Enantioselective Total Synthesis of Fortimicin B

Fortimicins, featuring a pseudodisaccharide scaffold, are an unusual class of aminoglycosides (AGs) with potent efficacy against several aminoglycoside-resistant bacterial strains. Notably, these molecules also exhibit lower inherent ototoxicity and nephrotoxicity than common aminoglycosides. Consequently, fortimicins are a promising type of protoypical molecules for the development of the next generation of aminoglycoside antibiotics. Here, we report the asymmetric total synthesis of fortimicin B in 12 steps (longest linear sequence, LLS) from readily available starting materials. An enantioselective Cu(II)-catalyzed inverse-electron-demand Diels-Alder (IEDDA) reaction of 2-pyrones and N-substituted 2-oxazolones was developed for the efficient synthesis of the fortamine fragment, which previously required a lengthy multistep synthesis owing to its complex stereochemistry. The 6-epi-purpurosamine B fragment was efficiently synthesized through a Cr(II)/Co(I)-mediated C─C bond coupling between aldehydes and alkyl halides. Within these two fragments, the stereoselective construction of the α-glycosidic bond of fortimicin B was realized via the gold(I)-catalyzed glycosylation. Overall, this study provides an efficient synthetic platform for future investigations into the structure-activity relationships of fortimicins.

Genomic, LC-MS, and FTIR Analysis of Plant Probiotic Potential of Bacillus albus for Managing Xanthomonas oryzae via Different Modes of Application in Rice (Oryza sativa L.)

Xanthomonas oryzae causes tremendous damage in rice plants (Oryza sativa L). Therefore, this study is focused on siderophore-producing Bacillus albus (CWTS 10) for managing BLB disease caused by X. oryzae. Both B. albus and its crude siderophore (methanolic and diethyl ether) extracts inhibited X. oryzae (10-12 mm). Fourier transform infrared spectroscopy (FTIR) analysis of the extracts indicated the presence of catecholate siderophore functional groups. Liquid chromatography-mass spectrometry (LC-MS) analysis revealed the presence of antimicrobial compounds such as 2-deoxystreptamine, miserotoxin, fumitremorgin C, pipercide, pipernonaline, gingerone A, and deoxyvasicinone. Complete genome sequencing revealed the gene clusters for antibiotic, siderophore, antibacterial, antifungal, and secondary metabolite production. An in vivo study revealed that bacteria (CWTS 10) and their siderophore extracts effectively inhibited X. oryzae. The mode of application of bacterial or siderophore extracts in terms of DI and DSI percentage was as follows: soak method > inoculation method > spray method. In addition to providing enhanced antagonistic activity, there was a significant increase in root and shoot length and weight (wet and dry) of treated plants compared to control plants challenged with X. oryzae. Thus, the results clearly indicate that siderophore-producing B. albus and its siderophore extracts strongly inhibited X. oryzae. However, further field experiments are required before being formulated to protect rice crops from X. oryzae.

Mechanistic plasticity in ApmA enables aminoglycoside promiscuity for resistance

The efficacy of aminoglycoside antibiotics is waning due to the acquisition of diverse resistance mechanisms by bacteria. Among the most prevalent are aminoglycoside acetyltransferases (AACs) that inactivate the antibiotics through acetyl coenzyme A-mediated modification. Most AACs are members of the GCN5 superfamily of acyltransferases which lack conserved active site residues that participate in catalysis. ApmA is the first reported AAC belonging to the left-handed β-helix superfamily. These enzymes are characterized by an essential active site histidine that acts as an active site base. Here we show that ApmA confers broad-spectrum aminoglycoside resistance with a molecular mechanism that diverges from other detoxifying left-handed β-helix superfamily enzymes and canonical GCN5 AACs. We find that the active site histidine plays different functions depending on the acetyl-accepting aminoglycoside substrate. This flexibility in the mechanism of a single enzyme underscores the plasticity of antibiotic resistance elements to co-opt protein catalysts in the evolution of drug detoxification.

