The identification of 3,6-dihydroxy-1,2-benzisoxazole (compound 1) as a potent antibacterial agent against multidrug-resistant Acinetobacter baumannii has prompted detailed investigation into its mechanism of action. While initial studies revealed that compound 1 exhibits strong growth inhibition in minimal media—where MIC values drop to as low as 6.25 µg/ml—the ability of 4-hydroxybenzoate (4-HB) to reverse this effect provided critical insight into potential molecular targets. This antagonism suggests that compound 1 interferes with bacterial metabolic pathways dependent on 4-HB, a precursor essential for ubiquinone biosynthesis and aerobic respiration.
To elucidate the precise target, we conducted a comprehensive analysis combining phenotypic assays, structural modeling, and biochemical reasoning. Phenotype microarray experiments using A. baumannii UNT197 and Pseudomonas aeruginosa UCBPP14 demonstrated that both 4-HB and its derivative, 4-hydroxybenzaldehyde, effectively counteracted the antibacterial activity of compound 1. This observation narrowed the focus to enzymes involved in 4-HB metabolism. Two key candidates emerged: chorismate pyruvate-lyase (CPL), responsible for converting chorismate into 4-HB, and 4-hydroxybenzoate octaprenyltransferase (UbiA), which catalyzes the prenylation of 4-HB during ubiquinone assembly.
Molecular docking simulations were performed using a homology model of A. baumannii CPL based on the crystal structure of E. coli CPL (PDB: 1FW9). The model exhibited high structural fidelity, with a root-mean-square deviation (RMSD) of only 0.780 Å. Docking results indicated that compound 1 binds strongly to the active site of CPL with a predicted binding affinity of -5.8 kcal/mol, forming five hydrogen bonds with conserved residues including Arg78, Glu157, Met35, and Leu116. These interactions mirror those observed between 4-HB and the native enzyme, suggesting that compound 1 acts as a competitive product inhibitor, displacing 4-HB from its binding site and disrupting the catalytic cycle.
This hypothesis is further supported by known biological behavior of CPL. In bacteria, the enzyme operates under a feedback inhibition mechanism where accumulated 4-HB stabilizes the protein and prevents reactivation until chorismate enters the secondary binding site.HAS2 Antibody Cancer By mimicking 4-HB’s binding pattern, compound 1 may lock CPL in an inactive conformation, halting the production of 4-HB and ultimately blocking ubiquinone synthesis. This would impair electron transport chain function, leading to respiratory deficiency and cell death—particularly under aerobic conditions.
An alternative, complementary mechanism involves inhibition of 4-HB octaprenyltransferase (UbiA). This membrane-bound enzyme requires both 4-HB and geranyl diphosphate for catalysis. The active site features two critical residues: Asp191, which facilitates proton abstraction to activate the benzene ring, and Arg72, which interacts with the meta-position substituent. Compound 1 satisfies both requirements through its C6 hydroxyl group and C3 hydroxyl, potentially allowing it to bind competitively or even undergo prenylation itself. If incorporated into the pathway, the resulting adduct might be non-functional due to steric or electronic incompatibility with downstream enzymes.
Sequence alignment confirmed conservation of Asp191 across multiple species, including A.Cytokeratin 6 Antibody MedChemExpress baumannii, but Arg72 was absent in humans, indicating possible selectivity for bacterial targets.PMID:34174043 This supports the notion that compound 1 could achieve a favorable therapeutic index by sparing mammalian cells while disrupting essential bacterial metabolism.
In summary, our data collectively point to dual or synergistic inhibition of 4-HB-utilizing enzymes as the primary mode of action of 3,6-dihydroxy-1,2-benzisoxazole. The structural mimicry of 4-HB, combined with strong binding predictions and functional validation via metabolite rescue, provides compelling evidence for targeting the ubiquinone biosynthetic pathway. These findings not only clarify the biological activity of this novel antibiotic but also open avenues for rational drug design—enabling the development of more selective and potent derivatives aimed at overcoming resistance in MDR A. baumannii.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com