Several bioluminescent proteins utilize coelenterazine as a substrate, and highresolution crystal structures have been determined for calcium-stimulated photoproteins like aequorin and obelin. Within these proteins, coelenterazine, while not covalently attached, resides in a snug cavity. It interacts with multiple residues that contribute to the efficiency of light emission and the modulation of its color. Typically, coelenteramide, the light emitter, is thought to be deprotonated at the 6-para hydroxyphenyl group (forming a phenolate ion), a process likely facilitated by a neighboring histidine residue (His22). This results in the emission of blue light in the range of 470-490 nm.