Eukaryotic pathway

Eukaryotic Pathways to Phosphatidylserine, Phosphatidylethanolamine, and Phosphatidylcholine Are Interrelated... 

Figure 21-5 A more complete outline of the biosynthesis of triacylglycerols, glycolipids, and phospholipids including characteristic eukaryotic pathways. Green lines indicate pathways utilized by both bacteria and eukaryotes. Structures of some of the compounds are shown in Fig. 21-4. The gray arrows show the formation of phosphatidylserine by exchange with ethano-lamine (Eq. 21-10).

A key question to keep asking during any discussion of bacterial cytochrome function, including that which follows, is To what extent does the primary evidence support the proposed conclusions, and how heavy is the dependence on analogy with known eukaryotic pathways ... 

Enzymes of the prokaryotic pathway are localized in plastids, whereas enzymes of the eukaryotic pathway - in the cytosol and ER. In the prokaryotic pathway, FA acyls are directly transferred from AGP to G3P, whereas in the eukaryotic pathway, FA are separated from ACP by acyl-ACP thioesterases and released free FA then are transported in the cytoplasm, where they are converted into acyl-CoA. During the synthesis of membrane and storage lipids acyl groups are used in the ER by acyltransferases of the eukaryotic G3P pathway [66]. In dependence on subcellular localization, these enzymes may differ in their structure, thus forming independent clusters in phylogenetic investigations [67]. 

Introduction Primary Fatty Acids Fatty Acids of Plant Vegetative Parts Biosynthesis Fatty Acid Biosynthesis The Two-Pathway Model of Lipid Biosynthesis The Second 3-Ketoacyl ACP Synthase Isozyme Biosynthesis of Unsaturated Fatty Acids The Prokaryotic Pathway The Eukaryotic Pathway Biosynthesis of Triacylglycerides Degradation of Fatty Acids Unusual Fatty Acids in Plants Fatty Acids from Unusual Starter Units Fatty Acids with Unusual Patterns of Unsaturation Hydroxy Fatty Acids Epoxy Fatty Acids... 

Research since about 1980 indicates that the leaves of higher plants utilize two distinctive pathways for glyceroli-pid biosynthesis. Palmitoleic acid (Cie) (13) and oleic acid (Ci8 i) (3) are synthesized de novo in the chloroplast and may be used directly for the bios)mthesis of chloroplast lipids via the prokaryotic pathway (see below) or exported from the chloroplast as CoA esters that are used in the eukaryotic pathway primarily at sites on the endoplasmic reticulum (Browse and Somerville, 1991). However, in all higher plants, a proportion of the diglycerol moiety of phosphatidylcholine, synthesized by the eukaryotic pathway, again enters the chloroplast where it contributes to the production of thy-lakoid membranes (Browse and Somerville, 1991). 

In many families of angiosperms, phosphatidylglycerol (9) is the only major product of the prokaryotic pathway. Other chloroplast lipids in these plants are synthesized only by the eukaryotic pathway. In some primitive angiosperm families, both pathways contribute to the s)mthesis of mono-galactosyldiacylglycerol (10) and other lipids (Browse and Somerville, 1991). 

The first committed step of the eukaryotic pathway is the hydrolysis of Cig acid and Cig i-ACP to the free fatty acids. These free fatty acids move through the two membranes of... 

TRIACYLGLYCEROLS PARTICIPATE IN THE EUKARYOTIC PATHWAY OF PUFAS BIOSYNTHESIS IN THE RED MICROALGA PORPHYRIDIUM CRUENTUM... 

The lipid composition of the HZ3 mutant demonstrated a three fold increase in the proportion of TAGs and a corresponding decrease in that of PC in comparison to the wild type (Table I). Moreover, the proportion of EPA of the mutant decreased from 41.1% (of total fatty acids) in the wild type to 26.5%. These alterations were most noticeable in MGDG where the proportion of EPA was reduced from 56 to 45.5%. The molecular species composition of MGDG demonstrated a decrease in the proportion of the major eukaryotic species 20 5/20 5 which was accompanied by increases in the prokaryotic species 18 2/16 0, 20 4/16 0 and 20 5/16 0 (Table I). DGDG which is entirely prokaryotic, was not significantly affected. We have interpreted this as an indication that the mutation affected the eukaryotic pathway. 

