The Charge-driven Engineering of Mnemiopsin 2: Impact of Serine 35 Mutation on Activity and Stability.

Bioluminescence in marine organisms, driven by photoproteins like mnemiopsin 2 from Mnemiopsis leidyi, relies on calcium-regulated light emission. This study investigates the functional and structural effects of site-directed mutagenesis at serine 35 in mnemiopsin 2, replacing it with lysine or glutamic acid to explore charge and polarity impacts on the structure and function of the photoprotein. Homology modeling and molecular docking were used and confirm that the S35K mutation enhances coelenterazine binding. In contrast, the S35E mutant displays weaker interactions with coelenterazine. The S35K mutation significantly increased the initial bioluminescent intensity to 208% of the wild-type. Strikingly, both mutations dramatically slowed the photoprotein’s decay rate, prolonging luminescence from 1.4 s in the wild-type to 5.2 s, indicating highly stabilized intermediate states compared with the WT. Additionally, S35E shifted the optimal pH of bioluminescent activity from 9 to 8.75. Thermal denaturation analyses showed higher values of the enthalpy change for the thermal unfolding reaction of the mutant, demonstrating more cooperativity in the stabilizing interactions at moderate temperatures. The observed cooperativity was confirmed by the fluorescence spectroscopy, revealing a more compact tertiary structure in the S35K mutant. These findings emphasize the critical role of charge-mediated structural dynamics and binding properties in modulating photoprotein properties, such as advanced imaging and biotechnological applications.