The Wonders of Bioluminescence
During the day, the sun lights the streets and alleys, meadows and grasslands, the surface
of dark blue seas. At night, moons, stars, and streetlamps illuminate the darkness. In some cases,
animals such as fireflies, glowworms, click beetles, jellyfish, and squid can even produce their
own light through chemical reactions.
Light production in fireflies is due to a type of chemical reaction called bioluminescence,
a process by which an organism produces and emits light through conversion of chemical energy
to light energy. The components involved in the production of light in a firefly include luciferin,
molecular oxygen, and adenosine triphosphate (ATP), which serves as the general source of
energy. Luciferase or luminase are generic terms for enzymes that catalyze the process of
bioluminescence in nature and are also involved in the light production of a firefly. Luciferase
catalyzes a variety of reactions using a variety of different substrates. In fireflies, luciferase
catalyzes an oxidative reaction involving ATP, luciferin and molecular oxygen. The result of the
reaction yields an electronically excited oxyluciferin species. One proposed mechanism of the
luciferin-luciferase reaction involves activation of the carboxylate group of luciferin by ATP to
form the adenylated luciferin. Then the alpha subunit is lost, allowing the reaction with
molecular oxygen. Cleavage of the dioxetanone ring yields the excited state of oxyluciferin.
The catalytic mechanism of firefly luciferase.
Firefly luciferase acts not only as a mono-oxygenase but also a synthetase to generate the
acyl adenylate. The catalytic mechanism of firefly luciferase is a multi-step process:
Step 1:
The firefly D-luciferin is transformed to the enzyme-bound luciferyl adenylate, releasing the
inorganic pyrophosphate when ATP and magnesium are present. The adenosine monophosphate,
AMP, is the substrate for the oxidation of firefly luciferase.
Step 2:
Although firefly luciferase functions as the mono-oxygenase, it behaves in a very unusual
manner without the apparent involvement of a metal or cofactor. The oxygen generates a ringed
species which later transforms into the electronic excited state and yields carbon dioxide. The
excited state of Luc⋅ Oxyluciferin and the carbon dioxide each contains an oxygen atom from
molecular oxygen.
Step 3:
The rapid loss of energy of the excited Luc⋅ Oxyluciferin to its ground state results in emission
of light. Because every reacted luciferin emits a photon, a majority of the energy is used towards
light production rather than heat production. This bioluminescence, also known as "cold light," is
96% efficient, far more efficient than an incandescent light with an efficiency rating of 10%. At
the same time, fireflies can also use dehydroluciferin to yield the acyl adenylate and inorganic
pyrophosphate. This step which has no further steps is considered as the limitation step for step 1
to inhibit the activity of the enzyme.
All known bioluminescence reactions require oxygen and the intermediacy of peroxides;
however, luciferases constitute a diverse group of unrelated enzymes acting upon chemically
different luciferins and employing a variety of cofactors.
The glow of fireflies serves many purposes. It acts to beget life, and it acts to protect it.
For example, many adult fireflies have flash patterns unique to their species and use them to
identify other members of their species as well as to distinguish between members of the
opposite sex. Several studies have shown that female fireflies choose mates depending upon
specific male flash pattern characteristics. Higher male flash rates, as well as increased flash
intensity, have been shown to be more attractive to females in two different firefly species.
Furthermore, the larvae use their glows as warning signals to communicate their distastefulness.
The ability of fireflies to control the rate at which they emit light are explained by two
theories: the Oxygen Control Theory and the Neural Activation Theory. As mentioned
previously, oxygen is required in the production of light. On one hand, the Oxygen Control
Theory explains how fireflies are able to regulate oxygen intake, therefore controlling the rate at
which light is emitted. The Neural Activation Theory, on the other hand, explains how fireflies
have neural-controlled structures structures called tracheal end cells that facilitate the chemical
reaction.
Insight into the mechanisms that govern luciferase allows scientists to apply
bioluminescence in the other areas of study. For example, Christopher Contag, professor of
microbiology and immunology at Stanford University, proposed using bioluminescence as an
indicator of life. Since ATP is a primary component of living organisms and a requirement in the
function of luciferase, bioluminescent activity can indicate whether or not ATP is present in a
culture or specimen. Additionally, NASA also considered using this technique to determine the
possibility of past lives in other planets by testing for ATP in planet samples.
Not only is bioluminescence a good indicator of life, it also serves as a good marker. To
test the strength of an antibiotic, luciferase can be added to tuberculosis samples along with the
antibiotic. If the antibiotic is potent enough, the bacteria would all die. If not, some bacteria
would still be alive and divide as indicated by a glow when luciferase reacts with oxygen and
ATP to produce light. Green fluorescent protein (GFP) from jellyfish are also great markers for
determining the location of proteins. When hit with blue light, GFP glows green. Therefore,
proteins of interest may be detected by staining cells with fluorescently labeled antibodies.
Bioluminescence is a natural wonder with the potential to be incorporated into
applications that can help further scientific research. However, it is sometimes overlooked. By
exploring the production and the different purposes of bioluminescence among organisms like
the firefly, one can appreciate the natural beauty that our earth has to offer us.