June 2006 Molecule of the Month by David Goodsell
Keywords: bioluminescence, luciferin, firefly
Do you remember the first time that you saw a firefly? If you live anywhere between the Rocky Mountains and the east coast of the US, you have probably chased fireflies since you were a child. If you live in other parts of the world, like me, you may have had the pleasure of discovering fireflies during a summer vacation. They are one of the delightful wonders of warm summer evenings.
Glow Little Glowworm, Glow
The cool yellowish light of fireflies is created by the enzyme luciferase, shown here from PDB entry 2d1s. The creation of light is not an easy process. It requires a lot of energy?a single photon of green light requires about the same energy as the breaking of eight ATP molecules. So, luciferase uses a very energetic process to create light. It has a cofactor, termed a luciferin, that forms a highly strained complex with oxygen, using an ATP molecule to help set everything up. When this oxygenated luciferin breaks, forming carbon dioxide in the process, it leaves behind a highly excited form that then emits the light.
Lighting the Way
Since the light-emitting reaction of luciferase is self-contained, needing only oxygen and ATP, clever researchers have used it as a tool in scientific research. It can be used inside cells as a sensor that reports the amount of ATP - if it is glowing, there must be ATP around. Luciferase has also been attached to other proteins to watch where they are in living organisms. The light is often too dim to follow in individual cells, but it has been used to label large collections of cancer cells. By watching for glowing cells, researchers follow a cancer as it grows and metastasizes, and test new anticancer therapies to see if they stop its growth.
Glow in the Dark
The chemical trick of bioluminescence has been discovered dozens of times during the
evolution of life, by bacteria, by fungi, by sea anemones, by dinoflagellates, and, of
course, by fireflies. In each case, there is a luciferase protein that uses a luciferin cofactor
to trap oxygen. The proteins are all very different, and the luciferins come in all shapes
and sizes, indicating that they have all separately evolved to perform the same function.
Two examples are shown here. The one on the left is from a bacterium (PDB entry
and the one on the right is from a dinoflagellate that is responsible for the greenish glow
seen in breaking waves (PDB entry
Note that both of these structures include only
the luciferase protein, and are missing their luciferin cofactors.
For more information on luciferase from a genomics perspective, take a look at the Protein of the Month at the European Bioinformatics Institute.
Exploring the Structure
The color of light that is emitted by luciferase is highly dependent on the amino acids that
surround the luciferin. PDB entry
(on the left) is the structure of luciferase from a
Japanese firefly. It normally emits a greenish-yellow light. But if you change one amino
acid from a serine to an asparagine, the color changes to red, as seen in PDB entry
(on the right). Surprisingly, this change is a fair distance from the luciferin, and the color
change is thought to be caused by slight changes in the packing of amino acids and a
change in the flexibility around the luciferin.
These pictures were created with RasMol. You can create similar pictures by clicking on the accession codes here and picking one of the options under Images and Visualization.
Additional information on luciferase
T. Wilson and J. W. Hastings (1998) Bioluminescence. Annual Review of Cell and
Developmental Biology 14, 197-230.
T. O. Baldwin (1996) Firefly luciferase: the structure is known, but the mystery remains. Structure 4, 223-228.
D. K. Welsh and S. A. Kay (2005) Bioluminescence imaging in living organisms. Current Opinion in Biotechnology 16, 73-78.
© 2014 David Goodsell & RCSB Protein Data Bank