Polyketide

Discovery

Polyketides can be produced in bacteria, fungi, plants, and certain marine organisms. Earlier discovery of naturally occurring polyketides involved the isolation of the compounds being produced by the specific organism using organic chemistry purification methods based on bioactivity screens. Later technology allowed for the isolation of the genes and heterologous expression of the genes to understand the biosynthesis. In addition, further advancements in biotechnology have allowed for the use of metagenomics and genome mining to find new polyketides using similar enzymes to known polyketides.

Biosynthesis

Polyketides are synthesized by multienzyme polypeptides that resemble eukaryotic fatty acid synthase but are often much larger. They include acyl-carrier domains plus an assortment of enzymatic units that can function in an iterative fashion, repeating the same elongation/modification steps (as in fatty acid synthesis), or in a sequential fashion so as to generate more heterogeneous types of polyketides.

Polyketide synthase

Polyketides are produced by polyketide synthases (PKSs). The core biosynthesis involves stepwise condensation of a starter unit (typically acetyl-CoA or propionyl-CoA) with an extender unit (either malonyl-CoA or methylmalonyl-CoA). The condensation reaction is accompanied by the decarboxylation of the extender unit, yielding a beta-keto functional group and releasing a carbon dioxide. The first condensation yields an acetoacetyl group, a diketide. Subsequent condensations yield triketides, tetraketide, etc. Other starter units attached to a coenzyme A include isobutyrate, cyclohexanecarboxylate, malonate, and benzoate.

PKSs are multi-domain enzymes or enzyme complex consisting of various domains. The polyketide chains produced by a minimal polyketide synthase (consisting of a acyltransferase and ketosynthase for the stepwise condensation of the starter unit and extender units) are almost invariably modified. Each polyketide synthases is unique to each polyketide chain because they contain different combinations of domains that reduce the carbonyl group to a hydroxyl (via a ketoreductase), an olefin (via a dehydratase), or a methylene (via an enoylreductase).

Termination of the polyketide scaffold biosynthesis can also vary. It is sometimes accompanied by a thioesterase that releases the polyketide via hydrating the thioester linkage (as in fatty acid synthesis) creating a linear polyketide scaffold. However, if water is not able to reach the active site, the hydrating reaction will not occur and an intramolecular reaction is more probable creating a macrocyclic polyketide. Another possibility is spontaneous hydrolysis without the aid of a thioesterase.

Post-tailoring enzymes

Further possible modifications to the polyketide scaffolds can be made. This can include glycosylation via a glucosyltransferase or oxidation via a monooxygenase. Similarly, cyclization and aromatization can be introduced via a cyclase, sometimes proceeded by the enol tautomers of the polyketide. These enzymes are not part of the domains of the polyketide synthase. Instead, they are found in gene clusters in the genome close to the polyketide synthase genes.

Classification

Polyketides are a structurally diverse family. There are various subclasses of polyketides including: aromatics, macrolactones/macrolides, decalin ring containing, polyether, and polyenes.

Polyketide synthases are also broadly divided into three classes: Type I PKSs (multimodular megasynthases that are non-iterative, often producing macrolides, polyethers, and polyenes), Type II PKSs (dissociated enzymes with iterative action, often producing aromatics), and Type III PKSs (chalcone synthase-like, producing small aromatic molecules).

In addition to these subclasses, there also exist polyketides that are hybridized with nonribosomal peptides (Hybrid NRP-PK and PK-NRP). Since nonribosomal peptide assembly lines use carrier proteins similar to those use in polyketide synthases, convergence of the two systems evolved to form hybrids, resulting in polypeptides with nitrogen in the skeletal structure and complex function groups similar to those found in amino acids.

Applications

Polyketide antibiotics, antifungals, cytostatics, anticholesteremic, antiparasitics, coccidiostats, animal growth promoters and natural insecticides are in commercial use.

Medicinal

There are more than 10,000 known polyketides, 1% of which are known to have potential for drug activity. Polyketides comprise 20% of the top-selling pharmaceuticals with combined worldwide revenues of over USD 18 billion per year.

Examples

• Macrolides
  • Pikromycin, the first isolated macrolide (1950)
  • The antibiotics erythromycin A, clarithromycin, and azithromycin
  • The antihelminthics ivermectin
• Ansamycins
  • The antitumor agents geldanamycin and macbecin,
  • The antibiotic rifamycin
• Polyenes
  • The antifungals amphotericin, nystatin and pimaricin
• Polyethers
  • The antibiotic monensin
• Tetracyclines
  • The antibiotic agent doxycycline
• Acetogenins
  • bullatacin
  • squamocin
  • molvizarin
  • uvaricin
  • annonacin
• Others
  • The immunosuppressants tacrolimus (FK506) (a calcineurin inhibitor) and sirolimus (rapamycin) (a mTOR inhibitor)
  • Radicicol and the pochonin family (HSP90 inhibitors)
  • The cholesterol lowering agent lovastatin
  • Discodermolide
  • Aflatoxin
  • Usnic acid
  • Anthracimycin
  • Anthramycin
  • Olivetolic acid (intermediate in cannabinoid pathways)
  • Colibactin, a genotoxin produced by some intestinal bacteria

Agricultural

Polyketides can be used for crop protection as pesticides.

Examples

• Pesticides
  • spinosad or spinosyn (an insecticide)
  • avermectin
  • polynactins
  • tetramycin

Industrial

Polyketides can be used for industrial purposes, such as pigmentation and dietary flavonoids.

Examples

• Pigments
  • azaphilones
  • hydroxyanthraquinones
  • naphthoquinones
• Flavonoids
  • curcumin
  • silymarin
  • daidzein

Biotechnology

Protein engineering has opened avenues for creating polyketides not found in nature. For example, the modular nature of PKSs allows for domains to be replaced, added or deleted. Introducing diversity in assembly lines enables the discovery of new polyketides with increased bioactivity or new bioactivity.

Furthermore, the use of genome mining allows for discovery of new natural polyketides and their assembly lines.