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Chemoenzymatic strategies that combine synthetic and enzymatic transformations offer efficient approaches to yield target molecules, which have been increasingly employed in the synthesis of bioactive natural products. In the biosynthesis of macrocyclic nonribosomal peptides, polyketides, and their hybrids, thioesterase (TE) domains play a significant role in late-stage macrocyclization. These domains can accept mimics of native substrates in vitro and exhibit potential for use in total synthesis. This review summarizes the recent advances of TE domains in the chemoenzymatic synthesis for these natural products that aim to address the common issues in classical synthetic approaches and increase synthetic efficiencies, which have the potential to facilitate further pharmaceutical research.

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AIMS: Biotechnological and chemical characterization of previously undescribed homologous siderophore-type macrocyclic polyketides from heterotrophic Shewanella algae Microbial Type Culture Collection (MTCC) 12715 affiliated with Rhodophycean macroalga Hypnea valentiae of marine origin, with significant anti-infective potential against drug-resistant pathogens. METHODS AND RESULTS: The heterotrophic bacterial strain in symbiotic association with intertidal macroalga H. valentiae was isolated to homogeneity in a culture-dependent method and screened for bioactivities by spot-over-lawn assay. The bacterial organic extract was purified and characterized by extensive chromatographic and spectroscopic methods, respectively, and was assessed for antibacterial activities with disc diffusion and microtube dilution methods. The macrocyclic polyketide compounds exhibited wide-spectrum of anti-infective potential against clinically significant vancomycin-resistant Enterococcus faecalis (VREfs), methicillin-resistant Staphylococcus aureus (MRSA), Pseudomonas aeruginosa and Klebsiella pneumonia with minimum inhibitory concentration of about 1-3 µg ml-1 , insomuch as the antibiotics chloramphenicol and ampicillin were active at ≥6·25 µg ml-1 . The studied compounds unveiled Fe3+ chelating activity, which designated that their prospective anti-infective activities against the pathogens could be due to their siderophore mechanism of action. In support of that, the bacterium exhibited siderophore production on bioassay involving the cast upon culture agar plate, and the presence of siderophore biosynthetic gene (≈1000 bp) (MF 981936) further corroborated the inference. In silico molecular modelling with penicillin-binding protein (PBP2a) coded by mecA genes of MRSA (docking score -11·68 to -12·69 kcal mol-1 ) verified their in vitro antibacterial activities. Putative biosynthetic pathway of macrocyclic polyketides through stepwise decarboxylative condensation initiated by malonate-acyl carrier protein further validated their structural and molecular attributes. CONCLUSIONS: The studied siderophore-type macrocyclic polyketides from S. algae MTCC 12715 with significant anti-infective potential could be considered as promising candidates for pharmaceutical and biotechnological applications, especially against emerging multidrug-resistant pathogens. SIGNIFICANCE AND IMPACT OF THE STUDY: This study exhibited the heterotrophic bacteria in association with intertidal macroalga as propitious biological resources to biosynthesize novel antibacterial agents.

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Heterotrophic Gamma-proteobacterium Shewanella algae MTCC 12715, associated with an intertidal red algae Hypnea valentiae, presented broad-spectra of antibacterial activities against pathogenic bacteria bringing about nosocomial infection. Bioassay-guided fractionation of the bacterial crude extract resulted in two undescribed macrocyclic polyketide analogs, with anti-infective activities against methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus faecalis (MIC 3.1-5.0 µg/mL). In order to identify the polyketide biosynthetic machinery termed type-I polyketide synthase (pks-I) encoding biologically active secondary metabolites in this strain, the ketosynthase-coding regions of DNA with ≈700 bp size, were amplified, and the partial sequence was submitted in the GenBank (accession number MH157093). The titled compounds were classified under macrocyclic polyketides bearing dodecahydropyrano-trioxacyclooctadecine-dione and trioxo-octadecahydro-1H-benzo[o]tetraoxacyclopentacosine-carboxylate functionalities. Structure-activity correlation analysis displayed that hydrophobic descriptor of the studied compounds could play a prominent role in its anti-infective property against the opportunistic pathogens. Further, in silico molecular docking studies were performed in the allosteric sites of penicillin-binding protein (PBP2a) coded by mecA genes of MRSA, and the best binding pose for each compound (docking score -8.47 kcal/mol and -9.58 kcal/mol, respectively) could be correlated with their in vitro antibacterial activities. The pks-I assisted biosynthetic pathway of macrocyclic polyketides through step-wise decarboxylative condensation initiated by malonate-acyl carrier protein corroborated their structural attributes. Chemical mining of the studied macroalgae-associated heterotrophic bacterium thus revealed the promising antagonistic properties of macrocyclic polyketides isolated from Shewanella algae MTCC 12715 against multidrug-resistant pathogens.

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Detailed analysis of the modular Type I polyketide synthase (PKS) involved in the biosynthesis of the marginolactone azalomycin F in mangrove Streptomyces sp. 211726 has shown that only nineteen extension modules are required to accomplish twenty cycles of polyketide chain elongation. Analysis of the products of a PKS mutant specifically inactivated in the dehydratase domain of extension-module 1 showed that this module catalyzes two successive elongations with different outcomes. Strikingly, the enoylreductase domain of this module can apparently be "toggled" off and on : it functions in only the second of these two cycles. This novel mechanism expands our understanding of PKS assembly-line catalysis and may explain examples of apparent non-colinearity in other modular PKS systems.

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Keeping it basic: Arginine provides the exotic 4-guanidinobutanoate starter unit for two different types of zwitterionic polyketide (an example for one type is shown in the picture) produced by the same Streptomyces bacterium. The three-step precursor pathway is initiated by a remarkable decarboxylating monooxygenase with high specificity for arginine.

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