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Obelin crystal structure: implications for the bioluminescence mechanism ES Vysotski2; Zhi-Jie Liu*1; J Rose1; BC Wang1; J Lee1 (1) Dept. of Biochemistry & Molecular Biology, Athens, GA 30605, USA; (2) Institute of Biophysics, RAS(SB), Krasnoyarsk, 660036 Russia      *(vys@bmb.uga.edu) The crystal structure of obelin has been refined to 1.1 angstrom resolution using data collected on a new C2 crystal form at the APS (Argonne). The structure was determined by molecular replacement using the 1.7 angstrom hexagonal crystal structure as a search model. An unexpected feature of this structure was that the bound coelenterazine ligand showed only a single oxygen bound at the C2-position, not the peroxide as predicted to account for the bioluminescence properties of obelin and other photoproteins. This is confirmed here and the H-bonded ligand structure is proposed to be coelenterazine hydrate. Soaking the crystals in solutions containing trace amounts of Ca++ ions, concentrations too low to produce any visible bioluminescence emission, results in a peroxide at the C2-position. This peroxide is evidently poised for dioxetane ring closure, the mechanism believed to lead to the oxidative decarboxylation and the excited product state. These results imply that from the stably bound coelenterazine hydrate the peroxide precursor to the light reaction follows some minor structural alteration, possibly due to the binding of the first of the three Ca++ required for the bioluminescence. Supported by ONR and RAS. [Poster: liu.zhijie.56132]

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