Luciferin - Chemical used by fireflies



Biological role of luciferin?

Luciferin: any organic compound that has been oxidized with luciferase (enzym) to produce a light. The luciferin-luciferase reaction is an enzyme-substrate reaction where luciferin is the substrate that needs to be oxidized and luciferase works as a catalyst for this reaction. (Britannica, 2010)

Reaction that produces light: 

ATP reacts with firefly luciferin, firefly luciferase, and ionic magnesium. This is then oxidized which releases energy and produces the light in the firefly. Different organisms will use different types of luciferins and luciferase; for fireflies their general name is firefly luciferin and firefly luciferase. (Britannica, 2010)

How do different biological molecules help cellular structures function?

Luciferase enzyme: 

Luciferase speeds up the process of oxidizing the luciferin substrate by performing the role of a catalyst. 

ATP: 

The ATP provides energy for this reaction to take place 

Luciferin: 

Luciferin is the main molecule (substrate) which emits light. The common name for the luciferin used in fireflies is called firefly luciferin. It is also called D-luciferin for fireflies. 

Oxygen: 

This reaction happens by the oxidation of luciferase-luciferyl-adenylate and pyrophosphate which is why molecular oxygen is essential. 

Magnesium Ions: 

Along with ATP, the magnesium ions are needed for the reaction to occur. Magnesium Ions help with the energy transfer from ATP to luciferin during the reaction. 

(Branham, 2005)

Chemical properties of firefly luciferin: 

  • Chemical formula C11H8N2O3S2 (Bowie, 2023) 

  • Chemical name S-2-6'-hydroxy-2'-benzothiazolyl-2-thiazoline-4-carboxylic acid (Bowie, 2023) 

  • Structure: Has two main structural components - a benzothiazole ring and a thiazoline ring (Oba et al, 2013)

  • Reactivity: Can be oxidized (Viviani et al, 2018)   

  • pH: Firefly luciferin is neutral/slightly basic. (Viviani et al, 2018)   

Physical properties of firefly luciferin: 

  • Melting point: Decomposes at 189.5-190C. Firefly luciferin doesnt really melt, instead it decomposes at 189.5-190C

  • Solubility: Slightly soluble in water at pH 6.5. It is also soluble in alkaline aqueous solutions, methanol, acetone, and ethyl acetate. 

  • Molecular weight: 280.32 g/mol

(Bowie, 2023)  

Shape and size of molecule: 

As stated previously, luciferin consists of 2 main structural components which are the benzothiazole ring and the thiazoline ring. These ring structures help with light emission and enzyme binding. The light emission happens through the ring as the shape allows the molecule to absorb UV light which plays a role in the reaction to emit light. Additionally since we need the luciferase as a catalyst, it is important that the shape of the molecule goes with the catalyst, and that is why the ring shapes are crucial. (Inouye, 2009). Also, the hydroxyl group in the benzothiazole ring helps with hydrogen bonding and reactivity. The hydroxyl group forms hydrogen bonds with the water molecules and other polar groups in the enzymes active site which is crucial for the positioning of the luciferin molecule during the reaction. The hydroxyl group is also needed for the oxidation of the entire reaction. (Oba et al, 2013)

Functional groups and properties: 

Firefly luciferin consists of 4 functional groups: 

  • Hydroxyl group (on the benzothiazole ring)

  • Carboxyl group 

  • Thiazoline ring 

  • Benzothiazole ring

Solubility: 

The hydroxyl and carboxyl group in this molecule increases the water solubility. Both hydroxyl and carboxyl are polar which makes the overall molecule polar making it compatible with the polar water molecule and increasing its solubility in water. Additionally, both of these groups form hydrogen bonds with water molecules, thus increasing the solubility in water. 

(Bowie, 2023). Being slightly soluble in water helps the luciferin move within and between cells where needed. This means that the luciferin can easily transport to the lantern of the firefly (where the light emitting organs are located) (Louis, 2014)

Reactivity: 

The hydroxyl group reacts with the oxygen molecule which results in an oxidation reaction. This reaction then produces the light in fireflies. The carboxyl group reacts with ATP where ATP gives energy to the carboxyl group to perform the reaction. Lastly, the thiazole and benzothiazole rings help with emitting light as their structure can absorb the UV light and contribute to the reaction. (Chapman, 2023)

Physical shape: 

As stated earlier, the molecule consists of 2 main rings (the thiazole and benzothiazole rings) which help with absorbing the UV light and contributing to the reaction. 

Overall function: 

With the help of ATP, the carboxyl group initiates the reaction. The hydroxyl group works with the oxygen molecule to oxidize the reaction. Both of the rings help with absorbing the UV light which is also needed for the emission of light. The shape of the rings and the arrangement of this molecule fits perfectly for luciferase to act as a catalyst and speed up the reaction. (Oba et al, 2013)

How does luciferin help fireflies survive? 

As we know, luciferin is used to produce light in fireflies. This light plays a key role for fireflies. It helps them with communication and mating, and as a defense mechanism. Fireflies are able to control the light flashes and create patterns. These patterns are used for communicating. Different species have different flash patterns, so they can use them to identify the same  species and potentially a mate. (Branham, 2005). They also use their lights as a defense mechanism in which the light acts as a warning symbol. Some fireflies are toxic to predators and the light helps to remind predators of this. (Lewis, 2016)

Applications of luciferin: 

Luciferin can be used to visualize and locate tumors in an animal body with the help of bioluminescence imaging (BLI). Researchers/scientists engineer a cancer cell which contains the firefly luciferase gene. They then inject this gene into an animal to form tumors. They also inject luciferin into the animal so the reaction can produce light.  This light is then captured by highly sensitive charged-couple device (CCD) cameras that allow researchers to non-invasively see the location and size of a tumor. This way researchers can track the growth of tumors in real time. Researchers often use this technology to study tumors. This procedure uses non-invasive methods, which means that they can study the tumor without surgery on the animal and it is less risky with fewer complications (Mizuta et al, 2024). Luciferin can also be used to detect the quality of water. As we know ATP is present in all living cells and that luciferin and luciferase react in the presence of ATP to emit light. This light is measured by a luminometer. The amount of light produced is directly proportional to the ATP concentration which correlates with the amount of microorganisms present. Compared to the traditional methods of assessing the water quality, this is a quick and easy to detect microorganisms in water (Ihssen et al, 2021).