Unusual shift in the visible absorption spectrum of an active ctenophore photoprotein elucidated by time-dependent density functional theory

Abstract

Active hydromedusan and ctenophore Ca 2+ -regulated photoproteins form complexes consisting of apoprotein and strongly non-covalently bound 2-hydroperoxycoelenterazine (an oxygenated intermediate of coelenterazine). Whereas the absorption maximum of hydromedusan photoproteins is at 460-470 nm, ctenophore photoproteins absorb at 437 nm. Finding out a physical reason for this blue shift is the main objective of this work, and, to achieve it, the whole structure of the protein-substrate complex was optimized using a linear scaling quantum-mechanical method. Electronic excitations pertinent to the spectra of the 2-hydroperoxy adduct of coelenterazine were simulated with time-dependent density functional theory. The dihedral angle of 60° of the 6-(p-hydroxy)-phenyl group relative to the imidazopyrazinone core of 2-hydroperoxycoelenterazine molecule was found to be the key factor determining the absorption of ctenophore photoproteins at 437 nm. The residues relevant to binding of the substrate and its adopting the particular rotation were also identified.

The Role of Tyr-His-Trp Triad and Water Molecule Near the N1-Atom of 2-Hydroperoxycoelenterazine in Bioluminescence of Hydromedusan Photoproteins: Structural and Mutagenesis Study

“…In the case of a similar mutation in obelin as well as in the double mutants including the replacement of Ser with Ala, these changes in the absorption spectrum are not observed. It is interesting to note that the absorption spectral maximum at 435 nm is characteristic of ctenophore photoproteins, the substrate-binding cavity of which is formed by completely different amino acids as compared to those in hydromedusan photoproteins [ 42 , 43 ].…”

“…In the case of a similar mutation in obelin as well as in the double mutants including the replacement of Ser with Ala, these changes in the absorption spectrum are not observed. It is interesting to note that the absorption spectral maximum at 435 nm is characteristic of ctenophore photoproteins, the substrate-binding cavity of which is formed by completely different amino acids as compared to those in hydromedusan photoproteins [42,43]. The wild-type obelin bioluminescence spectrum has a maximum at 480 nm with a shoulder at 400 nm conditioned by light emission from neutral species of coelenteramide [5,44], whereas the wild-type aequorin displays a monomodal spectrum with λ max = 470 nm (Table 2, Figure 2).…”

Ca2+-Triggered Coelenterazine-Binding Protein Renilla: Expected and Unexpected Features

“…The calculations of atomic and electronic structure of furimazine (FMZ) were performed using B3LYP functional [13] with the Solvation Model Based on Density (SMD) [14] and cc-pVDZ basis set. As shown in [15], the calculation of coelenterazine (CTZ) in methanol in SMD model was the most suitable for calculating optical properties. The absorption spectra of the substrate in the gas phase and solvents were calculated by the TD-DFT theory [16].…”