How cis-Acyltransferase Assembly-Line Ketosynthases Gatekeep for Processed Polyketide Intermediates

With the redefinition of polyketide synthase (PKS) modules, a new appreciation of their most downstream domain, the ketosynthase (KS), is emerging. In addition to performing its well-established role of generating a carbon-carbon bond between an acyl-CoA building block and a growing polyketide, it may gatekeep against incompletely processed intermediates. Here, we investigate 739 KSs from 92 primarily actinomycete, cis-acyltransferase assembly lines. When KSs were separated into 16 families based on the chemistries at the α- and β-carbons of their polyketide substrates, a comparison of 32 substrate tunnel residues revealed unique sequence fingerprints. Surprisingly, additional fingerprints were detected when the chemistry at the γ-carbon was considered. Representative KSs were modeled bound to their natural polyketide substrates to better understand observed patterns, such as the substitution of a tryptophan by a smaller residue to accommodate an l-α-methyl group or the substitution of four smaller residues by larger ones to make better contact with a primer unit or diketide. Mutagenesis of a conserved glutamine in a KS within a model triketide synthase indicates that the substrate tunnel is sensitive to alteration and that engineering this KS to accept unnatural substrates may require several mutations.

Polyether Cyclization Cascade Alterations in Response to Monensin Polyketide Synthase Mutations

The targeted manipulation of polyketide synthases has in recent years led to numerous new-to-nature polyketides. For type I polyketide synthases the response of post-polyketide synthases (PKS) processing enzymes onto the most frequently polyketide backbone manipulations is so far insufficiently studied. In particular, complex processes such as the polyether cyclisation in the biosynthesis of ionophores such as monensin pose interesting objects of research. We present here a study of the substrate promiscuity of the polyether cyclisation cascade enzymes in monensin biosynthesis in the conversion of redox derivatives of the nascent polyketide chain. LC-HRMS/MS² -based studies revealed a remarkable flexibility of the post-PKS enzymes. They acted on derivatized polyketide backbones based on the three possible polyketide redox states within two different modules and gave rise to an altered polyether structure. One of these monensin derivatives was isolated and characterized by 2D-NMR spectroscopy, crystallography, and bioactivity studies.

Mutational Biosynthesis of Hitachimycin Analogs Controlled by the β-Amino Acid-Selective Adenylation Enzyme HitB

Hitachimycin is a macrolactam antibiotic with an (S)-β-phenylalanine (β-Phe) at the starter position of its polyketide skeleton. (S)-β-Phe is formed from l-α-phenylalanine by the phenylananine-2,3-aminomutase HitA in the hitachimycin biosynthetic pathway. In this study, we produced new hitachimycin analogs via mutasynthesis by feeding various (S)-β-Phe analogs to a ΔhitA strain. We obtained six hitachimycin analogs with F at the ortho, meta, or para position and Cl, Br, or a CH₃ group at the meta position of the phenyl moiety, as well as two hitachimycin analogs with thienyl substitutions. Furthermore, we carried out a biochemical and structural analysis of HitB, a β-amino acid-selective adenylation enzyme that introduces (S)-β-Phe into the hitachimycin biosynthetic pathway. The K_M values of the incorporated (S)-β-Phe analogs and natural (S)-β-Phe were similar. However, the K_M values of unincorporated (S)-β-Phe analogs with Br and a CH₃ group at the ortho or para position of the phenyl moiety were high, indicating that HitB functions as a gatekeeper to select macrolactam starter units during mutasynthesis. The crystal structure of HitB in complex with (S)-β-3-Br-phenylalanine sulfamoyladenosine (β-m-Br-Phe-SA) revealed that the bulky meta-Br group is accommodated by the conformational flexibility around Phe328, whose side chain is close to the meta position. The aromatic group of β-m-Br-Phe-SA is surrounded by hydrophobic and aromatic residues, which appears to confer the conformational flexibility that enables HitB to accommodate the meta-substituted (S)-β-Phe. The new hitachimycin analogs exhibited different levels of biological activity in HeLa cells and multidrug-sensitive budding yeast, suggesting that they may target different molecules.

The ambruticins and jerangolids - chemistry, biology and chemoenzymatic synthesis of potent antifungal drug candidates

Covering: 1977 to 2020The ambruticins and jerangolids are myxobacterial reduced polyketides, which are produced via highly unusual biosynthetic pathways containing a plethora of non-canonical enzymatic transformations. Since the discovery of the first congeners in the late 1970s, they have been in the focus of drug development due to their good antifungal activity and low toxicity in mammals, which result from interaction with an unusual innercellular target in fungi. Despite significant efforts, which have led to the development of various total syntheses, their structural complexity has yet avoided full exploitation of their pharmacological potential. This article summarises biological, total and semisynthetic as well as biosynthetic studies on both compounds. An outlook on the biosynthesis-based approaches to them and their derivatives is presented. Due to the structural and biosynthetic characteristics of the ambruticins and jerangolids, chemoenzymatic processes that make use of their biosynthetic pathway enzymes are particularly promising to gain efficient access to derivative libraries for structure activity relationship studies.

Site directed mutagenesis as a precision tool to enable synthetic biology with engineered modular polyketide synthases

Modular polyketide synthases (PKSs) are a multidomain megasynthase class of biosynthetic enzymes that have great promise for the development of new compounds, from new pharmaceuticals to high value commodity and specialty chemicals. Their colinear biosynthetic logic has been viewed as a promising platform for synthetic biology for decades. Due to this colinearity, domain swapping has long been used as a strategy to introduce molecular diversity. However, domain swapping often fails because it perturbs critical protein-protein interactions within the PKS. With our increased level of structural elucidation of PKSs, using judicious targeted mutations of individual residues is a more precise way to introduce molecular diversity with less potential for global disruption of the protein architecture. Here we review examples of targeted point mutagenesis to one or a few residues harbored within the PKS that alter domain specificity or selectivity, affect protein stability and interdomain communication, and promote more complex catalytic reactivity.