Deconstruction of Iterative Polyketide Synthases

Abstract

The enormous progress of the last 25 years in natural products biochemistry has been dominated by investigation of giant microbial enzymes comprised of organized catalytic domains that function according to two distinct paradigms. Each is "programmed" to sequentially assemble simple precursor molecules over multiple steps without diffusible intermediates to synthesize highly elaborated products. Both synthetic strategies rely upon carrier proteins to hold and shuttle extending intermediates among catalytic domains in an ordered fashion. Collectively these megaproteins are impressive "supramolecular machines" exemplifying protein catalysis far more sophisticated than the classical view of "one gene, one enzyme, one reaction" [1], and are of central importance to the creation of small-molecule drugs for human and animal health. Assembly-line and iterative catalysis in natural product biosynthesisThe application of molecular biological techniques to natural product biosynthetic problems in the mid 1980's ushered in the modern era of research in the field. Seminal events include isolation of the penicillin gene cluster and characterization of the non-ribosomal peptide synthetase (NRPS) ACVS that assembles the classical Arnstein L,L,D-tripeptide precursor of the antibiotic [2], Fig. 1. Biochemical experiments on isolated wild-type NRPS enzymes had already established a linear mode of accreting ATP-dependent synthesis [3], but now isolation of the corresponding biosynthetic genes and their translation to domains of recognized function in loosely repeated sets, or "modules," underscored the exquisitely organized nature of the "assembly line" synthetic program executed by these large enzymes. The end of 1990 marked the watershed co-discovery of genes encoding three giant modular polyketide synthases (PKSs) that synthesize the 6-deoxyerythronolide core of the macrolide antibiotic erythromycin [4,5]. Together deoxyerythronolide B synthases (DEBS)1-3, totaling ca. 10 6 Da, opened the door to a second paradigm of assembly line megaenzymes whose synthetic program could be "read" from the N -to C-terminus.The parallel universes of NRPS and modular PKS enzymes in their canonical forms (there are many exceptions and variations on the theme) are extravagant extensions of the classical view of enzyme function enunciated 70 years ago [1]; that New Chemistry and New Opportunities from the Expanding Protein Universe Downloaded from www.worldscientific.com by NANYANG TECHNOLOGICAL UNIVERSITY on 08/20/15. For personal use only.