Full-quantum chemical calculations of the absorption maxima of oxyluciferin in the catalytic center of firefly luciferase containing adenosine monophosphate

Absorption maxima of firefly oxyluciferin (Oxyln) were calculated by taking into account all atoms (10,467–10,473) of Luciola cruciata luciferase (Luc) containing Oxyln, a cofactor adenosine monophosphate (AMP) and water molecules based on its X-ray diffraction data (PDB code: 2D1R). Energy minimization and absorption peak calculations were executed by the Quantum/Molecular Mechanics (QM/MM) method, and by the Intermediated Neglect of Differential Overlap for Spectroscopy (INDO/S) method, respectively. The present study focused on the charge of AMP present in the catalytic center of Luc and investigated the effect of its change on the absorption maxima of Oxyln. As a result, the tendencies of the absorption peaks were found to differ greatly according to the protonation state of Oxyln and to depend on the AMP charge: the absorption maxima of the monoanion and the neutral Oxyln shifted to the longer-wavelength region (approximately 600 nm) and to the shorter-wavelength region (around 400–500 nm), respectively. Although this trend does not differ from the calculation results for the isolated Oxyln, directly taking into account the specific interaction with the catalytic center of Luc, the absorption maxima of the monoanion and the neutral Oxyln shifted to the longer-wavelength region. The absorption maxima of both monoanion and neutral Oxyln shifted toward the longer-wavelength region on reducing the AMP negative charge from dianion to neutral. According to the results obtained by our “Full-QM” calculations considering the Oxyln–Luc interaction, it was concluded that the red light (∼620 nm) and the yellow–green light (∼560 nm) emission for the acidic and the neutral/basic pH, respectively, could be assigned to the monoanion and the neutral Oxyln. It was also concluded that the charge of AMP significantly affects the luminescent color.