Aequorin
Encyclopedia
Aequorin is a photoprotein
Photoprotein
Photoproteins refer to bioluminescent proteins from luminous organisms. These proteins do not exhibit a luciferin-luciferase reaction, that is, a normal enzyme-substrate reaction. Instead, these proteins display luminescence proportional to the amount of the protein...

 isolated from luminescent jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...

 (like various Aequorea species, e.g., Aequorea victoria
Aequorea victoria
Aequorea victoria, also sometimes called the crystal jelly, is a bioluminescent hydrozoan jellyfish, or hydromedusa, that is found off the west coast of North America. This species is thought to be synonymous with Aequorea aequorea of Osamu Shimomura, the discoverer of green fluorescent protein . ...

) and a variety of other marine organisms. It was originally isolated from the coelenterate by Osamu Shimomura
Osamu Shimomura
is a Japanese organic chemist and marine biologist, and Professor Emeritus at Marine Biological Laboratory in Woods Hole, Massachusetts and Boston University Medical School...

.

Structure

Aequorin is composed of two distinct units, the apoprotein
Apoprotein
Apoprotein can refer to:*Apoenzyme, the protein part of an enzyme without its characteristic prosthetic group.*Apolipoprotein, a lipid-binding protein that is a constituent of the plasma lipoprotein....

 apoaequorin, with an approximate molecular weight of 22 kDa
Atomic mass unit
The unified atomic mass unit or dalton is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of...

, and the prosthetic group coelenterazine
Coelenterazine
Coelenterazine is the luciferin, the light-emitting molecule, found in many aquatic organisms across seven phyla. It is the substrate in many luciferases and photoproteins including Renilla reniformis luciferase , Gaussia luciferase , aequorin, and obelin.-History:Coelenterazine was simultaneously...

, a luciferin
Luciferin
Luciferins are a class of light-emitting biological pigments found in organisms that cause bioluminescence...

.

Mechanism of Action

The two components of aequorin reconstitute spontaneously, forming the functional protein. The protein bears three EF hand
EF hand
The EF hand is a helix-loop-helix structural domain found in a large family of calcium-binding proteins. The EF-hand motif contains a helix-loop-helix topology, much like the spread thumb and forefinger of the human hand, in which the Ca2+ ions are coordinated by ligands within the loop...

 motifs that function as binding sites for Ca2+ ions. When Ca2+ occupies such sites, the protein undergoes a conformational change and converts through oxidation
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 its prosthetic group, coelenterazine
Coelenterazine
Coelenterazine is the luciferin, the light-emitting molecule, found in many aquatic organisms across seven phyla. It is the substrate in many luciferases and photoproteins including Renilla reniformis luciferase , Gaussia luciferase , aequorin, and obelin.-History:Coelenterazine was simultaneously...

, into excited coelenteramide
Coelenteramide
Coelenteramide is the oxidized product, or oxyluciferin, of the bioluminescent reactions in many marine organisms that utilize coelenterazine. It was first isolated as a blue fluorescent compound from Aequorea victoria after the animals were stimulated to emit light...

 and CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

. As the excited coelenteramide relaxes to the ground state, blue light (wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 = 469 nm) is emitted.

Uses in Biology and Medicine

Since the emitted light can be easily detected with a luminometer, aequorin has become a useful tool in molecular biology
Molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...

 for the measurement of intracellular Ca2+ levels. Cultured cells expressing the aequorin gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

 can effectively synthesize aequorin: However, recombinant
Recombinant DNA
Recombinant DNA molecules are DNA sequences that result from the use of laboratory methods to bring together genetic material from multiple sources, creating sequences that would not otherwise be found in biological organisms...

 expression yields only the apoprotein
Apoprotein
Apoprotein can refer to:*Apoenzyme, the protein part of an enzyme without its characteristic prosthetic group.*Apolipoprotein, a lipid-binding protein that is a constituent of the plasma lipoprotein....

, therefore it is necessary to add coelenterazine
Coelenterazine
Coelenterazine is the luciferin, the light-emitting molecule, found in many aquatic organisms across seven phyla. It is the substrate in many luciferases and photoproteins including Renilla reniformis luciferase , Gaussia luciferase , aequorin, and obelin.-History:Coelenterazine was simultaneously...

 into the culture medium of the cells to obtain a functional protein and thus use its blue light emission to measure Ca2+ concentration. Coelenterazine is a hydrophobic molecule, and therefore is easily taken up across plant and fungal cell walls, as well as the plasma membrane of higher eukaryotes, making aequorin suitable as a (Ca2+ reporter
Calcium in biology
Calcium plays a pivotal role in the physiology and biochemistry of organisms and the cell. It plays an important role in signal transduction pathways, where it acts as a second messenger, in neurotransmitter release from neurons, contraction of all muscle cell types, and fertilization...

) in plants, fungi, and mammalian cells.

Aequorin has a number of advantages over other Ca2+ indicators: It has a low leakage rate from cells, lacks phenomena of intracellular compartmentalization or sequestration, and does not disrupt cell functions or embryo development. Moreover the light emitted by the oxidation of coelenterazine does not depend on any optical excitation, so problems with auto-fluorescence are eliminated. The primary limitation of aequorin is that the prosthetic group coelenterazine is irreversibly consumed to produce light, and requires continuous addition of coelenterazine into the media. Such issues led to developments of other genetically encoded calcium sensors including the calmodulin
Calmodulin
Calmodulin is a calcium-binding protein expressed in all eukaryotic cells...

-based sensor cameleon, developed by Roger Tsien and the troponin
Troponin
400px|thumb|right|alt = Colored dice with checkered background|Ribbon representation of the human cardiac troponin core complex in the calcium-saturated form...

-based sensor, TN-XXL, developed by Oliver Griesbeck.

Commercial quantities available for oral supplementation in humans are available under the name Prevagen.
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