Discharged photoprotein obelin: Fluorescence peculiarities

Photoprotein obelin, the enzyme–substrate complex of polypeptide with 2-hydroperoxycoelenterazine, is responsible for bioluminescence of marine hydroid Obelia longissima. Addition of Ca2+ to the photoprotein triggers the bioluminescent reaction with light emission. The product of the bioluminescent reaction – enzyme-bound coelenteramide – is a fluorescent protein called ‘discharged’ obelin. It is stable and highly fluorescent. The paper considers dependence of its light-induced fluorescence on Ca2+ concentration. Increase of Ca2+ concentration enhanced the fluorescence intensity of discharged obelin; the dependence was found as linear in double logarithmic coordinates at Ca2+ concentration range 10−7–10−6 М, both in excitation and emission spectra. The spectra were divided into components; contributions of the components to experimental excitation and emission spectra depended on Ca2+ concentration. The data suggest enzymatic conformational transition in discharged obelin at ∼5 × 10−7 M of Ca2+ concentration. Spectra variations were attributed to acidity changes of discharged obelin chromophore (coelenteramide) in its fluorescent state S1∗.