A comparison of the two most famous groups of calcium-regulated photoproteins, cnidarians and ctenophores, showed unexpectedly high degree of structural similarity regardless of their low sequence identity. It was suggested these photoproteins can play an important role in understanding the structural basis of bioluminescence activity. Based on this postulate, in this study the cDNA of mnemiopsin from luminous ctenophore Mnemiopsis leidyi was cloned, expressed, purified and sequenced. The purified cDNA, with 621 base pairs, coded a 206 residues protein. Sequence of mnemiopsin showed 93.5 and 51% similarity to other ctenophore proteins and cnidarians, respectively. The cDNA encoding apo-mnemiopsin of M. leidyi was expressed in Escherichia coli. The purified apo-protein showed a single band on SDS-PAGE (molecular weight ~27 kDa). A semi-synthetic mnemiopsin was prepared using coelenterazine and EDTA and its luminescence activity was measured in the presence of CaCl(2). The results showed an optimum pH of 9.0 and lower calcium sensitivity compared to aequorin. Comparison of amino acid residues in substrate binding site indicated that binding pocket of ctenophores contains less aromatic residues than cnidarians. This can lead to a decline in the number of stacking interactions between substrate and protein which can affect the stability of coelenterazine in binding cavity. Structural comparison of photoproteins with low sequence identity and high 3D structural similarity, can present a new insight into the mechanism of light emission in photoproteins.

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Bioluminescence of a variety of marine organisms, mostly cnidarians and ctenophores, is carried out by Ca 2؉ -dependent photoproteins. The mechanism of light emission operates via the same reaction in both animal families. Despite numerous studies on the ctenophore photoprotein family, the detailed catalytic mechanism and arrangement of amino acid residues surrounding the chromophore in this family are a mystery. Here, we report the crystal structure of Cd 2؉ -loaded apo-mnemiopsin1, a member of the ctenophore family, at 2.15 Å resolution and used quantum mechanics/molecular mechanics (QM/MM) to investigate its reaction mechanism. The simulations suggested that an Asp-156 -Arg-39 -Tyr-202 triad creates a hydrogen-bonded network to facilitate the transfer of a proton from the 2-hydroperoxy group of the chromophore coelenterazine to bulk solvent. We identified a water molecule in the coelenteramidebinding cavity that forms a hydrogen bond with the amide nitrogen atom of coelenteramide, which, in turn, is hydrogen-bonded via another water molecule to Tyr-131. This observation supports the hypothesis that the function of the coelenteramide-bound water molecule is to catalyze the 2-hydroperoxycoelenterazine decarboxylation reaction by protonation of a dioxetanone anion, thereby triggering the bioluminescence reaction in the ctenophore photoprotein family.

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Photoproteins are responsible for light emission in a variety of marine ctenophores and coelenterates. The mechanism of light emission in both families occurs via the same reaction. However, the arrangement of amino acid residues surrounding the chromophore, and the catalytic mechanism of light emission is unknown for the ctenophore photoproteins. In this study, we used quantum mechanics/molecular mechanics (QM/MM) and site-directed mutagenesis studies to investigate the details of the catalytic mechanism in berovin, a member of the ctenophore family. In the absence of a crystal structure of the berovin-substrate complex, molecular docking was used to determine the binding mode of the protonated (2-hydroperoxy) and deprotonated (2-peroxy anion) forms of the substrate to berovin. A total of 13 mutants predicted to surround the binding site were targeted by site-directed mutagenesis which revealed their relative importance in substrate binding and catalysis. Molecular dynamics simulations and MM-PBSA (Molecular Mechanics Poisson-Boltzmann/surface area) calculations showed that electrostatic and polar solvation energy are +115.65 and -100.42 kcal/mol in the deprotonated form, respectively. QM/MM calculations and pKa analysis revealed the deprotonated form of substrate is unstable due to the generation of a dioxetane intermediate caused by nucleophilic attack of the substrate peroxy anion at its C3 position. This work also revealed that a hydrogen bonding network formed by a D158- R41-Y204 triad could be responsible for shuttling the proton from the 2- hydroperoxy group of the substrate to bulk solvent.