Biosynthetic enzymes polyketides

Many enzymes are involved in the synthesis of secondary metabolites. The modular biosynthetic enzymes polyketide synthase (PKS) and nonribosomal peptide synthetase (NRPS) are responsible for the generation of a multitude of structurally diverse and biologically important small-molecule natural products. A complex carbon structure is assembled sequentially from simple carbon building blocks (acyl-CoA and amino acids). The elongation of each carbon unit is catalyzed by... 

In this chapter, we will introduce an exciting class of natural product biosynthetic enzymes, the modular synthases, as well as their associated enzyme partners. We will discuss the use of metabolic engineering as a tool for small-molecule discovery and development, both through directed fermentation and combinatorial biosynthesis. In addition, we will review six classes of partner enzymes involved in the modification of polyketide (PK) and nonribosomal peptide (NRP) natural products. We believe that these enzymatic transformations hold great opportunities for synthetic chemists and will serve as the foundation for a new trend in both discovery and process chemistry. 

In this chapter, we describe the atomic resolution structural elucidation of several plant type III polyketide synthases, including chalcone synthase, 2-pyrone synthase, and stilbene synthase. Manipulation of the catalytic activity and specificity of these biosynthetic enzymes by using a structurally guided approach offers a novel... 

In carrot, Daucus carota (Umbelliferae), 6-methoxymellein was identified as a phytoalexin (Figure 29). It is biosynthesized by 6-hydroxymellein-O-methyltransferase from 6-hydroxymellein, which is biosynthesized from 1 mol of acetyl coenzyme A (CoA) and 4 mol of malonyl-CoA by a polyketide biosynthetic enzyme, 6-hydroxymellein hydroxylase.2 ... 

In nature, there are a large number of bioactive secondary metabolites produced by microorganisms and plants, probably for proliferation of the producer under living conditions. Our accumulative knowledge on biosynthetic pathways of natural products indicates that the unique backbone of natural products such as polyketides, polypeptides, and terpenes is constructed by a relatively small number of biosynthetic enzyme systems. The Diels-Alder reaction provides a further option to diversify secondary metabolites since this reaction... 

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. 

The potential for producing novel macrocyclic polyketides is even more promising, since, in this case, the physical and temporal functioning of biosynthetic enzymes has been implicated as being a sequential process (86). If substrate specificities can be overcome, then an enormous number of compounds could be produced representing all of the possible combinations of oxidation... 

There are at least three types of PKS. Type I PKSs catalyze the biosynthesis of macrolides such as erythromycin and rapamycin. As modular enzymes, they contain separate catalytic modules for each reaction catalyzed sequentially in the polyketide biosynthetic pathway. Type II PKSs have only a few active sites on separate polypeptides, and the active sites are used iteratively, catalyzing the biosynthesis of bacterial aromatic polyketides. Type III are fungal PKSs they are hybrids of type I and type II PKSs [49,50]. 

Members of the CHS/STS family of condensing enzymes are relatively modest-sized proteins of 40-47 kDa that function as homodimers. Each enzyme typically reacts with a cinnamoyl-CoA starter unit and catalyzes three successive chain extensions with reactive acetyl groups derived from enzyme catalyzed decarboxylation of malonyl-CoA.11 Release of the resultant tetraketide together with or prior to polyketide chain cyclization and/or decarboxylation yields chalcone or resveratrol (a stilbene). Notably, CHS and STS catalyze identical reactions up to the formation of the intermediate tetraketide. Divergence occurs during the termination step of the biosynthetic cascade as each tetraketide intermediate undergoes a distinct cyclization reaction (Fig. 12.2). 

These studies have much contributed to deepen our understanding of the biosynthetic mechanisms for polyketide production, particularly with the fundamental discovery of a novel RppA chalcone-synthase-related enzyme in bacteria (Funa 1999). 

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