Actinorhodin ACP

Figure 4 FPLC purification and ESMS analysis of actinorhodin ACP expressed in i) the absence, and ii) the presence of ACP ho o-synthase...

The Type II PKSs, of which the actinorhodin cluster is the model, consist minimally of an ACP and a heterodimeric ketosynthase complex and typically construct 2-4 ring aromatic molecules. Unlike modular systems, Type II PKSs are found as discrete proteins which probably associate as a complex and their reaction mechanism is iterative. Therefore, predicting the resulting size and cyclisation pattern of a Type II PKS product can be problematic, though enough systems have been studied that phylogenetic analysis is often helpful. 

Fig. 16. Molscript diagram showing the restrained minimised structure of actinorhodin apo ACP...

Figure 4. Organization of representative type 1, II, and III polyketide synthases. Upper modular arrangement of DEBS 1,2,3 subunits Center orientation and arrangement of open reading frames in actinorhodin gene cluster Lower chalcone synthase subunit. AT, acyltransferase ACP, acyl carrier protein KS, ketosynthase KR, ketoreductase DH, dehydratase ER, enoyl reductase TE, thioesterase TA, tailoring enzyme R/T, regulatory/transport related AR aromatase CY, cyclase.

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. 

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