Polyketide synthetases

Each synthetase module contains three active site domains The A domain catalyzes activation of the amino acid (or hydroxyacid) by formation of an aminoacyl- or hydroxyacyl-adenylate, just as occurs with aminoacyl-tRNA synthetases. However, in three-dimensional structure the A domains do not resemble either of the classes of aminoacyl-tRNA synthetases but are similar to luciferyl adenylate (Eq. 23-46) and acyl-CoA synthetases.11 The T-domain or peptidyl carrier protein domain resembles the acyl carrier domains of fatty acid and polyketide synthetases in containing bound phos-phopantetheine (Fig. 14-1). Its -SH group, like the CCA-terminal ribosyl -OH group of a tRNA, displaces AMP, transferring the activated amino acid or hydroxy acid to the thiol sulfur of phosphopan-tetheine. The C-domain catalyzes condensation (peptidyl transfer). The first or initiation module lacks a C-domain, and the final termination module contains an extra termination domain. The process parallels that outlined in Fig. 21-11.1... 

Three categories of synthetases are distinguished, based on their substrate specificity and mode of product synthesis. The two known types of polyketide synthetases (PKSs) (Type I and II) utilize acyl-coenzyme A (CoA) monomers while nonribosomal peptide synthetases (NRPSs) use amino acids and their analogs as substrates. Type I PKS and NRPS oligomerize these building blocks by a modular assembly-line arrangement while type II PKS iteratively assembles monomeric units. 

Bender CL, Alarcon-Chaidez F, Gross DC. Pseudomonas syringae phytotoxins mode of action, regulation, and biosynthesis by 35. peptide and polyketide synthetases. Microbiol. Mol. Biol. Rev. 1999 63 266-292. 

NRPS nonribosomal peptide synthetase PKS polyketide synthetase... 

FIGURE 2.5 In silica natural product discovery. Genome seannlng identified a type I polyketide synthetase in Streptomyces azuinensis, and the structure of the secondary metabolite produced was predicted. This was then verified by culture and isolation of the compound. 

The spatial arrangement of the polyketo acids leads to cyclization by aldol condensations, i.e., reaction of carbonyl groups with acidic CHg-groups, or by Claisen condensation, i.e., reaction of the ester group at the head of the molecule with one of the acidic CHg-groups. The actual type of condensation depends on the nature of the polyketide synthetase in question and the spatial arrangement of the polyketo acid directed by the enzyme. Thus several cyclic structures... 

Tillett D, Dittmann E, Erhard M, von Dohren H, Bomer T, Neilan BA (2000) Structural organization of microcystin biosynthesis in Microcystis aeruginosa PCC7806 an integrated peptide-polyketide synthetase system. Chem Biol 1 153-164... 

Powerful support for any biosynthetic pathway as well as detailed information on the reactions involved may be gained by isolation, purification and characterization of enzymes which will catalyse individual steps of biosynthesis. A number of examples are quoted in succeeding chapters. The most notable are associated with the polyketide synthetases (Section 3.2) and enzymes of steroid biosynthesis (Sections 4.2 and 4.4). 

Several enrichment experiments established the primitive biogenetic precursors of the aflatoxins. The directional mode of folding of the original polyketide and the anomalous nature of the aflatoxin biosynthesis were deduced from NMR spectroscopy, hence illustrating the utility of this technique. The sequence of the aflatoxin biosynthesis was elucidated by the identification of advanced intermediates, the use of blocked mutants and enzyme inhibitors, and conversion of potential precursors by whole cell and cell-free systems into the aflatoxins. Certain facets of aflatoxin biosynthesis require further attention among these are the nature of the polyketide synthetase the exact mechanism by which primary metabolites induce the enzymes of aflatoxin production the loss of the C(6) phenolic hydroxy group during the conversion of the polyhydroxyanthraquinones (e.g., averufin. 

---