– 163 is structurally flexible, facilitating greater diffusion of solvents into the active site, which contains a highly-conserved catalytic triad consisting of Aspartic Acid at residue 120, Glutamic Acid at residue 144, and Histidine at residue 285. Enzyme pathway Unlike photoproteins which stably bind coelenterazine and emit light upon addition of calcium, coelenterazine is normally bound by RrLBP, the luciferin-binding protein. When stimulated, a Ca2+ ion first interacts with RrLBP, causing it to release coelenterazine. Coelenterazine is then oxidized by RLuc into coelenteramide, releasing a single photon of blue light (480 nm) in the process. This photon is captured by the adjacent GFP, releasing a photon of green light. This pathway is summarized below. Mechanism The RLuc mediated chemical reaction involves the catalytic degradation of coelenterazine, and proceeds through a 1,2-dioxetane (also called dioxetanone or cyclic peroxide) intermediate. Based on studies using radioactively labelled oxygen species within the RLuc complex, it has been determined that the luciferin carbonyl oxygen is exchanged rapidly with oxygen from water prior to incorporation of an oxygen atom from O2 via a dioxetane intermediate. The resultant CO2 also rapidly exchanges its oxygens with those from the surrounding water. The general mechanism is depicted below. References EC 1.13.12 Enzymes of known structure