Biosynthetic Origin of the Octose Core and Its Mechanism of Assembly during Apramycin Biosynthesis

Apramycin is an aminoglycoside antibiotic isolated from Streptoalloteichus tenebrarius and S. hindustanus that has found clinical use in veterinary medicine. The apramycin structure is notable for its atypical eight-carbon bicyclic dialdose (octose) moiety. While the apramycin biosynthetic gene cluster (apr) has been identified and several of the encoded genes functionally characterized, how the octose core itself is assembled has remained elusive. Nevertheless, recent gene deletion studies have hinted at an N-acetyl aminosugar being a key precursor to the octose, and this hypothesis is consistent with the additional feeding experiments described in the present report. Moreover, bioinformatic analysis indicates that AprG may be structurally similar to GlcNAc-2-epimerase and hence recognize GlcNAc or a structurally similar substrate suggesting a potential role in octose formation. AprG with an extended N-terminal sequence was therefore expressed, purified, and assayed in vitro demonstrating that it does indeed catalyze a transaldolation reaction between GlcNAc or GalNAc and 6'-oxo-lividamine to afford 7'-N-acetyldemethylaprosamine with the same 6'-R and 7'-S stereochemistry as those observed in the apramycin product. Biosynthesis of the octose core in apramycin thus proceeds in the [6 + 2] manner with GlcNAc or GalNAc as the two-carbon donor, which has not been previously reported for biological octose formation, as well as novel inverting stereochemistry of the transferred fragment. Consequently, AprG appears to be a new transaldolase that lacks any apparent sequence similarity to the currently known aldolases and catalyzes a transaldolation for which there is no established biological precedent.

Synthesis of Gentamicins C1, C2, and C2a and Antiribosomal and Antibacterial Activity of Gentamicins B1, C1, C1a, C2, C2a, C2b, and X2

Complementing our earlier syntheses of the gentamicins B1, C1a, C2b, and X2, we describe the synthesis of gentamicins C1, C2, and C2a characterized by methyl substitution at the 6'-position, and so present an alternative access to previous chromatographic methods for accessing these sought-after compounds. We describe the antiribosomal activity of our full set of synthetic gentamicin congeners against bacterial ribosomes and hybrid ribosomes carrying the decoding A site of the human mitochondrial, A1555G mutant mitochondrial, and cytoplasmic ribosomes and establish structure-activity relationships with the substitution pattern around ring I to antiribosomal activity, antibacterial resistance due to the presence of aminoglycoside acetyl transferases acting on the 6'-position in ring I, and literature cochlear toxicity data.

Chemo-enzymatic Synthesis of Pseudo-trisaccharide Aminoglycoside Antibiotics with Enhanced Nonsense Read-through Inducer Activity

Aminoglycosides (AGs) are broad-spectrum antibiotics used to treat bacterial infections. Over the last two decades, studies have reported the potential of AGs in the treatment of genetic disorders caused by nonsense mutations, owing to their ability to induce the ribosomes to read through these mutations and produce a full-length protein. However, the principal limitation in the clinical application of AGs arises from their high toxicity, including nephrotoxicity and ototoxicity. In this study, five novel pseudo-trisaccharide analogs were synthesized by chemo-enzymatic synthesis by acid hydrolysis of commercially available AGs, followed by an enzymatic reaction using recombinant substrate-flexible KanM2 glycosyltransferase. The relationships between their structures and biological activities, including the antibacterial, nephrotoxic, and nonsense readthrough inducer (NRI) activities, were investigated. The absence of 1-N-acylation, 3',4'-dideoxygenation, and post-glycosyl transfer modifications on the third sugar moiety of AGs diminishes their antibacterial activities. The 3',4'-dihydroxy and 6'-hydroxy moieties regulate the in vitro nephrotoxicity of AGs in mammalian cell lines. The 3',4'-dihydroxy and 6'-methyl scaffolds are indispensable for the ex vivo NRI activity of AGs. Based on the alleviated in vitro antibacterial properties and nephrotoxicity, and the highest ex vivo NRI activity among the five compounds, a kanamycin analog (6'-methyl-3''-deamino-3''-hydroxykanamycin C) was selected as a novel AG hit for further studies on human genetic disorders caused by premature transcriptional termination.

The isolation of water-soluble natural products - challenges, strategies and perspectives

Covering period: up to 2019Water-soluble natural products constitute a relevant group of secondary metabolites notably known for presenting potent biological activities. Examples are aminoglycosides, β-lactam antibiotics, saponins of both terrestrial and marine origin, and marine toxins. Although extensively investigated in the past, particularly during the golden age of antibiotics, hydrophilic fractions have been less scrutinized during the last few decades. This review addresses the possible reasons on why water-soluble metabolites are now under investigated and describes approaches and strategies for the isolation of these natural compounds. It presents examples of several classes of hydrosoluble natural products and how they have been isolated. Novel stationary phases and chromatography techniques are also reviewed, providing a perspective towards a renaissance in the investigation of water-soluble natural products.

