Photoproteins are a type ofenzyme produced bybioluminescent organisms. They add to the function of theluciferins whose usual light-producing reaction iscatalyzed by the enzymeluciferase.
The term photoprotein was first used to describe the unusual chemistry of the luminescent system ofChaetopterus (a marinePolychaete worm).[1] This was meant to distinguish them from other light-producing proteins because these do not exhibit the usualluciferin-luciferase reaction.[2]
Photoproteins do not display typicalenzyme kinetics as seen inluciferases. Instead, when mixed with luciferin, they display luminescence proportional to the amount of the photoprotein. For example, the photoproteinaequorin produces a flash of light when luciferin and calcium are added, rather than the prolonged glow that is seen for luciferases when luciferin is added. In this respect, it may appear that photoproteins are not enzymes, when in fact they docatalyze their bioluminescence reactions. This is due to afast catalytic step, which produces the light, and a slow regeneration step, where the oxyluciferin is freed and another molecule of luciferin is then enabled to bind to the enzyme.[3] Because of thekinetically slow step, each aequorin molecule must "recharge" with another molecule ofluciferin before it can emit light again, and this makes it appear as though it is not behaving as a typical enzyme.
Photoproteins form a stable luciferin-photoprotein complex, often until the addition of another required factor such asCa2+ in the case ofaequorin.