Modern Industrial Microbiology and Biotechnology

Metabolic Pathways for the Biosynthesis of Industrial Microbiology Products 93referred to as the shikimate pathway. In view of this central role of chorismic acid,however, the route is more widely known as the shikimate-chorismate route. Theshikimate-chorismate route is an important route for the formation of aromaticsecondary products in the bacteria and actinomycetes. Examples of suchsecondary products include chloramphenicol and novobiocin. The route is lessused in fungi, where the polyketide pathway is more common for the synthesis ofaromatic secondary products.(iv) The polyketide pathway: polyketide biosynthesis is highly characteristic of thefungi, where more secondary metabolites are produced by it than by any other.Indeed most of the known polyketide-derived natural products have been obtainedfrom the fungi, a much smaller number being obtained from bacteria and higherplants. The triose (C 3 ) derived from glucose in the EMP pathway is converted viapyruvic acid to acetate, which occupies a central position in both primary andsecondary synthesis. The addition of CO 2 to an acetate group gives a malonategroup. The synthesis of polyketides is very similar to that of fatty acids. In thesynthesis of both groups of compounds acetate reacts with malonate with the lossof CO 2 . By successive further linear reactions between the resulting compound andmalonate, the chain of the final compound (fatty acid or polyketide) can besuccessively lengthened.However, in the case of fatty acid the addition of each malonate molecule isfollowed by decarboxylation and reduction whereas in polyketides these latterreactions do occur. Due to this a chain of ketones or a b-polyketomethylene (hencethe name polyketide) is formed (Fig. 5.10). The polyketide (b - poly-ketomethylene)Fig. 5.10Formation of Polyketideschain made up of repeating C-CH 2 or ‘C 2 units’, is a reactive protein-boundintermediate which can undergo a number of reactions, notably formation intorings. Polyketides are classified as triketides, tetraketides, pentaketides, etc.,depending on the number of ‘C 2 units’. Thus, orsellenic acid which is derived fromthe straight chain compound in Fig. 5.11 with four ‘C 2 -units’ is a tetraketide.Although the polyketide route is not common in actinomycetes, a modifiedpolyketide route is used in the synthesis of tetracyclines by Streptomyces griseus.(v) Terpenes and steroids: The second important biosynthetic route from acetate is thatleading via mevalonic acid to the terpenes and steroids. Microorganisms