Computational Characterization of Ligand Recognition and Inhibition in a 1,4-Dioxane-Degrading Propane Monooxygenase

Abstract

1,4-Dioxane is a widespread groundwater contaminant frequently co-occurring with chlorinated solvents. The group-6 propane monooxygenase (PRM) from Mycobacterium dioxanotrophicus PH-06 degrades dioxane efficiently, yet the molecular determinants underlying its broad substrate spectrum and inhibition behavior remain unresolved. Here, we combined AlphaFold2-based structure prediction, molecular docking, and alchemical free energy calculations to systematically characterize ligand binding in PH-06 PRM. The catalytic center (residues 87–265), coordinated by two histidines and four glutamates, exhibited calculated binding free energies for five substrates (C2–C4 alkanes and cyclic ethers) that significantly correlated with experimentally determined degradation rates (p < 0.05), supporting the ability of ligand binding thermodynamics to capture major substrate-recognition trends in PH-06 PRM. In addition, we identified a remote binding site (residues 329–446) approximately 30 Å from the catalytic center. Thermodynamic analysis indicates that this remote site can attract methane and ethane, potentially limiting their access to the catalytic center. Comparative binding analyses of three common chlorinated solvent inhibitors (i.e., trichloroethylene [TCE], 1,1-dichloroethylene [DCE], and 1,1,1-trichloroethane [TCA]) revealed distinct site preferences consistent with their experimentally observed inhibition modes. Noncompetitive inhibitors (DCE and TCA) displayed stronger affinity for the remote site, whereas the competitive inhibitor TCE exhibited comparable affinity for both binding regions. Sequence conservation, contact probability analysis, and Gene Ontology enrichment further suggest the structural relevance of the remote site and its possible role in ligand recognition and inhibitor selectivity. These findings demonstrate that the AlphaFold-guided free energy calculations can capture experimentally observed substrate and inhibitor trends in PH-06 PRM, while identifying a remote ligand-binding pocket that warrants further mechanistic investigation.