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Firefly, also named Photinus pyralis, is a bioluminescent insect. They are able to produce light by a very energetic process, in order to attract its mate. The enzyme responsible of this light producing is luciferase, also known as luciferin-4-monooxygenase (EC: 1.13.12.7). It has various applications in the biotechnology field. In fact, it is used in chemical biology and drug assays. For example, they help to detect protein-protein interactions, to track cells in vivo in order to analysis in real-time the development of a disease in molecular level, to monitor the transcriptional and post-transcriptional regulation of specifics gens, to control apoptosis, to label cancer cells, to detect environmental contamination, to follow protein trafficking, … As there is no light production by mammals, luciferase is a really a great tool for researchers. But this light emission depends on the environmental conditions. Luciferase acts also as a ligase.  

Thus, this oxydo-reductase is involved in severals reactions.

=====Light emission=====

The wavelength of the light can vary with the pH : at the physiological pH, the emitted light is green and at a lower pH, the color is red.

=====Fatty-acyl-CoA synthesis=====

Luciferase is a 62kDa protein. It contains 550 amino acids. The enzyme can be divide into two domains. On the one hand, the major portion, corresponding to the N-terminal.  On the other hand, the small portion, corresponding to the C-terminal.  Those two domains are separated by a large cleft.

The amino acids of C terminal sequence is composed of contiguous residues and form a type of lid upon the N terminal domain.  It contains two β sheets: one is composed of two short antiparallel strands and the other one is composed of 3 antiparallel strands, mixed with 3 α helices. Those helices are put toward the outside.It is α + β structure.  

The amino acids of C terminal sequence is composed of non contiguous residues. It falls into three subdomains (noted A, B, and C).It has an antiparallel β barrel, two β sheets, which are framed by α helices. Each of the two β sheets subdomains (A and B)are composed of 8 β strands and 6 helices. The β sheet A has 5 parallel and 3 antiparallel β strands. The β sheet B has 6 parallel and 2 antiparallel β strands. Those β sheets create a groove, closed on one end by the β barrel.   

The most known reaction of luciferase is the light emission where luciferase uses luciferin, ATP and O2 as substrates. The color of emitted light varies according to the pH, which can be explained by the luciferase structure. But there are some other reactions which can uses fatty acids and coenzyme A. So the active site of the luciferase can theorically bind all these compounds.

The active site is not strictly highlighted according to the actual state of studies but some residues and motifs strongly modified have been determined and this conformation enables to find the active site. Many of these conserved residue are located on the core of the β-barrel and on the small C-terminal domain and in the surface of the N-terminal domain, which forms a <scene name='60/604470/Depression_c-ter_and_n-ter/1'>depression</scene>. However, this depression is too large to enable interactions between residues and substrates, so it is thought that a conformational change occurs and sandwiches the substrates, forming the active site. This conformational change provide a suitable environment for light production because of water molecules will be excluded from the active site, favouring intramolecular reactions. Residues also follow a <scene name='60/604470/Cleft/1'>cleft</scene> caused by of the <scene name='60/604470/Beta_sheet_b/3'>Beta-sheet B</scene> against the <scene name='60/604470/Beta_barrel/3'>Beta-barrel</scene>.

Fatty acids are highly similar to luciferin. Therefore, luciferase can use the luciferin binding site to bind fatty acids. That is why they can be used as substrates by luciferase and then, very high similar reaction as for luciferin occurs.

When the pH is low, the color of light changes. This is probably due to the hydrogen bonds network between substrates, residues of the cleft and water. Indeed, this network triggers to an external electrostatic potential which stabilizes a charge created during the reaction of light emission. The lower pH leads to a weakening of the network, so the energy which can be emitted decreases and the wavelength of photon increases and becomes more red.