Characterization of the phenolate-keto oxyluciferin/luciferase interactions in the S₁ state by QM/MM energy...

Unraveling the nature of the interaction on the complex formed by the oxyluciferin chromophore in the first electronically excited state and the luciferase enzyme can lead to a more profound understanding of fireflies' bioluminescence, which is a natural process with a large amount of applications in medicine. In this work, we have studied the interaction of the phenolate-keto oxyluciferin (OLU) chromophore and the luciferase protein, and we have found that the interaction energy is governed mostly by electrostatic terms. Nonetheless, non-electrostatic contributions are non-negligible due to the high presence of π-system-containing amino acids within the bioluminescent pocket. Furthermore, we have outlined a methodology for performing energy decomposition analysis in the excited state using a multiscale hybrid electrostatic-embedding QM/MM approach, stressing that the OLUFF force field is not suitable for this task from a force field-base approach. Moreover, we have analyzed how the proximity of PHE249 and SER349 affect the different terms of the interaction energy, unraveling that the most relevant terms of their contributions are polarization and electrostatic, respectively. This strategy can be extended to investigate the interactions of the other three potential emitters within the OLU/luciferase complex, as well as complexes with mutated proteins. Such analyses may help clarify the factors governing color modulation in the bioluminescent process and provide valuable insights for tuning the OLU/luciferase complex to enhance its applicability.