Polyketide synthase dehydratase domain

Figure 5 Domain organization of the erythromycin polyketide synthase. Putative domains are represented as circles and the structural residues are ignored. Each module incorporates the essential KS, AT, and ACP domains, while all but one include optional reductive activities. AT, acyltransferase ACP, acyl carrier protein KS, (3-ketoacyl synthase KR, P-ketoacyl reductase DH, dehydratase ER, enoyl reductase TE, thioesterase.

Figure 2 Conventional modular type I PKS paradigm, (a) Individual domains in a full type I polyketide synthase extension module. Homodimeric contacts are made in the N-terminal docking, ketosynthase, dehydratase, enoyi reductase, and C-terminal docking domains, (b) PKS system for 10-deoxymethynolide and narbonolide generation.

Figure 1 Hypothetical pentaketide biosynthetic system, which illustrates the enzymatic logic of type I modular polyketide synthases (PKSs) and the catalytic role of acyl transferase (AT) domains. Each AT domain selects substrates from the cellular pool and tethers them as thioesters to acyl carrier protein (ACP) domains. In a typical PKS module, the AT and ACP domains are present in all modules. The ketosynthase (KS) domain is present in all chain extension modules. The dehydratase (DH), enoyl reductase (ER), and ketoreductase (KR) domains are optional domains. The final thioesterase (TE) domain catalyzes the release of the product from the PKS.

Since the PKS (polyketide synthase) gene cluster for actinorhodin (act), an antibiotic produced by Streptomyces coelicolor[ 109], was cloned, more than 20 different gene clusters encoding polyketide biosynthetic enzymes have been isolated from various organisms, mostly actinomycetes, and characterized [98, 100]. Bacterial PKSs are classified into two broad types based on gene organization and biosynthetic mechanisms [98, 100, 102]. In modular PKSs (or type I), discrete multifunctional enzymes control the sequential addition of thioester units and their subsequent modification to produce macrocyclic compounds (or complex polyketides). Type I PKSs are exemplified by 6-deoxyerythronolide B synthase (DEBS), which catalyzes the formation of the macrolactone portion of erythromycin A, an antibiotic produced by Saccharopolyspora erythraea. There are 7 different active-site domains in DEBS, but a given module contains only 3 to 6 active sites. Three domains, acyl carrier protein (ACP), acyltransferase (AT), and P-ketoacyl-ACP synthase (KS), constitute a minimum module. Some modules contain additional domains for reduction of p-carbons, e.g., P-ketoacyl-ACP reductase (KR), dehydratase (DH), and enoyl reductase (ER). The thioesterase-cyclase (TE) protein is present only at the end of module 6. 

Modular PKSs are large multifunctional enzymes. Active sites (domains) within these enzymes ketosynthases (KS), acyltransferases (AT), dehydratases (DH), enoyl reductases (ER), ketoreductases (KR), acyl carrier proteins (AGP) and thioesterases (TE) are organized into modules such that each module catalyzes the stereospecific addition of a new monomer onto a growing polyketide chain and also sets the reduction level of the carbon atoms of the resulting intermediate [70]. In 1994, the heterologous expression of the complete erythromycin polyketide synthase was accomplished. The recombinant... 

O. Vergnolle, F. Hahn, A. Baerga-Ortiz, P.F. Leadlay, J.N. Andexer, Stereoselectivity of isolated dehydratase domains of the borreUdin polyketide synthase implications for cis double bond formation. ChemBioChem 12, 1011-1014 (2011)... 

The basic principle of polyketide assembly is highly related to that of fatty acid biosynthesis [14, 16]. In both biosynthetic systems, an acyl-primed ketosynthase (KS) catalyzes chain extension by decarboxylative Claisen condensation with malonate activated by its attachment to coenzyme A or an acyl carrier protein (ACP) via a thioester bond (Scheme 2.2). hi fatty acid synthases (FASs), the resulting ketone is rednced to the corresponding alcohol by a ketore-ductase (KR), dehydrated by action of a dehydratase (DH) to give the alkene with snbseqnent donble-bond reduction by an enoyl rednctase (ER) yielding the saturated system (cf. Section 3.2). The latter can then be transferred onto the KS domain and enter the next cycle of chain extension and complete rednction. This homologation process facilitates the assembly of long-chain satnrated fatty acids, for example, palmitic acid, after seven cycles, which will ultimately be released from the catalytic system by saponification of the... 

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