photoproteins

Photoproteins are a family of small, calcium-regulated luminescent proteins found primarily in cnidarians such as jellyfish and hydroids. They contain a bound luciferin cofactor, most commonly coelenterazine, and emit light when intracellular calcium levels rise. The emission results from a Ca2+-triggered oxidation of the bound coelenterazine within the photoprotein, with the energy released as blue light, typically peaking around 470 nanometers. After light emission, the chromophore is oxidized and the photoprotein becomes inactive until it is reloaded with fresh coelenterazine.

The best-known member is aequorin, from Aequorea victoria; other well-studied photoproteins include obelin from Obelia. These proteins are typically monomeric, with a molecular weight around 22–25 kDa, and each molecule binds a single coelenterazine as a prosthetic group. The photoprotein’s light emission is extremely rapid and localized to sites of Ca2+ concentration increases, and it does not require ATP or other cofactors besides oxygen.

Applications of photoproteins include use as calcium indicators in cells and whole organisms. Aequorin-based reporters generate luminescent signals proportional to intracellular Ca2+ concentrations, enabling real-time monitoring of calcium transients without external illumination. They have played a significant role in early calcium imaging and continue to be used in combination with imaging systems capable of detecting luminescence, and sometimes alongside fluorescent reporters for dual measurements. Biochemical properties such as rapid response, single-turnover emission, and reloadability with coelenterazine contribute to their utility in physiological studies.