Detection and Expression of Biosynthetic Genes in Actinobacteria ...

BERVANAKIS, G.Chapter 1: INTRODUCTION1.3.1.1 Polyketide Biosynthesis PathwayThe key enzyme reactions in the polyketide biosynthetic pathway are shown in figure5, the initiation of polyketide biosynthesis begins with carboxylic acid extender unitswhich may consist of acetate, propionate or butyrate. The carboxylated extender unitsare transferred from coenzyme A (CoA) to the 4’-phosphopantetheine arm of the acylcarrier protein [ACP] by the actions of the acyltransferase [AT] (Figure 5; stage 1).The polyketide chain is attached via a thioester linkage to a cysteine residue in theactive site of the ketosynthase [KS]. A linear chain is assembled, whereby eachcarboxylic acid incorporates two carbon atoms carrying a β-keto group, acondensation reaction occurs which is catalyzed by ketosynthase [KS] (Figure 5;stage 2), the β-carbonyl can be subjected to all, part, or none of a series of stepscatalyzed by a ketoreductase [KR], dehydratase [DH], and enoylreductase [ER](Figure 5; stage 3). Following condensation cycles, the polyketide chain is releasedfrom the PKS via the action of the thioesterase [TE] (Figure 5; stage 4) (Hopwood &Sherman, 1990).Stage 2Stage 3Stage 1Stage 4Figure 5. Illustration depicting the key enzyme reactions in polyketide biosynthesis.Condensation begins by the attachment of the polyketide chain to a ketosynthase(KS). An acyltransferase (AT) transfers the carboxylated extender unit from CoA theacyl carrier protein (ACP). The specificity of the AT determines the choice ofextender unit (R 2 ). The solid line arrows in the centre of the biosynthetic scheme,indicate the degree of β-ketoreduction can vary at any given carbonyl. The polyketidechain is released from the PKS by a thioesterase [TE] (adapted from Khosla, 1998).1.3.1.2 Polyketide Synthetases (PKSs)_____________________________________________________________________13