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Protein–ligand interaction studies are useful to determine the molecular mechanism of the binding phenomenon, leading to the establishment of the structure–function relationship. Here, we report the binding of well-known antibiotic sulfonamide drugs (sulfamethazine, SMZ; and sulfadiazine, SDZ) with heme protein myoglobin (Mb) using spectroscopic, calorimetric, ζ potential, and computational methods. Formation of a 1:1 complex between the ligand and Mb through well-defined equilibrium was observed. The binding constants obtained between Mb and SMZ/SDZ drugs were on the order of 104 M–1. SMZ with two additional methyl (−CH3) substitutions has higher affinity than SDZ. Upon drug binding, a notable loss in the helicity (via circular dichroism) and perturbation of the three-dimensional (3D) protein structure (via infrared and synchronous fluorescence experiments) were observed. The binding also indicated the dominance of non-polyelectrolytic forces between the amino acid residues of the protein and the drugs. The ligand–protein binding distance signified high probability of energy transfer between them. Destabilization of the protein structure upon binding was evident from differential scanning calorimetry results and ζ potential analyses. Molecular docking presented the best probable binding sites of the drugs inside protein pockets. Thus, the present study explores the potential binding characteristics of two sulfonamide drugs (with different substitutions) with myoglobin, correlating the structural and energetic aspects.

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Protein-drug binding study addresses a broad domain of biological problems associating molecular functions to physiological processes composing and modifying safe and coherent drug therapeutics. Comparison of the binding and thermodynamic aspect of sulfa drugs, sulfamethazine (SMZ) and sulfadiazine (SDZ) with the protein, lysozyme (Lyz) was carried out using spectroscopic, molecular docking, and dynamic simulation studies. The fluorescence quenching and apparent binding constant for the binding reaction were calculated to be in the order of 10 4 M À1 , slightly higher for SMZ as compared to that of SDZ and the binding stoichiometry values show 1:1 drug binding with each protein molecule. The binding was an enthalpy-driven spontaneous exothermic reaction favored by a negative enthalpy and a positive entropy contribution for both the complexes. The binding from the fluorescence quenching data suggests a static quenching mechanism dominated by non-polyelectrolytic components. Synchronous fluorescence denoted a conformational change in the tryptophan moiety of the protein. Molecular docking and dynamic simulation study provided a clearer view of the interaction pattern, where the drug resides on the binding pocket of the protein structure. Overall the protein, Lyz binding of SMZ was slightly more favored over SDZ.

Binding of the well-known antibiotic drug sulfamethazine (SMZ) with tetrameric heme protein, hemoglobin (Hb), was studied using spectroscopic, calorimetric and molecular docking techniques. SMZ belongs to the sulfonamide group of medicines with versatile applications. Nevertheless, it can be biologically harmful if used in excess or in an uncontrolled manner. The binding induced absorbance and fluorescence data indicated a ground state complex formation between the drug and the protein with 1 : 1 stoichiometry. Drug induced conformational perturbation of Hb structure was investigated with circular dichroism and synchronous fluorescence. The binding constant obtained from spectroscopy was in the order of 10 4 M À 1 which was further confirmed by isothermal titration calorimetry. This may be higher compared to that of the oxygenation in Hb; thus SMZ binding can subsequently interrupt the oxygen transporting property of this iron protein. FRET analysis showed that the distance between SMZ and Hb is ∼ 3.75 nm, which is suitable for an effective binding. The MD simulation substantiated the mode and site of binding by confirming the factors contributing to the binding energy. It shows that SMZ binds to the central cavity close to subunit α 1helix of Hb. Residues around the drug forms hydrogen bonding and increases hydrophobicity to stabilize the SMZÀ Hb complex. The binding interaction appears to be dominated by H-bond formation and electrostatic and hydrophobic forces. This is in agreement with the thermodynamic analyses and contributes to the binding energy.