Photinus pyralis (north american firefly)
A fascinating sight during summer nights is the points of light floating in the air, glowing on and off periodically. This light is caused by insects, fireflys. The process by which they generate their own light is called bioluminescence and the protein which generates the light is called luciferase. Fireflys are not the only organism capable to produce light and this here is not the only way to do it. The production of light by luciferase proceeds in three distinct steps. The compound which ultimately produces the light is a small molecule called luciferin. Luciferin has a carboxylate group (COOH). The protein splits adenosine triphosphate (ATP) into adenosine monophosphate (AMP) and a diphosphate group (P) and attaches the AMP to the carboxylate of luciferin. In the next step oxygen is used to cleave off this AMP, generate carbon dioxide and water and leave luciferin with just one carbonyl (CO) group instead of the initial carboxylate. The oxygen in this carbonyl is in an excited state, one of its electrons occupies a position with too much energy. This electron then moves to its ground state and the energy difference is emitted as visible light.
Luciferase is a monomeric protein with a single polypeptide chain. In the PDB structure it may appear as if it were more than one chain, but that is not true. Some parts are just missing for technical reasons. It does however consist of two very distinct domains, clearly visible, forming a cleft. Residues involved in the light production step are found in both domains but to far away from each other to catalyze the reaction. It is therefore assumed that the two domains will move to come closer together when luciferin and ATP enter the cleft as substrates.
Protein Data Bank (PDB)
author: Arno Paehler