associated. Plants employ separate enzymes to carry out the biosynthetic reactions. 

Wilson, R.F. H.H. Weissinger J.A. Buck G.D. Faulkner. Involvement of phospholipids in polyunsaturated fatty acid synthesis in developing soybean cotyledons. Plant Physiol. 1980, 66, 545-549. 

The studies on fatty acid synthesis in higher plants over the last 25 years have led to a consensus about the individual reactions and their localization in the cell. This consensus is that the enzyme system for fatty acid synthesis is procaryotic in nature, that is the enzymes are soluble and separable, and that the system is localized entirely in the plastld. Thus the membranes of the mitochondria, the endoplasmic reticulum, the plasmalemma, the tonoplast, the nuclear membrane, and the Golgi apparatus all depend for their fatty acid components on the activities of the plastids. In outline the reactions of fatty acid synthesis may be summarized ... 

The studies of the individual enzymes of fatty acid synthesis in higher plants has shown that the two reductive steps, p-ketoacyl ACP reductase and enoyl ACP reductase have different cofactor requirements. As a result the synthesis of fatty acids depends on the availability of both NADH and NADPH. While the provision of NADPH can be attributed to the photosynthetic reactions, the source of NADH in the chloroplast is less certain. Takahama etal (8) have demonstrated that the content of NADPH in the chloroplast is influenced by illumination as expected, but there is no such fluctuation of the oxidation state of NAD/NADH. The production of NADH to be utilized in fatty acid synthesis would therefore appear to depend on dark reactions. One possibility would be by the action of pyruvate dehydrogenase, which would generate not only the NADH required for reduction in fatty acid synthesis but also the precursor acetyl CoA. 

Rochester, C. and Silver, J. 1983. Unsaturated fatty acid synthesis in simflower (Helianthus annuus L.) seeds in response to night temperature. Plant Cell Reports 2 229-231. 

Fatty acid synthesis in bacteria and plants is a multistep process. One transformation in the process involves the reduction of the ketone unit in acetoacetyl AGP (94) with the enzyme [3-ketoacyl-ACP reductase and NADPH (91) to give (3-hydroxybutaryl-ACP (96) and NADP" (92). AGP is the acyl carrier protein, and it is bound to the acetoxy unit via a phosphopantetheine group (marked in cyan in 94). 

Acetyl-CoA carboxylase (ACCase) catalyses the ATP-dependant carboxylation of acetyl-CoA to form malonyl-CoA, thus providing the essential substrate for fatty acid biosynthesis. Dicotyledonous plants contain two forms of ACCase a multifunctional enzyme (typel) wich is presumed to be cytosolic, and a multi-subunit complex (typell) located in the plastid wich is responsible for de novo fatty acid synthesis. In prokaryotes, the ACCase is a type II enzyme comprising biotin carboxylase (BC), biotin carboxyl carrier protein (BCCP) and a carboxyl transferase with two subunits (CTa and CTp). The cDNA encoding the B.napus CXp and BCCP have already been cloned (Elborough et a/.,1995) as have the cDNAs encoding the BC and BCCP from tobacco and Arabidopsis respectively (Shorrosh et al, 1995 Choi et al.y 1995). 

Plant acetyl CoA carboxylase [EC 6.4.1.3] is one of the pivotal enzymes of fatty acid biosynthesis in both seed and leaf tissue and is thought to be an important regulatory step of de novo fatty acid synthesis in chloroplasts (Post Beittenmiller et al., 1992). Its central role reflects its importance as a target for commercial herbicides. Two forms of ACCase are present in dicot plants. The chloroplast is thought to be the site for de novo fatty acid synthesis in mesophyll cells and BCCP has been shown to reside within the chloroplast. It is therefore reasonable to suppose that a type II Brassica napus ACCase is mainly associated with de novo lipid synthesis. Specific herbicides differentiate between the two forms of acetyl CoA carboxylase (ACCase) found in dicotyledonous plants. 

Kang F and Rawsthome S (1994). Starch and fatty acid synthesis in plastids from developing embryos of oilseed rape Brassica napus L.). Plant J. 6 795-805. 

Smith, R.G., Gauthier, D.A., Dennis, D.T. and Turpin, D.H. (1992) Malate- and pymvate-dependent fatty acid synthesis in leucoplasts from developing castor endosperm., Plant Physiol 98, 1233-1238. 

The general mechanism of fatty acid biosynthesis in plants is well recognized (Harwood, 1988, 1989 Stumpf, 1989 Hills and Murphy, 1991 Somerville and Browse, 1991). De novo synthesis involves the conversion of acetyl-CoA to Cie and Cig fatty acyl moieties in seed and mesocarp... 

Harwood J L, Stumpf P K 1971 Fat metabolism in higher plants. XLIII. Control of fatty acid synthesis in germinating seeds. Arch Biochem Biophys 142 281-291... 

Macey M J K, Stumpf P K 1968 Fat metabolism in higher plants. 36 Long chain fatty acid synthesis in germinating peas. Plant Physiol 43 1637-1647... 

Bao, X. et al. (2000) Understanding in vivo carbon precursor supply for fatty acid synthesis in leaf tissue. Plant J. 22, 39-50... 

In animal cells and yeasts, multienzyme complexes localised in cytosol, referred to as type I fatty acid synthase (FAS I), carry out the bulk of the de novo fatty acid synthesis. In animals, it occurs primarily in the liver, adipose tissue, central nervous system and lactating mammary gland. FAS I contains seven distinct catalytic centres and is arranged around a central acyl carrier protein (ACP) containing bound pantothenic acid (see Section 5.9.1). In prokaryotes and plants, distinct soluble enzymes localised in mitochondria and plastids, referred to as type II fatty acid synthase (FAS II), carry out the reactions. 

Fan, J., Yan, C., Andre, C., Shanklin, J. et al (2012) Oil accumulation is controlled by carbon precursor supply for fatty acid synthesis in Chlamydomonas reinhardtii. Plant Cell Physiol, 53, 1380-1390. 

Chloroplasts play a central role in fatty add metabolism in the leaf cell not only do they synthesize fatty acids-and may In fact be the sole site of this synthesis-but they are also able to desaturate fatty acids. Desaturation of 18 0 to 18 1 occurs while the acyl group is esterified to AGP in the stroma the desaturase is a soluble protein, and requires both a source of electrons--reduced ferredoxin is sufficient-and oxygen (Jacobson et al, 1974 Mckeon and Stumpf, 1982). All other fatty acid desaturation in plant cells, in both the plastid and elsewhere, appears to occur while the fatty acids are esterified to glycerolipids (Roughan and Slack, 1982). This is in stark contrast to animal systems In which fatty acids esterified to CoA are the predominant substrates for desaturation. 

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