Structural Biochemistry/Polyketide Synthases
Polyketide synthases (PKSs) are enzymes that produce polyketides—secondary metabolites found primarily in bacteria and fungi, but can also be found in plants and some animals.
These enzymes are made up of several domains where each domain has a specific function. Examples of PKS domains are:
- Acyltransferase (AT)
- Acyl Carrier Protein (ACP)
- Ketosynthase (KS)
- Ketoreductase (KR)
- Dehydratase (DH)
- Enoylreductase (ER)
- Methyltransferase (MT)
- Sulfhydrolase (SH)
- Thioesterase (TE)
PKS can be broken up into three different types:
- Type I PKSs are large proteins,
- Iterative Type I PKSs
- Modular Type I PKSs
- Type II PKSs are monofunctional proteins (each domain is on its own protein)
- Type III PKSs, which do not use ACP domains
PKSs produce small molecules that are currently being studied for various pharmacological uses. The most common molecules produced are antibiotics like tetracyclin and macrolide. Other PKS products are also being studied for their antifungal and anticancer/antitumor properties.
Examples of Polyketide Synthases
Lovastatin is a fungal polyketide produced by the highly reducing iterative type I PKS originally isolated from Aspergillus Terreus. It was found that lovastatin is a cholesterol-lowering agent and was then used to create the semi-synthetic drug simvastatin.
18 genes are thought to encode for the formation of lovastatin but the two PKSs, in particular are responsible for the molecules production. These two PKSs are LovB and LovF. LovB is highly reducing iterative type I PKS containing KS, AT, DH, MT, KR, and ACP domains. This particular PKS creates a portion of the lovastatin molecule known as dihydromonacolin L. Dihydromonacolin L is a nonaketide that requires nine Malonyl-CoA molecules, Niicotinamide adenine dinucleotide phosphate (NADPH), and S-Adenosyl methionine (SAM).
Lovstatin begins by attaching an acetyl starter unit derived from Malonyl-CoA to the ACP active site. Here the Ketosynthase (KS) domain catalyzes the ketoreduction up to the nonaketide. The Dehydratase (DH) domain is active until the hexaketide. The Methyl Transferase (MT) domain takes a methyl group from SAM and attaches it to the building dihydromonacolin L molecule. The Enoyl Reductase (ER) domain comes from LovC and reduces the keto groups to olefinic functionality at the tetra-, penta-, and hepta-ketides. It is also suspected that the hexaketide undergoes a Diels-Alder cycloaddition by the Diels-Alderase contained in LovB.
The product of LovB, dihydromonacolin L, and Lov F are joined together by LovD to give the final lovastatin product.
DEBS (6-deoxyerythronolide B synthase)
DEBS (6-deoxyerythronolide B synthase) is a Type 1 polyketide synthase responsible for the production of the macrolide 6-Deoxyerythronolide B (6dEB), a aglycone precursor of the antibiotic erythromycin. It is commonly found in the actinomycete bacteria, Saccharopolyspora erythraea. DEBS contains three large polypeptides, DEBS 1, -2, and -3, that are appropriately arranged to provide the necessary reaction sites for the biosynthesis of 6dEB. Each of these three polypeptides exist as a dimer of two modules, resulting in a total of six modules. Each of the six modules is comprised of, at the very least, an Acyl carrier protein (ACP) site, a Ketosynthase (KS), and an acyltransferase (AT), all of which are essential components in the synthesis of 6dEB. Additional components serving the function of further reduction reactions include Dehydrotase (DH), Enol Reductase (ER), and Ketoreductase (KR). In addition to these elementary components, module 1 contains a Loading Didomain (LDD) and module 6 contains a terminal Thioesterase. The processive action of all of the these components provides the basis for the synthesis of the polyketide, 6dEB.