Natural product libraries biosynthesis

Merging chemical synthesis and biosynthesis as a strategy of the total synthesis of natural products and natural product libraries 12AG(E)4012. Palladium-mediated total synthesis ofbioactive natural products 13S1271. Recent contributions from the asymmetric aza-Michael reaction to alkaloids total synthesis 13NPR1211. 

Figure 10.42 Combinatorial biosynthesis structure of a hypothetical three-member discrete library of modified natural products.

Combinatorial biosynthesis, which is also known as genetic engineering of natural product biosynthetic pathways, involves a series of methods that establish novel enzyme combinations that are used for the generation of encoded libraries of bioactive small molecules [14-21]. The feasibility of this approach was first demonstrated by Hop wood et al. [22]. 

Diverse polyketide-type libraries were synthesized using a-chiral aldehydes attached by a silyl linker to a hydromethylpolysyrene resin (Figure 11.24). " Aldol chain extension with chiral ketone modules, and subsequent ketone reduction and manipulation on the solid support led to elaborated stereopentad sequences found in natural products such as 6-deoxyerythronolide B and discoder-molide. Based on the biosynthesis of erythromycin, the same methodology was used in the combinatorial synthesis of polyketide sequence mimetics with a great variety of chain-extending units. " ... 

Importantly, these genomic libraries can be enriched for the appropriate gene transcripts if the tissue or cell type from which the genetic material is harvested conesponds to the site of natural product production. For example, 25% of the clones in cDNA obtained exclusively from the oil gland secretory cell of peppermint Mentha x piperita) appear to be involved in oil metabolism [28]. In another example, mRNA for construction of a C. roseus cDNA Ubraiy obtained from epidermal cells resulted in a collection enriched in genes involved in alkaloid biosynthesis [21]. 

A set of 60 cDNAs were isolated by subtractive hybridization of Nicotiana tabacum roots before and after removal of the flowers and young leaves, a process known as topping [32], Topping is known to increase the amount of nicotine and other related alkaloids in the plant. A number of the subtracted cDNAs that were unique to the topped plants corresponded to known alkaloid biosynthetic enzymes. The functions of additional cDNAs observed in the subtracted library were not clear and remain under investigation. Further study of these new clones may provide insights into alkaloid natural product biosynthesis in N. tabacum. 

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