Photoprotein refers to broadly to a type of protein from bioluminescent organisms. The term was first used to describe the unusual chemistry of the Chaetopterus luminescent system. This was meant to distinguish them from other light-producing proteins because these do not exhibit a luciferin-luciferase reaction, that is, a normal enzyme-substrate reaction. That is to say that they do not display typical enzyme kinetics as seen in luciferases.
Instead, these proteins display luminescence proportional to the amount of the protein. For example, the photoprotein aequorin will produce a flash of light when calcium is added rather than a 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 do catalyze their bioluminescence reactions. This is due to a fast catalyic step, which produces the light, and a slow regeneration step, where the oxyluciferin is freed and another molecule of the luciferin will bind to the enzyme. Because of the kinetically slow step, each protein molecule must "recharge" with another molecule of luciferin before it can emit light again, and this makes it appear as though it is not behaving as a typical enzyme.
This can be considered analogous to flash bulbs for photograph used during the mid-20th century. During use, the magnesium filaments in each bulb would heat up to make a single flash (fast step), then the operator had to wait for the bulb to cool and exchange it out with a new bulb (slow step).
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