Evaluation of Luciferase Thermal Stability by Arginine Saturation in the Flexible Loops

Abstract

Background: The firefly luciferase enzyme is widely used in protein engineering and diverse areas of biotechnology, but the main problem with this enzyme is low-temperature stability. Previous reports indicated that surface areas of thermostable proteins are rich in arginine, which increased their thermal stability. In this study, this aspect of thermophilic proteins evaluated by mutations of surface residues to Arg. Here, we report the construction, purification, and studying of these mutated luciferases. Methods: For mutagenesis, the QuikChange site-directed mutagenesis was used and the I108R, T156R, and N177R mutant luciferases were created. In the following, the expression and purification of wild-type and mutant luciferases were conducted and their kinetic and structural properties were analyzed. To analyze the role of these Arg in these loops, the 3D models of these mutants’ enzymes were constructed in the I-TASSER server and the exact situation of these mutants was studied by the SPDBV and PyMOL software. Results: Overall, the optimum temperature of these mutated enzymes was not changed. However, after 30 min incubation of these mutated enzymes at 30°C, the I108R, T156R, N177R, and wild-type kept the 80%, 50%, 20%, and 20% of their original activity, respectively. It should be noted that substitution of these residues by Arg preserved the specific activity of firefly luciferase. Conclusion: Based on these results, it can be concluded that T156R and N177R mutants by compacting local protein structure, increased the thermostability of luciferase. However, insertion of positively charged residues in these positions create the new hydrogen bonds that associated with a series of structural changes and confirmed by intrinsic and extrinsic fluorescence spectroscopy and homology modeling studies.