Primary to secondary metabolism

FROM PRIMARY TO SECONDARY METABOLISM THE KEY BUILDING BLOCKS... 

Understanding mechanisms controlling metabolon localization in plastids of different membrane architectures Little is known about metabolon structure, assembly, and membrane targeting. The carotenoid biosynthetic pathway exists on plastid membranes. However, plastids have different membrane architectures and therefore tissue- and plastid-specific differences in membrane targeting of the biosynthetic metabolon can be expected. Localization in chloroplasts that harbor both thylakoid and envelope membranes differs from the envelope membranes in endosperm amy-loplasts. In fact, localization on both thylakoid and envelope membranes implies that the carotenoid pathway is really not a single pathway, but a duplicated pathway that may very well have membrane-specific roles with regard to functions in primary and secondary metabolism. 

Conventionally, central and special metabolic pathways are distinguished. Central pathways are common to the decomposition and synthesis of major macromolecules. Actually, they are much alike in all representatives of the living world. Special cycles are characteristic of the synthesis and decomposition of individual monomers, macromolecules, cofactors, etc. Special cycles are extremely diversified, especially in the plant kingdom. For this reason, the plant metabolism is conventionally classified into primary and secondary metabolisms. The primary metabolism includes the classical processes of synthesis and deeradation of major macromolecules (proteins, carbohydrates, lipids, nucleic acids, etc.), while the secondary metabolism ensuing from the primary one includes the conversions of special biomolecules (for example, alkaloids, terpenes, etc.) that perform regulatory or other functions, or simply are metabolic end byproducts. 

The key reaction that links primary and secondary metabolism is provided by the enzyme phenylalanine ammonia lyase (PAL) which catalyzes the deamination of l-phenylalanine to form iran.v-cinnamic acid with the release of NH3 (see Fig. 3.3). Tyrosine is similarly deaminated by tyrosine ammonia lyase (TAL) to produce 4-hydroxycinnamic acid and NH3. The released NH3 is probably fixed by the glutamine synthetase reaction. These deaminations initiate the main phenylpropanoid pathway. 

Much attention has been paid to the last step of the formation of monoter-penes and sesquiterpenes, which is catalysed by terpenoid synthases. Over 30 complementary DNAs (cDNAs) encoding plant terpenoid synthases involved in the primary and secondary metabolism have been cloned, characterised, and the proteins heterologously expressed [6]. However, because geranyl diphosphate and farnesyl diphosphate are not readily available substrates, their biotransformation by terpenoid synthases is not economically viable. As a result, considerable effort has been put into engineering the total plant terpenoid biosynthetic pathway in recombinant microorganisms. 

Lloyd JC, Zakhleniuk OV. 2004. Responses of primary and secondary metabolism to sugar accumulation revealed by microarray expression analysis of the Arabidopsis mutant, pho3. J Exp Bot 55 1221-1230. 

Secondary metabolic reactions may be catalyzed by enzymes of primary metabolism, enzymes specific to secondary metabolism or enzymes that occur spontaneously. 

Because CYP1A2 is predominantly involved in caffeine s primary and secondary metabolism, a urinary MR approach has been applied with an expectation that this would provide a simple and convenient noninvasive means of assessing the isoform s metabolic activity. Over 12 different urinary molar MRs have been suggested as putative trait measures of CYP1A2 activity, generally based on a 0- to 24-hour urine collection following an oral dose of caffeine, although spot ... 

Figure 9.4 Pathways of the sum of all biochemical processes is called metabolism. It is distinguished between primary and secondary metabolism. Primary metabolism contains all pathways necessary to keep the cell alive while in secondary metabolism compounds are produced and broken down that are essential for the entire organism.

The catabolic and anabolic pathways that are responsible for the formation of many of the biomarker compounds discussed in this chapter occur through an intermediary metabolism via glycolysis and the citric acid cycle (Voet and Voet, 2004). The biosynthetic pathways of these compounds can be divided into primary and secondary metabolism (figure 9.4). Many of these compounds are not used as chemical biomarkers in estuarine research but are shown here simply to illustrate their relationship with the biomarkers discussed in this chapter. For more details on the biosynthetic pathways illustrated here, please refer to Voet and Voet (2004) and Engel and Macko (1993). 

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