Enzymatic competition and cooperation branch the caerulomycin biosynthetic pathway toward different 2,2'-bipyridine members

Discovery and biosynthesis of karnamicins as angiotensin converting enzyme inhibitors

Angiotensin-converting enzyme inhibitors are widely used for treatment of hypertension and related diseases. Here, six karnamicins E₁-E₆ (1-6), which bear fully substituted hydroxypyridine and thiazole moieties are characterized from the rare actinobacterium Lechevalieria rhizosphaerae NEAU-A2. Through a combination of isotopic labeling, genome mining, and enzymatic characterization studies, the programmed assembly of the fully substituted hydroxypyridine moiety in karnamicin is proposed to be due to sequential operation of a hybrid polyketide synthase-nonribosomal peptide synthetase, two regioselective pyridine ring flavoprotein hydroxylases, and a methyltransferase. Based on AlphaFold protein structures predictions, molecular docking, and site-directed mutagenesis, we find that two pyridine hydroxylases deploy active site residues distinct from other flavoprotein monooxygenases to direct the chemo- and regioselective hydroxylation of the pyridine nucleus. Pleasingly, karnamicins show significant angiotensin-converting enzyme inhibitory activity with IC₅₀ values ranging from 0.24 to 5.81 μM, suggesting their potential use for the treatment of hypertension and related diseases.

Investigation of 2,2'-Bipyridine Biosynthesis Reveals a Common Two-Component System for Aldehydes Production by Carboxylate Reduction

Here, we report a two-component enzymatic system that efficiently catalyzes the reduction of a carboxylate to an aldehyde in the biosynthesis of 2,2'-bipyridine antibiotics caerulomycins. The associated paradigm involves the activation of carboxylate by ATP-dependent adenylation protein CaeF, followed by its reduction catalyzed by CaeB2, a new class of NADPH-dependent aldehyde dehydrogenase (ALDH) that directly reduces AMP-conjugated carboxylate, which is distinct from the known aldehyde-producing enzymes that reduce ACP- or CoA-conjugated carboxylates.

Structure and Function of a Dual Reductase-Dehydratase Enzyme System Involved in p-Terphenyl Biosynthesis

We report the identification of the ter gene cluster responsible for the formation of the p-terphenyl derivatives terfestatins B and C and echoside B from the Appalachian Streptomyces strain RM-5-8. We characterize the function of TerB/C, catalysts that work together as a dual enzyme system in the biosynthesis of natural terphenyls. TerB acts as a reductase and TerC as a dehydratase to enable the conversion of polyporic acid to a terphenyl triol intermediate. X-ray crystallography of the apo and substrate-bound forms for both enzymes provides additional mechanistic insights. Validation of the TerC structural model via mutagenesis highlights a critical role of arginine 143 and aspartate 173 in catalysis. Cumulatively, this work highlights a set of enzymes acting in harmony to control and direct reactive intermediates and advances fundamental understanding of the previously unresolved early steps in terphenyl biosynthesis.

Caerulomycin and collismycin antibiotics share a trans-acting flavoprotein-dependent assembly line for 2,2'-bipyridine formation

Linear nonribosomal peptide synthetases (NRPSs) and polyketide synthases (PKSs) template the modular biosynthesis of numerous nonribosomal peptides, polyketides and their hybrids through assembly line chemistry. This chemistry can be complex and highly varied, and thus challenges our understanding in NRPS and PKS-programmed, diverse biosynthetic processes using amino acid and carboxylate building blocks. Here, we report that caerulomycin and collismycin peptide-polyketide hybrid antibiotics share an assembly line that involves unusual NRPS activity to engage a trans-acting flavoprotein in C-C bond formation and heterocyclization during 2,2'-bipyridine formation. Simultaneously, this assembly line provides dethiolated and thiolated 2,2'-bipyridine intermediates through differential treatment of the sulfhydryl group arising from L-cysteine incorporation. Subsequent L-leucine extension, which does not contribute any atoms to either caerulomycins or collismycins, plays a key role in sulfur fate determination by selectively advancing one of the two 2,2'-bipyridine intermediates down a path to the final products with or without sulfur decoration. These findings further the appreciation of assembly line chemistry and will facilitate the development of related molecules using synthetic biology approaches.

Activation and enhancement of caerulomycin A biosynthesis in marine-derived Actinoalloteichus sp. AHMU CJ021 by combinatorial genome mining strategies

Background

Activation of silent biosynthetic gene clusters (BGCs) in marine-derived actinomycete strains is a feasible strategy to discover bioactive natural products. Actinoalloteichus sp. AHMU CJ021, isolated from the seashore, was shown to contain an intact but silent caerulomycin A (CRM A) BGC-cam in its genome. Thus, a genome mining work was preformed to activate the strain’s production of CRM A, an immunosuppressive drug lead with diverse bioactivities.

Results

To well activate the expression of cam, ribosome engineering was adopted to treat the wild type Actinoalloteichus sp. AHMU CJ021. The initial mutant strain XC-11G with gentamycin resistance and CRM A production titer of 42.51 ± 4.22 mg/L was selected from all generated mutant strains by gene expression comparison of the essential biosynthetic gene-camE. The titer of CRM A production was then improved by two strain breeding methods via UV mutagenesis and cofactor engineering-directed increase of intracellular riboflavin, which finally generated the optimal mutant strain XC-11GUR with a CRM A production titer of 113.91 ± 7.58 mg/L. Subsequently, this titer of strain XC-11GUR was improved to 618.61 ± 16.29 mg/L through medium optimization together with further adjustment derived from response surface methodology. In terms of this 14.6 folds increase in the titer of CRM A compared to the initial value, strain XC-GUR could be a well alternative strain for CRM A development.

Conclusions

Our results had constructed an ideal CRM A producer. More importantly, our efforts also had demonstrated the effectiveness of abovementioned combinatorial strategies, which is applicable to the genome mining of bioactive natural products from abundant actinomycetes strains.

Studies on the preparation of aminobipyridines and bipyridine sultams via an intramolecular free radical pathway

A variety of aminated bipyridines and bipyridine sultams are prepared by intramolecular radical [1,5]-ipso and [1,6]-ortho substitutions, using a sulfonamide as a linker to connect the pyridyl radical to the pyridine under attack. For the cases studied, different regiochemistries are observed depending on the initial position of the sulfonamide linker.

Results show that intramolecular radical [1,5]-ipso and [1,6]-ortho substitutions offer a method for the preparation of aminobipyridines or bipyridine sultams.

Caerulomycins from Actinoalloteichus cyanogriseus WH1-2216-6: isolation, identification and cytotoxicity

SAR study of 42 caerulomycins from A. cyanogriseus revealed that 6-aldoxime and 4-O-glycosidation are respectively essential for their activity and selectivity.

Discovery of caerulomycin/collismycin-type 2,2'-bipyridine natural products in the genomic era

2,2'-Bipyridine (2,2'-BP) is the unique molecular scaffold of the bioactive natural products represented by caerulomycins (CAEs) and collismycins (COLs). CAEs and COLs are highly similar in the chemical structures in which their 2,2'-BP cores typically contain a di- or tri-substituted ring A and an unmodified ring B. Here, we summarize the CAE and COL-type 2,2'-BP natural products known or hypothesized to date: (1) isolated using methods traditional for natural product characterization, (2) created by engineering the biosynthetic pathways of CAEs or COLs, and (3) predicted upon bioinformatics-guided genome mining. The identification of these CAE and COL-type 2,2'-BP natural products not only demonstrates the development of research techniques and methods in the field of natural product chemistry but also reflects the general interest in the discovery of CAE and COL-type 2,2'-BP natural products.