Nosiheptide analogues as potential antibacterial agents via dehydroalanine region modifications: Semi-synthesis, antimicrobial activity and molecular docking study

Selective biosynthesis of a rhamnosyl nosiheptide by a novel bacterial rhamnosyltransferase

Nosiheptide (NOS) is a thiopeptide antibiotic produced by the bacterium Streptomyces actuosus. The hydroxyl group of 3-hydroxypyridine in NOS has been identified as a promising site for modification, which we therefore aimed to rhamnosylate. After screening, Streptomyces sp. 147326 was found to regioselectively attach a rhamnosyl unit to the 3-hydroxypyridine site in NOS, resulting in the formation of a derivative named NOS-R at a productivity of 24.6%. In comparison with NOS, NOS-R exhibited a 17.6-fold increase in aqueous solubility and a new protective effect against MRSA infection in mice, while maintaining a similar in vitro activity. Subsequently, SrGT822 was identified as the rhamnosyltransferase in Streptomyces sp. 147326 responsible for the biosynthesis of NOS-R using dTDP-L-rhamnose. SrGT822 demonstrated an optimal reaction pH of 10.0 and temperature of 55°C, which resulted in a NOS-R yield of 74.9%. Based on the catalytic properties and evolutionary analysis, SrGT822 is anticipated to be a potential rhamnosyltransferase for use in the modification of various complex scaffolds.

Design of a chimeric glycosyltransferase OleD for the site-specific O-monoglycosylation of 3-hydroxypyridine in nosiheptide

To identify the potential role of the 3-hydroxyl group of the pyridine ring in nosiheptide (NOS) for its antibacterial activity against Gram-positive pathogens, enzymatic glycosylation was utilized to regio-selectively create a monoglycosyl NOS derivative, NOS-G. For this purpose, we selected OleD, a UDP glycosyltransferase from Streptomyces antibioticus that has a low productivity for NOS-G. Activity of the enzyme was increased by swapping domains derived from OleI, both single and in combination. Activity enhancement was best in mutant OleD-10 that contained four OleI domains. This chimer was engineered by site-directed mutagenesis (single and in combination) to increase its activity further, whereby variants were screened using a newly-established colorimetric assay. OleD-10 with I117F and T118G substitutions (FG) had an increased NOS-G productivity of 56%, approximately 70 times higher than that of wild-type OleD. The reason for improved activity of FG towards NOS was structurally attributed to a closer distance (<3 Å) between NOS/sugar donor and the catalytic amino acid H25. The engineered enzyme allowed sufficient activity to demonstrate that the produced NOS-G had enhanced stability and aqueous solubility compared to NOS. Using a murine MRSA infection model, it was established that NOS-G resulted in partial protection within 20 h of administration and delayed the death of infected mice. We conclude that 3-hydroxypyridine is a promising site for structural modification of NOS, which may pave the way for producing nosiheptide derivatives as a potential antibiotic for application in clinical treatment.

Selenium in Peptide Chemistry

In recent years, researchers have been exploring the potential of incorporating selenium into peptides, as this element possesses unique properties that can enhance the reactivity of these compounds. Selenium is a non-metallic element that has a similar electronic configuration to sulfur. However, due to its larger atomic size and lower electronegativity, it is more nucleophilic than sulfur. This property makes selenium more reactive toward electrophiles. One of the most significant differences between selenium and sulfur is the dissociation of the Se-H bond. The Se-H bond is more easily dissociated than the S-H bond, leading to higher acidity of selenocysteine (Sec) compared to cysteine (Cys). This difference in acidity can be exploited to selectively modify the reactivity of peptides containing Sec. Furthermore, Se-H bonds in selenium-containing peptides are more susceptible to oxidation than their sulfur analogs. This property can be used to selectively modify the peptides by introducing new functional groups, such as disulfide bonds, which are important for protein folding and stability. These unique properties of selenium-containing peptides have found numerous applications in the field of chemical biology. For instance, selenium-containing peptides have been used in native chemical ligation (NCL). In addition, the reactivity of Sec can be harnessed to create cyclic and stapled peptides. Other chemical modifications, such as oxidation, reduction, and photochemical reactions, have also been applied to selenium-containing peptides to create novel molecules with unique biological properties.