Using chill-sensitivity as a selection tool we isolated a mutant of P. cruentum deficient in EPA production. The lipid and fatty acid composition of the mutant, which showed a reduced level of eukaryotic molecular species of MGDG support our hypothesis that the mutant is deficient in the eukaryotic pathway. 

Browse, J., Warwick, N., Somerville, C.R., and Slack, C.R. Fluxes through the prokaryotic and the eukaryotic pathways of lipid synthesis in the 16 3 plant Arabidopsis thaliana, Biochem. J. 235 (1986), 25-31. 

Autoradiography of the labelled fatty acids in position sn-2 of MGDG (not shown) indicated only traces of label in the area covered by 16 0 (prokaryotic galactolipid) in EC while this area was well labelled in NY 18 2 and 18 3 were well labelled in both cultivars (eukaryotic pathway). 

In plants, de novo fatty acid biosynthesis occurs exclusively in the stroma of plastids, whereas, with the exception of plastidial desaturation, modification of fatty acid residues including further desaturation and triacylglrycerol (TAG) assembly are localized in the cytosol/endoplasmic reticulum (ER). The primary fatty acids formed in the plastid (palmitic, stearic, and oleic acid) are used in the plastidic prokaryotic pathway for membrane lipid synthesis or diverted to the cytoplasmic eukaryotic pathway for the synthesis of membrane lipids or storage TAGs (1). Movement of glycerolipids is believed to occur in the reverse direction between the cytosol/ER and the plastids in the highly regulated manner (2). 

Triacylglycerols Participate in the Eukaryotic Pathway of PUFAs Biosynthesis in the Red Microalga Porphyridium cruentum. 

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). 

It is concluded that linolenic acid is synthesized via PC molecular species in the eukaryotic pathway of pea leaves and via M6D6 molecular species in the eukaryotic pathway of spinach leaves. 

I A GENERAL FRAME FOR THE STUDY OF POLYUNSATURATED FATTY ACID BIOSYNTHESIS IN PLANTS THE PROKARYOTIC AND EUKARYOTIC PATHWAYS [3-7 ]. 

To overcome progressively the numerous difficulties linked to linoleate desaturation studies, we began with in vivo studies [22j. As a labelled precursor of linolenic acid, we have used C-oleate, given externally to entire leaves, to follow the labelling of the metabolic intermediates of the sole eukaryotic pathway. To perform the labelling experiments, we have used... 

The schema of fig. 1 is proposed to summarize the results. In pea leaves only "eukaryotic pathways" would be present for MGDG biosynthesis and linolenic acid would be synthesized mostly in endoplasmic reticulum, while esterified to PC molecules. In spinach leaves, both "prokaryotic and eukaryotic pathways" would function simultaneously but linolenic acid would be formed mostly in chloroplasts, while esterified to MGDG molecules. 

The eukaryotic pathway for MGDG synthesis (6) predicts that the DAG... 

Arabidopsis is a typical 16 3-plant in which both the prokaryotic and eukaryotic pathways [4] contribute to the production of chloroplast lipids. We have investigated the pattern of lipid metabolism in wild type Arabidopsis and calculated the fluxes of carbon involved [5]. An abbreviated version of this analysis is shown in Fig. la. For every 1000 fatty acid molecules synthesized in the chloroplast 390 enter the prokaryotic pathway in the chloroplast envelope while 610 are exported as CoA esters to enter the eukaryotic pathway. Of these 340 are reimported into the chloroplast. Overall, almost equal amounts of chloroplast lipids are produced by each pathway. However, the quantities of individual lipids synthesized by the two routes are very different. All the chloroplast phosphatidylglycerol (PG) and over 70% of the monogalactosyldiacylglycerol (MGD) is derived from the prokaryotic pathway while digalactosyldiacylglycerol is synthesized mainly on the eukaryotic pathway [5]. In this paper we have outlined how four of the Arabidopsis mutants have changed the way we view the operation of the two pathways involved in leaf membrane lipid synthesis. More detailed information on each mutant can be found elsewhere [1-3, 5,6 and in preparation]. 

---