Molecular Insight into the Binding Property and Mechanism of Sulfamethoxazole to Extracellular Proteins of Anammox Sludge

Antibiotics are widely found in nitrogen-containing
wastewater,
which may affect the operation stability of anaerobic ammonium oxidation
(anammox)-based biological treatment systems. Extracellular polymeric
substances (EPSs) of anammox sludge play a pivotal role in combining
with antibiotics; however, the exact role and how the structure of
the leading component of EPSs (i.e., extracellular proteins) changes
under antibiotic stress remain to be elucidated. Here, the interaction
between sulfamethoxazole and the extracellular proteins of anammox
sludge was investigated via multiple spectra and molecular simulation.
Results showed that sulfamethoxazole statically quenched the fluorescent
components of EPSs, and the quenching constant of the aromatic proteins
was the largest, with a value of 1.73 × 104 M–1. The overall binding was an enthalpy-driven process,
with ΔH = −75.15 kJ mol–1, ΔS = −0.175 kJ mol–1 K–1, and ΔG = −21.10
kJ mol–1 at 35 °C. The O-P-O and CO
groups responded first under the disturbance of sulfamethoxazole.
Excessive sulfamethoxazole (20 mg L–1) would decrease
the ratio of α-helix/(β-sheet + random coil) of extracellular
proteins, resulting in a loose structure. Molecular docking and dynamic
simulation revealed that extracellular proteins would provide abundant
sites to bind with sulfamethoxazole, through hydrogen bond and Pi-Akyl
hydrophobic interaction forces. Once sulfamethoxazole penetrates into
the cell surface and combines with the transmembrane ammonium transport
domain, it may inhibit the NH4+ transport. Our
findings enhance the understanding on the interaction of extracellular
proteins and sulfamethoxazole, which may be valuable for deciphering
the response property of anammox sludge under the antibiotic stress.