What is Bioluminescence?
Bioluminescence is light produced by chemical reactions in a living organism. It is sometimes referred to as “cold light” because heat is not generated or used in its generation. Bioluminescence is produced by over 80% of the species inhabiting the deep sea.
Firefly bioluminescence
Probably the best known example of bioluminescence is the glow of fireflies. Fireflies are actually beetles of the family Lampyridae.
The enzyme luciferase acts on the substrate luciferin to release light as described below.
MgATP + luciferin + luciferase (E) to E-luciferyl-adenylate + PPi to E-Luciferyl-adenylate + O2 to E + MgAMP + CO2 + oxyluciferin + light
The efficiency of conversion of chemical energy to light energy is very high with almost one photon of light produced for every molecule of luciferin. This contrasts with the incandescent light bulb where only about 10% of the energy is emitted as light (most of the rest is wasted as heat).
Bioluminescence in bacteria
The reaction to produce bioluminescence in bacteria is similar to that in fireflies, Bacterial substrates are a NADH-reduced riboflavin phosphate (FMNH2), and a long chain fatty aldehyde. These are then oxidized in the presence of oxygen and the luciferase enzyme. The resulting complex interacts with the aldehyde as a monooxygenase to form an excited but highly stable intermediate, which decays slowly, resulting in the emission of light. This intermediate can last 10 to 20 seconds at 20 degrees Celsius meaning that the turnover of the luciferase enzyme is extremely slow.
FMNH2 + RCHO + O2 to FMN + H2O + RCOOH + light (490 nm)
The light emitted at 490 nm is in the blue-green range (Blue: 440 – 490 nm; Green: 490 – 570 nm)
Other bioluminescent systems
Aequorin
In 1921, Harvey reported that squeezed extracts from the margin of the umbrella of the jellyfish Aequorea glowed blue for some hours, in one case for nine hours. Aequorin is a photoprotein (not considered a luciferase) isolated from jellyfish including the Aequorea species from which it got its name. It is composed of two subunits, apoaequorin and coelenterazine. In the presence of calcium ions, coelenterazine, a member of the luciferin family, enters an excited state through oxidation and blue light is then emitted (469 nm).
The story of the discovery of the aequorin bioluminescent system (PDF file)
Decapod shrimp
In 1978, Oplophorus luciferase secreted by the deep-sea shrimp Oplophorus gracilirostris was isolated and coelenterazine was identified as a luciferin. Thus, Oplophorus luciferase catalyzes the luminescence reaction of coelenterazine to coelenteramide to emit blue light (454 nm).
The use of coelenterazine to coelenterazide by a luciferase has also been reported in the anthozoan sea pansy Renilla and the myctophid fish Neoscopelus
Light modifying proteins
These proteins shift the wavelength of the bioluminescent system to a longer or shorter wavelength.
Green fluorescent protein (GFP)
GFP is a light modifier protein. In the crystal jellyfish, Aequorea victoria, blue light is emitted by aequorin as described above. This blue light stimulates GFP shifting the light emitted to green (509 nm).
Yellow Fluorescent Protein
Among various kinds of luminous bacteria, marine bacterium Vibrio fischeri strain Y-1b is unusual because it emits yellow light 535 nm, which is attributable to a secondary emitter named yellow fluorescent protein (YFP). Yellow light is emitted under well oxygenated conditions but is rapidly turned off under low oxygen concentration because it is converted into a reduced form that is non-fluorescent.
Next see how bioluminescence occurs in larger organisms and what structures are used to control it
References
Hastings, J. W., and Gibson, Q. H. (1963) J. Biol. Chem. 238, 2537-2554
Hastings, J. W. (1968) Annu. Reu. Biochem. 37, 597-630
J. Bacteriol., Jan 1998, 59-64, Vol 180
Photochemistry and Photobiology, 2006, 82: 587-592
Harvey, E.N. (1921) Biol. Bull. 41, 280–287.
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