Nosiheptide Harbors Potent In Vitro and Intracellular Inhbitory Activities against Mycobacterium tuberculosis

Multidrug-resistant tuberculosis (MDR-TB) is often associated with poor clinical outcomes. In this study, we evaluated the potential of nosiheptide (NOS) as a new drug candidate for treating Mycobacterium tuberculosis infections, including MDR-TB. The antimicrobial susceptibility testing was performed to determine the MICs of NOS against 18 reference strains of slowly growing mycobacteria (SGM) and 128 clinical isolates of M. tuberculosis. The postantibiotic effects (PAE) and interaction with other antituberculosis drugs of NOS were also evaluated using M. tuberculosis H37Rv. Fifteen out of the 18 tested reference strains of SGM had MICs far below 1 μg/mL. From the 128 M. tuberculosis clinical isolates, the MIC₅₀ and MIC₉₀ were 0.25 μg/mL and 1 μg/mL, respectively; the tentative epidemiological cutoff (ECOFF) was defined at 1 μg/mL. Furthermore, a Lys89Thr mutation was found in one M. tuberculosis isolate with a MIC of NOS >8 μg/mL. After 24 h of incubation, NOS at 1 μg/mL inhibited 25.79 ± 1.22% of intracellular bacterial growth, which was comparable with the inhibitory rate of 25.71 ± 3.67% achieved by rifampin at 2 μg/mL. Compared to rifampicin and isoniazid (INH), NOS had a much longer PAE, i.e., a value of about 16 days. In addition, a partial synergy between NOS and INH was observed. NOS has potent inhibitory activities against M. tuberculosis in vitro as well as in macrophages. Furthermore, the long PAE and partial synergistic effect with INH, in addition to the added safety of long-term use as a feed additive in husbandry, provide support for NOS being a promising drug candidate for tuberculosis treatment. IMPORTANCE This study is aimed at chemotherapy for MDR-TB, mainly to explore the anti-TB activity of the existing chemotherapeutic reagent. We found that NOS has potent inhibitory activities against M. tuberculosis in vitro regardless of the drug-resistant profile. Furthermore, NOS also showed the long PAE and partial synergistic effect with INH and is nontoxic, providing support for its promise as a drug candidate for drug-resistant tuberculosis treatment.

In vitro and intracellular inhibitory activities of nosiheptide against Mycobacterium abscessus

The high level of inherent drug resistance of Mycobacterium abscessus makes the infection caused by it very difficult to be treated. The objective of this study was to evaluate the potential of nosiheptide (NOS) as a new drug candidate for treating M. abscessus infections. The microplate AlamarBlue assay was performed to determine the minimum inhibitory concentrations (MICs) of NOS for 28 reference strains of rapidly growing mycobacteria (RGM) and 77 clinical isolates of M. abscessus. Time-kill kinetic and post-antibiotic effect (PAE) of NOS against M. abscessus was evaluated. Its bactericidal activity against M. abscessus in macrophages was determined by an intracellular colony numerating assay. NOS manifested good activity against the reference strains of RGM and M. abscessus clinical isolates in vitro. The MICs of NOS against M. abscessus clinical isolates ranged from 0.0078 to 1 μg/ml, and the MIC₅₀ and MIC₉₀ were 0.125 μg/ml and 0.25 μg/ml, respectively. The pattern of growth and kill by NOS against M. abscessus was moderate with apparent concentration-dependent characteristics, and the PAE value of NOS was found to be ~6 h. Furthermore, NOS had low cell toxicity against the THP-1 cell line after 48 h of exposure (IC₅₀ = 106.9 μM). At 4 μg/ml, NOS exhibited high intracellular bactericidal activity against M. abscessus reference strains with an inhibitory rate of 66.52% ± 1.51%, comparable with that of clarithromycin at 2 μg/ml. NOS showed suitable inhibitory activities against M. abscessus in vitro and in macrophages and could be a potential drug candidate to treat M. abscessus infection.