Enzymes of secondary metabolism

Xenoblotics can alter various metabolic pathways In plants. Chemical alteration of phenolic compounds for agricultural purposes has been suggested to have potential (130. 131). Since several major plant enzymes of secondary metabolism are Involved In resistance to pathogens, alteration of secondary metabolism could Influence disease development. Herbicides, plant growth regulators (synthetic and natural), and enzyme Inhibitors such as those presented for PAL (Fig. 

Strack, D. and Mock, H.P. (1993) Hydroxycinnamic acids and lignins, in Methods in Plant Biochemistry, Vol. 9, Enzymes of Secondary Metabolism (eds P.M. Dey and J.B. Harborne). Academic Press, London, pp. 45-97. 

Occurrence of genes encoding key enzymes of secondary metabolism... 

To date over 30 plant terpenoid synthases have been cloned as cDNAs, and many of these were found to encode enzymes of secondary metabolism (43). Isolation and analysis of six genomic clones encoding monoterpene ((—)-pinene and (—)-limonene), sesquiterpene ((E)-a-bisabolene and S-selinene) and diterpene (abietadiene) synthases from Abies grandis, and a diterpene (taxadiene) synthase from Taxus brevifolia have been reported (44). Overexpression of a cotton farnesyl diphosphate synthase (EPPS) in transgenic Artemesia annua has resulted in 3- to 4-fold increase in the yield of the sesquiterpenoid anti-malarial drug, artemisinin, in hairy roots (45). 

This approach assumes some knowledge, or at least some presumption, of the general biosynthetic pathway by which the metabolite is formed. It is usually possible to make a good guess at the biosynthetic origins of a molecule, particularly when biosynthetic information is available on a structurally similar molecule, and these theories can be tested by the use of isotopically labeled precursors. It also relies on the fact that the biosynthetic enzymes are not so specific as to exclude all but the natural precursor. Fortunately, the enzymes of secondary metabolism tend to be less specific than those of primary metabolism. 

De Luca V. Enzymology of indole alkaloid biosynthesis. In Lea PJ, editOT. Enzymes of Secondary Metabolism. Dey PM, Harbome JB, editos. Methods in Plant Biochemistry, voL 9. London, San Diego J few York, Boston, Sydney, Tokyo, Toronto Academic Press 1993. p. 345 8. 

Zenk, M.H. Chasing the enzymes of secondary metabolism plant cell culture s as a pot of gold. Phytochemistry, 1991, 30, 3861-3863. 

I hope that my arguments, which plead for a view of metabolism in terms of subcellular structures, may stimulate our thinking. What I am getting at in emphasizing these corapartmentatlons is also that preparing cell subfractions may greatly facilitate the task to find some of the still unknown enzymes of secondary metabolism. 

Cataboirte repression Inhibition of enzyme synthesis by increased concentrations of certain metabolic products. Enzymes subject to Cr. are formed in reponse to metabolic events (utilization of new nutrients by catabolic enzymes, synthesis of secondary products in certain developmental phases of microorganisms). C. r. is probably present in all organisms, although the molecular mechanisms are diverse. Examples of C.r. are glucose repression of catabolic enzymes in . coU and the enzymes of secondary metabolism in microorganisms, e.g. those for the s thesis of penicillin, actinomycin and riboflavin. 

Enzymes of secondary metabolism. (Synthesis and metabolism of substances found only in certain cells or organisms)... 

The structural genes encoding secondary metabolic enzymes usually are located in the genome. Neither the DNA of plastids nor of mitochondria is known to code for any enzymes of secondary metabolism. In a few procaryotes, however, structural genes of secondary metabolic pathways may reside in plasmids (Hopwood 1981). The SCP 1 sex plasmid of Streptomyces coelicolor for instance, carries at least four or five structural genes for the production of the antibiotic methylenomycin A. 

Organelles separated from cytoplasm by a double membrane are the nucleus, mitochondria, and plastids. Secondary products and enzymes of secondary metabolism are rare in nuclei and mitochondria, but common in plastids. The envelope and the invaginations of the inner membrane, which traverse these organelles, and the nonparticulate fraction obtained after chloroplast rupture... 

Table 5. Sites of location of secondary products and enzymes of secondary metabolism in plastids...

Enzymes that enable the cell to perform special functions, e.g., enzymes of secondary metabolism, enzymes with a function in morphogenesis, special catabolic enzymes that degrade drugs, special nutrients, etc. 

However, cell division and cell specialization are not always mutually exclusive. There are instances in which specialized cells may be built de novo by so-called quantal mitoses, i.e., formation of daughter cells being determined to differ from their mother cells or from each other. These cells may already contain enzymes of secondary metabolism from the time point of their synthesis. 

A Determination in vitro of an activity increase of enzymes of secondary metabolism before or at the beginning of the phase of secondary product formation. 

De Luca, V. (1993) Indole alkaloid biosynthesis. In Lea, P., ed. Methods in plant biochemistry. Enzymes of secondary metabolism, Vol. 9, pp 345-368, Academic Press... 

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