The logic behind the target is elegant. The AgrA protein is a response regulator whose C-terminal LytTR domain, spanning residues 138 to 238, binds to DNA and switches on the expression of numerous virulence genes. Blocking that domain with a small molecule would, in principle, silence the virulence program without imposing the intense survival pressure that drives conventional antibiotic resistance. Crucially, the LytTR domain is widespread among bacteria but essentially absent from higher organisms, which reduces the risk of off-target effects in human cells. Because experimental three-dimensional structures were unavailable for the AgrA LytTR domains of Chlamydia trachomatis, Enterococcus faecalis, Listeria monocytogenes and Macrococcus canis, the team used homology models built in an earlier study, with the crystal structure of the Staphylococcus aureus AgrA LytTR domain (PDB ID 4G4K) serving as the template and as a fifth test case.

The two candidate molecules were chosen from the literature on natural products. Alkaloids are a chemically diverse class of plant secondary metabolites with a long clinical pedigree, ranging from the antimalarial quinine to the central nervous system stimulant brucine, and both lanoginosine and liriodenine have documented biological activities, including reported anti-quorum-sensing effects. After drawing the two-dimensional structures and optimizing their geometry with the PM7 semiempirical method in MOPAC2016, the researchers docked each ligand into the DNA-interacting region of all five AgrA proteins using AutoDock 4.2 with the Lamarckian genetic algorithm, treating both protein and ligand as rigid and generating ten conformations per run ranked by predicted binding affinity.

The docking results were strikingly consistent. Both alkaloids achieved binding free energies below −8.5 kcal/mol with every AgrA protein tested. Liriodenine bound to the Macrococcus canis protein at −9.90 kcal/mol, the strongest docking score in the study, while lanoginosine reached −9.26 kcal/mol against the Enterococcus faecalis protein. For comparison, the well-known experimental AgrA inhibitor savirin shows a reported docking energy of only −7.2 kcal/mol. The atomic details of the interactions reveal why the compounds bind so well. In the Chlamydia trachomatis complex, liriodenine forms two hydrogen bonds, one between an oxygen of its benzodioxole moiety and the NH group of Thr164 at 2.30 angstroms and another between its carbonyl oxygen and Arg201 at 2.31 angstroms, supplemented by pi-sigma, pi-alkyl and pi-donor hydrogen bond contacts with residues including Leu171 and Leu186. In the Enterococcus faecalis complex, lanoginosine hydrogen bonds with Ser164 and Leu186 and engages in multiple pi-cation interactions with Arg198.

Docking, however, is only a static snapshot. To test whether the binding poses hold up under realistic thermal motion, the team ran 50-nanosecond molecular dynamics simulations for all ten protein-ligand complexes using GROMACS via the WebGRO server, with the GROMOS96 43a1 force field, an SPC water model, neutralizing sodium or chloride ions, physiological salt concentration of 0.15 M, 300 K temperature and 1 bar pressure. They tracked root mean square deviation (RMSD), root mean square fluctuation (RMSF), radius of gyration (Rg) and solvent accessible surface area (SASA) across 1000 saved frames per simulation. A stable protein-ligand complex is generally expected to keep its backbone RMSD within about 0.3 nanometers.

The simulations largely passed that test. Five complexes, including Listeria monocytogenes-liriodenine, Macrococcus canis-liriodenine, Macrococcus canis-lanoginosine and both Staphylococcus aureus complexes, achieved average RMSD values below 0.3 nanometers, with the 4G4K-lanoginosine complex reaching an exceptionally low 0.24 ± 0.036 nanometers. The remaining complexes hovered near the threshold, such as the Chlamydia trachomatis-liriodenine pair at 0.32 ± 0.029 nanometers. RMSF analysis showed that only a handful of individual residues, mostly near the flexible termini of the domain, exceeded 0.4 nanometers of fluctuation, while average RMSF values ranged from 0.15 to 0.20 nanometers across all systems. Radius of gyration values of roughly 1.23 to 1.33 nanometers indicated that the proteins retained their compact folded architecture throughout the simulations, and SASA values remained steady, further supporting structural integrity.

