title: Binding of the iminium and alkanolamine forms of sanguinarine to lysozyme: spectroscopic analysis, thermodynamics, and molecular modeling studies.
abstract: Sanguinarine (SGR) exists in charged iminium (SGRI) and neutral alkanolamine (SGRA) forms. The binding of these two forms to the protein lysozyme (Lyz) was investigated by fluorescence, UV-vis absorbance and circular dichroism spectroscopy, and in silico molecular docking approaches. Binding thermodynamics were studied by microcalorimetry. Both forms of sanguinarine quenched the intrinsic fluorescence of Lyz, but the quenching efficiencies varied on the basis of binding that was derived after correction for an inner-filter effect. The equilibrium binding constants at 25 ± 1.0 °C for the iminium and alkanolamine forms were 1.17 × 10(5) and 3.32 × 10(5) M(-1), respectively, with approximately one binding site for both forms of the protein. Conformational changes of the protein in the presence of SGR were confirmed by absorbance, circular dichroism, three-dimensional fluorescence, and synchronous fluorescence spectroscopy. Microcalorimetry data revealed that SGRI binding is endothermic and predominantly involves electrostatic and hydrophobic interactions, whereas SGRA binding is exothermic and dominated by hydrogen-bonding interactions. The molecular distances (r) of 3.27 and 3.04 nm between the donor (Lyz) and the SGRI and SGRA acceptors, respectively, were calculated according to Förster's theory. These data suggested that both forms were bound near the Trp-62/63 residues of Lyz. Stronger binding of SGRA than SGRI was apparent from the results of both structural and thermodynamic experiments. Molecular docking studies revealed that the putative binding site for the SGR analogues resides at the catalytic site. The docking results are in accordance with the spectroscopic and thermodynamic data, further validating the stronger binding of SGRA over SGRI to Lyz. The binding site is situated near a deep crevice on the protein surface and is close to several crucial amino acid residues, including Asp-52, Glu-35, Trp-62, and Trp-63. This study advances our knowledge of the structural nature and thermodynamic aspects of binding between the putative anticancer alkaloid sanguinarine and lysozyme.
authors.affiliation: Biophysical Chemistry Laboratory, Chemistry Division and ‡Structural Biology and Bioinformatics Division, CSIR-Indian Institute of Chemical Biology , Kolkata 700 032, India.
authors.forename: Chandrima
authors.initials: C
authors.lastname: Jash
authors.forename: Pavan V
authors.initials: PV
authors.lastname: Payghan
authors.forename: Nanda
authors.initials: N
authors.lastname: Ghoshal
authors.forename: Gopinatha
authors.initials: G
authors.lastname: Suresh Kumar
mesh_terms: D000470:Alkaloids / Q000737:chemistry / Q000378:metabolism; D000588:Amines / Q000737:chemistry*; D000818:Animals; D053119:Benzophenanthridines / Q000737:chemistry / Q000378:metabolism*; D001665:Binding Sites; D002151:Calorimetry; D020134:Catalytic Domain; D002645:Chickens; D002942:Circular Dichroism; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D007097:Imines / Q000737:chemistry*; D007546:Isoquinolines / Q000737:chemistry / Q000378:metabolism*; D062105:Molecular Docking Simulation; D009113:Muramidase / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding; D013056:Spectrophotometry, Ultraviolet; D013696:Temperature; D013816:Thermodynamics
chemical_list: D000470:Alkaloids; D000588:Amines; D053119:Benzophenanthridines; D007097:Imines; D007546:Isoquinolines; C005705:sanguinarine; D009113:Muramidase