Oblein, a protein from the bio-luminescent hydroid, Obleia longissima, is a calcium-regulated photoprotein. The addition of calcium regulates, but is not necessary for bio-luminescence and there is no requirement for an additional substrate (e.g. molecular oxygen). Calcium-regulated photoproteins are found in and are responsible for the light emission of a variety of bio-luminescent marine organisms, mostly coelenterates. Obelin consists of a single polypeptide chain to which the substrate, 2-hydroxy-coelenterazine is tightly, but not covalently bound. Calcium binding results in blue bio-luminescence via oxidative carboxylation of the substrate to the excited state coelenteramide product through several intermediates.
Defined by 5 residues: HIS:A-22TRP:A-92TYR:A-138HIS:A-175TYR:A-190