Crystal structure of semi-synthetic obelin-v after calcium induced bioluminescence implies coelenteramine as the main reaction product

Coelenterazine- v (CTZ- v ), a synthetic vinylene-bridged π-extended derivative, is able to significantly alter bioluminescence spectra of different CTZ-dependent luciferases and photoproteins by shifting them towards longer wavelengths. However, Ca 2+ -regulated photoproteins activated with CTZ- v display very low bioluminescence activities that hampers its usage as a substrate of photoprotein bioluminescence. Here, we report the crystal structure of semi-synthetic Ca 2+ -discharged obelin- v bound with the reaction product determined at 2.1 Å resolution . Comparison of the crystal structure of Ca 2+ -discharged obelin- v with those of other obelins before and after bioluminescence reaction reveals no considerable changes in the overall structure. However, the drastic changes in CTZ-binding cavity are observed owing to the completely different reaction product, coelenteramine- v (CTM- v ). Since CTM- v is certainly the main product of obelin- v bioluminescence and is considered to be a product of the “dark” pathway of dioxetanone intermediate decomposition, it explains the low bioluminescence activity of obelin and apparently of other photoproteins with CTZ- v .