Researchers describe a new fireworm bioluminescence system
A collaborative effort by an international team of scientists led to the discovery of a new luciferin from a fireworm. The characterization of three key low molecular weight components of its bioluminescence system, presented in an article published in the PNAS, provides a better understanding of the molecular mechanisms of bioluminescence in this organism."This work is an important step in a major project to fully characterize a novel bioluminescent system, including the enzyme luciferase, the luciferin substrate, the main reaction products, the light emission mechanism and the biosynthesis pathways of luciferin and its analogues. The newly discovered molecules and mechanisms presented in this work have the potential to stimulate the development of new bioluminescence-based applications in the future," said Aleksandra Tsarkova, a leading author and researcher at the Shemyakin-Ovchinnikov Institute of Organic Chemistry of the Russian Academy of Sciences (Russia).
Bioluminescence, the emission of "cold light" by living organisms, is based on a dependent enzymatic biochemical oxidation reaction in which energy is released as light. Out of 40 presumed bioluminescence systems, the structures of luciferin substrates were elucidated for only nine of them, while a complete description of the luciferin biosynthesis pathway and corresponding enzymes was provided for only two.
In recent years, a wide variety of bioluminescence-based screening methods have been developed because of their usefulness in dynamically monitoring a variety of cellular functions. Over the years, the popularity of bioluminescence imaging techniques has increased due to their high sensitivity and specificity compared to other known imaging technologies. However, each of these techniques has its own limitations and disadvantages imposed by the bioluminescence system used. Thus, researchers are looking for new natural bioluminescence systems to overcome these limitations.
Since 2014, Russian scientists, in collaboration with their colleagues from Japan, Brazil, Spain, the United States and the United Kingdom, have been determining the structures and mechanisms of action of two new luciferins from fungal and Siberian earthworm bioluminescence systems. Today, in an article published in the Proceedings of the National Academy of Sciences, researchers presented the structure of a 10th luciferin. This article describes the results of a multi-year research project, supported by the Russian Scientific Foundation, including the characterization of three key low molecular weight components of the Odontosyllis bioluminescence system, as well as the oxidation pathways of luciferin leading to light emission.
Called "fireworms", odontosyllis are tiny marine polychaetes (~20 mm) that produce a bright blue-green bioluminescence . During the breeding season, normally in summer, swarms appear near the shore for a short period of time at night. These fascinating luminous swarms were observed and described by Christopher Columbus during his voyage of 1492. The study of the Odontosyllis bioluminescence system began in the middle of the 20th century, but despite numerous attempts, including that of the Nobel Prize winner Osamu Shimomura (1928-2018), the biochemical basis of the Odontosyllis light emission process remains largely unknown.
In 2018, the identification of the Odontosyllis luciferase gene was performed by partial protein purification and RNA sequence analysis using small frozen samples. However, the characterization of the chemical structure of luciferin required a much higher amount of worm biomass.
"The first obstacle to overcome was the collection of Odontosyllis samples, a process limited by the small size of the worms and the particularities of their life cycle, which allowed us to obtain only a few grams of specimens per year. However, thanks to the efforts of the late Professor Shoji Inoue, who dedicated himself to the study of the bioluminescence of Japanese Odontosyllis undecimdonta, we obtained 80 grams of freeze-dried Odontosyllis worms that he collected alone for 17 years and kept in the freezer for future research," said Yuichi Oba from the Department of Environmental Biology at Chubu University (Japan).
Due to the difficulty of obtaining the pure substrate, the susceptibility of luciferin to decomposition and the extreme sensitivity of luciferin to UV light presented the following challenge for the studies of the Odontosyllis bioluminescence system. A specially designed purification procedure produced tiny amounts of unstable luciferin, which was subjected to a battery of tests to determine its structure. NMR spectroscopy, mass spectrometry and X-ray diffraction have been used to reveal a very unusual tricyclic heterocyclic luciferin substrate containing three sulfur atoms in different electronic states. The unique structure of Odontosyllis luciferin provides a key insight into a brand new chemical base of bioluminescence, as this new molecule does not share any structural similarities with other known luciferins.
In addition to luciferin, two other crucial molecules have been isolated from the biomass of Odontosyllis and identified: Odontosyllis oxyluciferin and the non-specific oxidation product of luciferin, called respectively green and pink, because of their colours. Together, the structures of these low molecular weight components of the Odontosyllis bioluminescence system have allowed researchers to propose chemical transformation pathways for enzymatic and non-specific oxidation of luciferin. Odontosyllis oxyluciferin is the only green primary emitter described for any known bioluminescent marine organism.
The results are published in the Proceedings of the National Academy of Sciences and provide essential information on the chemical transformations underlying enzymatic and non-specific oxidation of luciferin. The newly discovered molecules and mechanisms presented in this work have the potential to stimulate the development of new bioluminescence-based applications in the future.