Microbial Enzymatic Synthesis of Amikacin Analogs With Antibacterial Activity Against Multidrug-Resistant Pathogens

With the constant emergence of multidrug-resistant gram-negative bacteria, interest in the development of new aminoglycoside (AG) antibiotics for clinical use has increased. The regioselective modification of AG scaffolds could be an efficient approach for the development of new antibiotics with improved therapeutic potency. We enzymatically synthesized three amikacin analogs containing structural modifications in the amino groups and evaluated their antibacterial activity and cytotoxicity. Among them, 6′-N-acyl-3^″-N-methylated analogs showed improved antibacterial activity against the multidrug-resistant gram-negative bacteria tested, while exhibiting reduced in vitro nephrotoxicity compared to amikacin. This study demonstrated that the modifications of the 6′-amino group as well as the 3^″-amino group have noteworthy advantages for circumventing the AG-resistance mechanism. The regiospecific enzymatic modification could be exploited to develop novel antibacterial agents with improved pharmacological potential.

Heptose-containing bacterial natural products: structures, bioactivities, and biosyntheses

Covering: up to 2020Glycosylated natural products hold great potential as drugs for the treatment of human and animal diseases. Heptoses, known as seven-carbon-chain-containing sugars, are a group of saccharides that are rarely observed in natural products. Based on the structures of the heptoses, the heptose-containing natural products can be divided into four groups, characterized by heptofuranose, highly-reduced heptopyranose, d-heptopyranose, and l-heptopyranose. Many of them possess remarkable biological properties, including antibacterial, antifungal, antitumor, and pain relief activities, thereby attracting great interest in biosynthesis and chemical synthesis studies to understand their construction mechanisms and structure-activity relationships. In this review, we summarize the structural properties, biological activities, and recent progress in the biosynthesis of bacterial natural products featuring seven-carbon-chain-containing sugars. The biosynthetic origins of the heptose moieties are emphasized.

Pyridoxal-5'-phosphate-dependent enzyme GenB3 Catalyzes C-3',4'-dideoxygenation in gentamicin biosynthesis

Background

The C-3′,4′-dideoxygenation structure in gentamicin can prevent deactivation by aminoglycoside 3′-phosphotransferase (APH(3′)) in drug-resistant pathogens. However, the enzyme catalyzing the dideoxygenation step in the gentamicin biosynthesis pathway remains unknown.

Results

Here, we report that GenP catalyzes 3′ phosphorylation of the gentamicin biosynthesis intermediates JI-20A, JI-20Ba, and JI-20B. We further demonstrate that the pyridoxal-5′-phosphate (PLP)-dependent enzyme GenB3 uses these phosphorylated substrates to form 3′,4′-dideoxy-4′,5′-ene-6′-oxo products. The following C-6′-transamination and the GenB4-catalyzed reduction of 4′,5′-olefin lead to the formation of gentamicin C. To the best of our knowledge, GenB3 is the first PLP-dependent enzyme catalyzing dideoxygenation in aminoglycoside biosynthesis.

Conclusions

This discovery solves a long-standing puzzle in gentamicin biosynthesis and enriches our knowledge of the chemistry of PLP-dependent enzymes. Interestingly, these results demonstrate that to evade APH(3′) deactivation by pathogens, the gentamicin producers evolved a smart strategy, which utilized their own APH(3′) to activate hydroxyls as leaving groups for the 3′,4′-dideoxygenation in gentamicin biosynthesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01558-7.

Development of 6'-N-Acylated Isepamicin Analogs with Improved Antibacterial Activity Against Isepamicin-Resistant Pathogens

The development of new aminoglycoside (AG) antibiotics has been required to overcome the resistance mechanism of AG-modifying enzymes (AMEs) of AG-resistant pathogens. The AG acetyltransferase, AAC(6′)-APH(2″), one of the most typical AMEs, exhibiting substrate promiscuity towards a variety of AGs and acyl-CoAs, was employed to enzymatically synthesize new 6′-N-acylated isepamicin (ISP) analogs, 6′-N-acetyl/-propionyl/-malonyl ISPs. They were all active against the ISP-resistant Gram-negative bacteria tested, and the 6′-N-acetyl ISP displayed reduced toxicity compared to ISP in vitro. This study demonstrated the importance of the modification of the 6′-amino group in circumventing AG-resistance and the potential of regioselective enzymatic modification of AG scaffolds for the development of more robust AG antibiotics.