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Mnemiopsin, a Ca2+-regulated photoprotein from ctenophore Mnemiopsis leidyi, as coelentrate photoproteins emits flash blue light upon reacting with coelenterazine. In contrast to coelenterate photoproteins, there is a little information about the structure of chromophore binding site and bioluminescence mechanism in ctenophore photoproteins. In this study, three important amino acid residues in coelenterazine binding cavity of mnemiopsin were substituted by corresponding residues in the well-known coelentrate photoproteins. W59K, N105W and L127W mutants were constructed and characterized for investigation of hydrogen bond network around the important rings of coelenterazine. All three mutants are completely inactivated. In addition, the results of structural studies including CD, intrinsic and extrinsic fluorescence together with theoretical studies showed that these mutants, especially for N105W and L127W, have found different structural features. These results suggest the presence of the residues in binding cavity and/or a mechanistic role for these residues. It seems that arrangement of amino acid residues in the binding cavity of coelenterate and ctenophore photoproteins are different, so that the replacement of these residues with their corresponding residues in other group (such as mutations in this study) perturbs the structural integrity needed for bioluminescence activity.

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Aims: Studies based on thermal stability are considered as one of the methods for investigating the physicochemical properties of proteins in biotechnology. The aim of this study was to evaluate the effect of replacement of Arginine 39 amino acid with lysine on the heat denaturation of mnemiopsin photoprotein 1.
Materials and Methods: In the current experimental study, R39K mutated mnemiopsin was compared with wild protein (in which arginine 39 amino acid was converted to the lysine amino acid). In order to investigate the effect of mutation on the content of the secondary structure, a rotation interpolation method was used. To investigate the possible changes in the rate of thermal stability of mutated and wild proteins, heat denaturation measurements were performed by differential scanning calorimeter. Bioinformatics software were used to compare the structure of two types of proteins.
Findings: The mutated R39K compression decreased in comparison with wild protein. No significant change was observed in the values of thermodynamic parameters, especially T_m. The upward movement of arginine 187 amino acid in the mutated protein decreased the thermal stability of this protein. Increasing the accessible surface of lysine 188 in the mutated protein increased its stability.
Conclusion: In thermal stability of the R39K mutated protein, various factors are effective, including the molecular movements of amino acids, their accessible surface, and the content of the secondary structure of protein stabilizing. This mutation reduces the mutated R39K compression rather than the wild protein; increasing ASA related to Lys188 amino acid in the mutated R39K compared with wild protein increases protein stability, but reducing the amount of secondary structure in this mutated, accompanied by an increase in the molecular upward movement in the Arg187 amino acid serves to reduce the stability of this mutated.