Chloroplast glycerolipids

As noted above, chloroplasts and other plastids are enriched in galactolipids (Fig. 1). They also contain a unique sulfolipid, sulfoquinovosyldiacylglycerol, whose head group is a modified galactose. The phospholipid components of plastids are less abundant. Phosphatidylglycerol, the most prominent phospholipid contributor to the thylakoid membrane system, comprises less than 10% of chloroplast glycerolipids, while plastidial phosphatidylcholine is limited primarily to the organelle s outer membrane. 

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).

In the plastids, acyltransferases provide a direct route for entrance of acyl groups from ACP to membrane lipids. Since this is the standard pathway for phosphatidic acid synthesis in E. coli and cyanobacteria, both the enzymes of phosphatidic acid synthesis in plastids and the glycerolipid backbones they produce are termed prokaryotic . In both chloroplasts and non-green plastids, the glycerol-3-phosphate acyltransferase is a soluble enzyme that, unlike the E. coli enzyme, shows preference for 18 1-ACP over 16 0-ACP. The lysophosphatidic acid acyltransferase, which is a component of the inner envelope of plastids, is extremely selective for 16 0-ACP. The presence of a 16-carbon fatty acid at the... 

The competitive binding experiments of Tischer and Strotmann (4) suggest that the phenylureas, biscarbamates, triazines, tria-zinones, and pyridazinones inhibit electron transport by interaction with the same component of PS II. Action at this site seemed to account for the phytotoxicity of pyrazon [5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone]. In addition to action at this site, compounds with molecular substitutions onto the structure of pyrazon (Figure 1) also interfere with the formation of chloroplast membrane lipids, namely the chlorophylls, carotenoids, and glycerolipids. 

Pyrazon inhibited PS II (Table I) but did not interfere with the accumulation of chloroplast pigments (Table II) or influence the composition of glycerolipids (Table III). Therefore, even though the other substituted pyridazinones also inhibit PS II, their actions on chloroplast pigments and/or glycerolipids are not likely to result from PS II inhibitions. Further evidence that inhibition of PS II does not result in membrane lipid changes is presented in Table IV. 

The interaction of the D1 and D2 proteins with other components of PS2 has been demonstrated by analysis of the composition of the isolated reaction centre preparation and reconstitution with the extrinsic 33 kD (manganese stabilising) protein, artifical electron donor and acceptor compounds, quinones and diacyl glycerolipids. The isolated reaction centre compiex was shown to contain three proteins in addition to D1 and D2 and these were identified as the products of the chloroplast genes psb E, F and 1. This preparation could bind the 33 kD protein under specific conditions and comparison with binding to a more complex PS2 core preparation indicated that binding is not greatly influenced by polypeptides other than those in the isolated reaction centre. 

DE NOVO BIOSYNTHESIS OF EUKARYOTIC GLYCEROLIPIDS IN CHLOROPLASTS OF TRANSGENIC TOBACCO PLANTS... 

De Novo Biosynthesis of Eukaryotic Glycerolipids in Chloroplasts of Transgenic Tobacco Plants. 

S.2 Fatty acyl-CoA transferases. The enzyme systems involved with fatty acyl-CoA utilization in the cytosol appear to be membrane-bound. Consequently, detailed knowledge of their individual structure, specificity and genetic control is generally lacking due to the particular inability to obtain ready isolation and purification of the relevant proteins. Studies, however, support the concept of the operation of the eukaryotic pathway for the production of glycerolipids and polyunsaturated fatty acid (Browse et al., 1990 Stymne et al., 1990). While this pathway may contribute a significant quantity of fatty acid for use in membrane synthesis in the plastid (chloroplast) (Browse et al., 1990), its major importance would seem to lie with the production of unsaturated oils (Frentzen, 1986). On the other hand the occurrence of the prokaryotic pathway in the plastid permits more direct membrane lipid formation in both 16 3 and 18 3 plants (Browse et al., 1990 Somerville and Browse, 1991). Different sets of acyltransferase may be associated with the two pathways (Hills and Murphy, 1991). 

The incorporation of 1- C-acetate and 2- C-pyruvate by circular spinach leaf sections (j0TO.5 cm), which had been freed of the lower epidermis and infiltrated in buffer medium is shown in Table 1. Similar to chloroplast suspensions the efficiency of lipid incorporation was significantly higher from acetate. Furthermore, radioactivity from 1- C-acetate appeared to accumulate in glycerolipids while that from... 

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. 

Acyl-(acyl-carrier protein) glycerol-3-phosphate acyl transferase (glycero-P acyltransferasc) in higher-plant chloroplasts transfers the acyl group from acyl-(acyl-carrier protein) to the C-1 position of glycerol 3-phosphate to synthesize 1-acylglycerol 3-phosphate. Since this reaction is the first step of glycerolipid synthesis in the chloroplasts, it is of special interest to study this enzyme. 

Pyrenocine A has bimodal effects on onion root and spinach chloroplast lipid metabolism. Low concentrations and shorter preincubation periods promote lipid synthesis from 14C-acetate while high concentrations and longer preincubation periods inhibit lipid synthesis. Maximum stimulation of lipid synthesis (approximately 75%) was observed when onion roots were preincubated for. 5 h with 25 Aig/ml pyrenocine A while maximum inhibition (approximately 80%) was observed when onion roots were pretreated with 100 ug/ml pyrenocine A for 24 h (Tables 1 and II). Over the range of onion treatments with pyrenocine A, there was a marked increase in the proportions of glycerolipid synthesized at the expense of sterols (50%... 

Other predications are (1) that SL In 16 3 plants must be synthesised from a sugar nucleotide and a mixture of procaryotic and eucaryotic DAGs, and (11) that dlsaturated molecular species of plant glycerolipids will be synthesised by the procaryotic pathway alone. Whereas there is no convincing evidence for dlsaturated PC or PE In plants. It Is now known that dlsaturated molecular species of PG may account for up to 50% of the total chloroplast PG in some plants S. 

Attached, expanding leaves of A. llvldus Incorporated 25% of the labeled acetate supplied Into total lipids within 60 minutes. The labeled glycerolipids were mainly PC, PG and MGD 60, 12 and 12% respectively. Saturated fatty acids accounted for more than 70% of the label In PG, but for only 14 and 8% of that In PC and MGD. Chloroplasts Isolated from expanding leaves of A. llvldus Incorporated labeled acetate primarily Into UFA but also into glycerolipids. Whereas the UFA typically contained 80% 18 1, the glycerides... 

We have analyzed the polar lipid composition of mitochondria (from both green and non-green tissues), peroxisomes, chloroplasts and non-green plastids. Some experiments have also been performed with isolated cells from sycamore, cultivated in liquid suspension. In this last case, a specific parameter for the culture, i.e. the oxygen content of the medium, has to be taken into account as far as the fatty acid content of the membrane glycerolipids is concerned . 

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