Shining the light
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Bioluminescence is a widespread natural phenomenon in which visible light is emitted by a living organism. Ca2+-regulated photoproteins are found and responsible for the light emission in a variety of bioluminescent marine organisms, mostly in coelenterates. They are members of the broad EF-hand calcium binding protein family. Upon binding calcium, Ca2+-regulated photoprotein undergoes conformational changes, converting itself into a luciferase that catalyzes the oxidation of coelenterazine by the bound molecular oxygen, yielding visible blue light, carbon dioxide and Ca2+-discharged photoprotein. X-ray diffraction experiments are capable of capturing snap shots of molecular conformations trapped in the crystal and revealing structural details at near atomic resolution. The structural studies of photoproteins from the jellyfish Aequorea (aequorin) and the hydroid Obelia (obelin) demonstrate that conformations of apophotoproteins are controlled by binding various ligands such as Ca2+, coelenterazine, and coelenteramide which are necessary components for photoprotein function and unambiguously conclude the hydroperoxy-coelenterazine binding state of the luciferin. A proton relay mechanism is proposed for the Ca2+-triggered bioluminescence reaction of photoproteins. New insights are obtained into how the protein environment could induce different ionic states of the bound coelenteramide whose excited states are responsible for the different bioluminescence and fluorescence spectra of photoproteins. A new approach of structure determination using diffraction signals of weak anomalous scatterers, sulfur and calcium, pushes X-ray crystallographic methodology to a higher level and could be of general interest.