Photoprotein
Obelin
Ca2+-regulated photoproteins
(Fig 1.)
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Abstract:
Obelin is a marine based calcium regulated
bioluminescent photoprotein from the organism Obelia Longissima. Members of the
genus Obelia are commonly found in the first few hundred feet of sea water
frequently on alage and regularly on submerged
substrata in tidal areas. Bioluminescence is most frequently found in marine
life and may serve as a method of communication, attraction, repulsion, or
camouflage. The word “photoprotein” is used to differentiate certain classes of
bio-luminescence which use enzymatic reactions to produce photons, specifically
luciferase and the substrate luciferin.
Many photoproteins exist in nature but very few have been studied in great detail. Obelin is a calcium activated photoprotein yet its luminescence is not completely governed by Ca2+. Very weak emissions are know to exist in its absence, yet the addition of Ca2+ has been shown to significantly increase the intensity of emission [1]. The binding of Ca2+ occurs in photocytes, light emitting cells, resulting in the oxidation of coelenterazine, an imidazolopyrazine derivative, which is firmly but not covalently bound to Obelin forming coelenterazine-oxygen substrate. This is then decarboxilated to form an exited state of coelenteramide along with CO2. Coelenteramide’s spectroscopic decay emission of the first excited singlet state generates blue luminescence at approximately 485 nm (max) [2]. This characteristic of photoproteins leads to an obvious application in the detection of calcium concentrations within cells. Obelin is a 22.2 kDa single subunit protein, with three binding sites for calcium, composed of 195 amino acids, and a crystal structure has been found with 1.7 A resolution. This further supports the proposed action of coelenterazine-oxygen complex which is necessary for light emission [3].
Citations:
Figure 1. was generated via
Pymol and the use of PDB code 1SL7
[1] Ohmiya, Y., and T. Hirano. 1996. Shining the light: the
mechanism of the bioluminescence reaction of calcium-binding photoproteins.
Chemistry & Biology. 3:337-347.
[2] D. Leung et al. Crystal Structure of a Ca2_-discharged Photoprotein Vol.
279, No. 32, Issue of August 6, pp. 33647-33652, 2004
[3] Z-J. Lui, et al. Structure of the Ca21-regulated photoprotein obelin at 1.7 Ã… resolution determined directly from its sulfur substructure Protein Science ~2000!, 9:2085-2093