Phenolic lipids biosynthesis

The combination of a root hair specific EST approach and expression analysis was an effective strategy for isolating candidate polyketide synthases potentially involved in sorgoleone biosynthesis. As a result of these efforts, two novel type III polyketide synthases have been identified that preferentially use long chain acyl Co-A s and are potentially involved in sorgoleone biosynthesis. These candidate polyketide synthases can form pentadecatriene resorcinol, an intermediate in sorgoleone biosynthesis. Furthermore, these efforts may aid in the identification of other polyketide synthases responsible for the biosynthesis of phenolic lipids in other plant species. 

The pathways of carbon flow in eukaryotes are more complex than in prokaryotes, mainly because the C2 units produced within mitochondria can be exported (e.g. for lipid biosynthesis) only as part of a citrate unit produced in the citric acid cycle. The branch point in carbon flow to either isoprene or acetate can lead to additional isotopic fractionation. In general, it seems that n-alkyl hpids in a particular organism are depleted in 13C by c. 1.5%o relative to isoprenoids produced from the same substrate (Hayes 1993). In higher plants, the phenolic precursors of lignin derive from glucose (Fig. 2.29), so it is not surprising that the carbon isotopic composition of lignin reflects the major photosynthetic pathway involved (C3 or C4 Benner et al. 1987). 

Kolattukudy PE (1977) Lipid polymers and associated phenols, their chemistry, biosynthesis, and role in pathogenesis. In Loewus FA, Runeckles VC (eds) Recent advances in phytochemistry. Plenum Publishing, New York, p 185... 

Plant metabolism can be separated into primary pathways that are found in all cells and deal with manipulating a uniform group of basic compounds, and secondary pathways that occur in specialized cells and produce a wide variety of unique compounds. The primary pathways deal with the metabolism of carbohydrates, lipids, proteins, and nucleic acids and act through the many-step reactions of glycolysis, the tricarboxylic acid cycle, the pentose phosphate shunt, and lipid, protein, and nucleic acid biosynthesis. In contrast, the secondary metabolites (e.g., terpenes, alkaloids, phenylpropanoids, lignin, flavonoids, coumarins, and related compounds) are produced by the shikimic, malonic, and mevalonic acid pathways, and the methylerythritol phosphate pathway (Fig. 3.1). This chapter concentrates on the synthesis and metabolism of phenolic compounds and on how the activities of these pathways and the compounds produced affect product quality. 

Information on the biosynthesis of the phenolic glycolipids is very limited.220 The carbon atoms in the methyl-branched structures in mycocerosic acids are derived from propanoate.221-222 So also are those in phthiocerol.223 224 The aromatic ring can be derived from tyrosine.224 The methoxyl residue in phenolphthiocerol presumably comes from methionine, by analogy with the known source in the related lipid phthiocerol.225 226 Rainwater and Kolatukuddy226 demonstrated two steps in the biosynthesis of mycocerosic acids, and isolated the enzymes involved, but the biosynthetic steps leading to phenolphthiocerol itself are not known. 

FIGURE 10.5 Biosynthesis of phenolic acids. (Adapted from Duthie, G. and Crozier, A., Curr. Opin. Lipid, 11,43, 2000 Hakkinen, S., Flavonols and phenolic acids in berries and berry products. Doctoral dissertation, Kuopio University Publications, Kuopio, Finland, D. Med. ScL, 221, 92, 2000.)... 

Packter, N. M., Biosynthesis of acetate-derived phenols (polyketides), in Lipids Structure and Function (P. K. Stumpf ed.). 

Packter, N. M., Biosynthesis of acetate-derived phenols (polyke-tides), in Lipids Structure and Function (P. K. Stumpf, ed.), Vol. 4 of The Biochemistry of Plants (P. K. Stumpf and E. E. Conn, eds.), 535-570, Academic Press, New York, 1980. 

Whether the occurrence of an enzyme in a particulate fraction really represents its in vivo location can be substantiated by the following properties (a) close binding to membrane lipids, cf. cyclopenase (D 8.4.2), which is a lipoprotein of the plasma membrane (b) cooperation with other enzymes, cf. the catalytic facilitation in the biosynthesis of cyanogenic glycosides, cinnamic and benzoic acids (A 3.1) and (c) in situ examination by cytochemical methods, cf. the localization of phenol oxidases (C 2.3.1), peroxidases (C 2.4) and thioglucosidases (D 9.4). 

Approaches such as those described above which rely on the isolation and detection of polyprenyl substituted phenols and quinones in lipid extracts are restricted by the minute quantities of compounds present and the difficulties, frequently encountered, in establishing a precursor relationship between the suspected intermediate and ubiquinone. Gibson and his colleagues approached the problem of ubiquinone biosynthesis in Escherichia coli K-12 by the isolation and examination of mutants unable to form ubiquinone. Five classes of ubiquinone deficient mutants were... 

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