Photoproteins are a type of

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

produced by

bioluminescent Bioluminescence is the emission of light during a chemiluminescence reaction by living organisms. Bioluminescence occurs in multifarious organisms ranging from marine vertebrates and invertebrates, as well as in some fungi, microorganisms inc ...

organisms. They add to the function of the

luciferins Luciferin () is a generic term for the light-emitting compound found in organisms that generate bioluminescence. Luciferins typically undergo an enzyme-catalyzed reaction with molecular oxygen. The resulting transformation, which usually involv ...

whose usual light-producing reaction is

catalyzed Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

by the enzyme

luciferase Luciferase is a generic term for the class of oxidative enzymes that produce bioluminescence, and is usually distinguished from a photoprotein. The name was first used by Raphaël Dubois who invented the words ''luciferin'' and ''luciferase'' ...

History

The term photoprotein was first used to describe the unusual chemistry of the luminescent system of ''

Chaetopterus ''Chaetopterus'' or the parchment worm or parchment tube worm is a genus of Marine (ocean), marine polychaete worm that lives in a tube it constructs in sediments or attaches to a rocky or coral reef substrate. The common name arises from the par ...

'' (a marine

Polychaete Polychaeta () is a paraphyletic class of generally marine Annelid, annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called c ...

worm). This was meant to distinguish them from other light-producing proteins because these do not exhibit the usual

luciferin Luciferin () is a generic term for the light-emitting chemical compound, compound found in organisms that generate bioluminescence. Luciferins typically undergo an enzyme-catalyzed reaction with Oxygen, molecular oxygen. The resulting transforma ...

reaction.

Reaction kinetics

Photoproteins do not display typical

enzyme kinetics Enzyme kinetics is the study of the rates of enzyme catalysis, enzyme-catalysed chemical reactions. In enzyme kinetics, the reaction rate is measured and the effects of varying the conditions of the reaction are investigated. Studying an enzyme' ...

as seen in luciferases. Instead, when mixed with luciferin, they display luminescence proportional to the amount of the photoprotein. For example, the photoprotein

aequorin Aequorin is a calcium-activated photoprotein isolated from the hydrozoan ''Aequorea victoria''. Its bioluminescence was studied decades before the protein was isolated from the animal by Osamu Shimomura in 1962. In the animal, the protein occur ...

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 do

catalyze Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

their bioluminescence reactions. This is due to a

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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. Because of the kinetically slow step, each aequorin molecule must "recharge" with another molecule of

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 as Ca²⁺ in the case of

References

{{reflist Bioluminescence Proteins