Why does aequorin require calcium or strontium ions to emit light?

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time-lapse photo of fireflies — Bioluminescence Time-lapse image of fireflies in the Catskill Mountains, New York. Firefly light is a cold light, with approximately 100 percent of the energy given off as light and only a minute amount as heat. Only light in the visible spectrum is emitted.

Aequorin is a bioluminescent protein found in jellyfish of the genus Aequorea. While the specific biochemical mechanism involves a complex interaction between the protein and its prosthetic group, coelenterazine, the addition of calcium or strontium ions is essential to trigger the light-emitting reaction.1

These ions act as a necessary catalyst or "trigger" that induces a conformational change in the aequorin molecule. This structural shift allows the protein to oxidize its bound coelenterazine, resulting in the release of energy in the form of light. Because this reaction is highly sensitive to the presence of these specific ions, aequorin is widely utilized in scientific research as a tool to detect and map the distribution of calcium within living cells.1