To quantify binding strength more rigorously than docking scores allow, the researchers applied MM/PBSA calculations to every trajectory using the g_mmpbsa tool, summing van der Waals, electrostatic, polar solvation and surface area terms. Here liriodenine emerged as the stronger binder overall, with binding free energies ranging from −71.08 to −134.317 kJ/mol across the model proteins. The most dramatic case was Enterococcus faecalis, where liriodenine reached −134.317 kJ/mol versus −74.453 kJ/mol for lanoginosine, a gap driven largely by van der Waals contributions of −153.048 kJ/mol for liriodenine. Against the Staphylococcus aureus template the difference was even starker: −99.558 kJ/mol for liriodenine compared with just −33.124 kJ/mol for lanoginosine. Principal component analysis and free energy landscape calculations reinforced the picture, with the Chlamydia trachomatis-liriodenine complex displaying the most pronounced energy minimum, a signature of superior conformational stability.

The study then turned to quantum chemistry to probe the electronic personalities of the two molecules. Density functional theory calculations at the M06-2X/def2-TZVP level yielded a full set of global reactivity descriptors. Lanoginosine has the higher HOMO energy at −7.20 eV versus −7.52 eV for liriodenine, meaning it donates charge more readily, while liriodenine’s lower LUMO makes it the better electron acceptor. Liriodenine also showed higher electronegativity (4.67 eV), greater chemical hardness (5.70 eV versus 5.42 eV) and a larger HOMO-LUMO gap, marking it as the more chemically stable and less reactive of the pair, whereas lanoginosine’s softer, smaller-gap profile suggests it interacts more easily with biological macromolecules, which are themselves soft. Both compounds shared an identical global softness of 0.09 eV⁻¹, implying comparable propensity for bonding with their receptor proteins.

Finally, ADMET screening on the SwissADME platform addressed the practical question of whether these hits could ever become drugs. Both alkaloids showed high predicted gastrointestinal absorption, blood-brain barrier permeability and a consensus Log P of 2.88, indicating good lipophilic balance for membrane permeation. Neither molecule has a hydrogen bond donor, lanoginosine has five hydrogen bond acceptors and liriodenine four, and their topological polar surface areas of 57.65 and 48.42 square angstroms fall comfortably within drug-like ranges. Both compounds violated none of the Lipinski, Ghose or Veber criteria for oral drug-likeness, and their log Kp values of −5.78 and −5.57 cm/s suggest high skin permeability. Differences remain worth noting: lanoginosine inhibits four cytochrome P450 isoforms while liriodenine inhibits two, and liriodenine is a P-glycoprotein substrate.

The authors are careful to frame this as a computational discovery stage rather than a therapeutic breakthrough. No laboratory or animal experiments have yet confirmed that lanoginosine or liriodenine actually silence agr-dependent virulence in living bacteria, and the team explicitly calls for in vitro and in vivo validation, exploration of additional natural and synthetic alkaloids, alternative density functionals and force fields, and extension to other quorum sensing proteins. Still, the convergence of evidence is compelling: strong docking across five bacterial species, stable dynamics, favorable MM/PBSA energies, sensible electronic profiles and clean drug-likeness all point in the same direction. If experimental work bears out these predictions, the study would add two well-characterized natural product scaffolds to the growing arsenal of anti-virulence strategies, a class of therapies that disarms pathogens by muting their chemical conversations rather than killing them, and that may prove far harder for bacteria to resist.

Subject of Research: Computational identification of natural alkaloid inhibitors of the AgrA LytTR domain in bacterial quorum sensing

Article Title: Exploration of binding mechanism of two potential natural alkaloids as AgrA inhibitors of quorum sensing: molecular docking, molecular dynamics simulation, ADMET and DFT analysis

Article References: Basak, H. K., Paswan, U., Podder, S., Pal, A., & Chatterjee, A. (2026). Exploration of binding mechanism of two potential natural alkaloids as AgrA inhibitors of quorum sensing: molecular docking, molecular dynamics simulation, ADMET and DFT analysis. Discover Chemistry, 3(1), Article 490. https://doi.org/10.1007/s44371-026-00925-w

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00925-w

Keywords: quorum sensing, AgrA protein, LytTR domain, molecular docking, molecular dynamics simulation, MM/PBSA, density functional theory, ADMET, alkaloids, liriodenine, lanoginosine, antibiotic resistance