Bioluminescence chemistry of fireworm Odontosyllis

Abstract

Marine polychaetes Odontosyllis undecimdonta, commonly known as fireworms, emit bright blue-green bioluminescence. Until the recent identification of the Odontosyllis luciferase enzyme, little progress had been made toward characterizing the key components of this bioluminescence system. Here we present the biomolecular mechanisms of enzymatic (leading to light emission) and nonenzymatic (dark) oxidation pathways of newly described O. undecimdonta luciferin. Spectral studies, including 1D and 2D NMR spectroscopy, mass spectrometry, and X-ray diffraction, of isolated substances allowed us to characterize the luciferin as an unusual tricyclic sulfur-containing heterocycle. Odontosyllis luciferin does not share structural similarity with any other known luciferins. The structures of the Odontosyllis bioluminescent system's low molecular weight components have enabled us to propose chemical transformation pathways for the enzymatic and nonspecific oxidation of luciferin.

Keywords: Odontosyllis luciferin; bioluminescence; heterocycles; oxyluciferin.

Fig. 1.

O. undecimdonta worms and components of its bioluminescence system. (A) The fireworm in daylight and in the dark. (B) Light emission of lyophilized worms in water. (C) NMR tube with the purified oxyluciferin (Green), visible light. (D) Fluorescence of Green, UV light. (E) NMR tube with the purified product of nonenzymatic oxidation of luciferin (Pink), visible light.

Fig. 2.

UV-visible spectra of Odontosyllis luciferin (blue line), Odontosyllis oxyluciferin (Green; green line), and the product of luciferin nonspecific oxidation (Pink; magenta line). Spectra were measured in the following conditions: luciferin in MeCN/H₂O 35%/65% containing 0.1% trifluoroacetic acid; Green in MeCN/H₂O 50%/50% containing 0.1% trifluoroacetic acid; Pink in MeCN/H₂O 25%/75% containing 0.1% trifluoroacetic acid.

Fig. 3.

¹H NMR spectra of compounds from Odontosyllis. (A) Luciferin in methanol-d₄ at 10 °C, 800 MHz. (B) Oxyluciferin in H₂O + 10% D₂O at 15 °C, pH 3.3 (800 MHz) and pH 6.0 (600 MHz). (C) Pink in H₂O + 10% D₂O at 15 °C, pH 2.5 (800 MHz). Impurities are labeled by asterisks; signals of the compounds are labeled with numbers (SI Appendix, Table S1 and Figs. S6, S8, and S14).

Fig. 4.

Structures of Odontosyllis oxyluciferin (Green) and product of nonspecific luciferin oxidation (Pink). (A) General view of Odontosyllis luciferin nonenzymatic oxidation product (Pink) as revealed by X-ray diffraction; nonhydrogen atoms are shown as thermal ellipsoids (P = 50%). (B) General view of oxyluciferin (Green) as revealed by X-ray diffraction; nonhydrogen atoms are shown as thermal ellipsoids (P = 50%).

Fig. 5.

Chemical structures of Odontosyllis luciferin, oxyluciferin (Green), and a product of nonspecific luciferin oxidation (Pink). Biochemical and chemical transformations lead to the oxyluciferin bioluminescence emitter and the nonspecific oxidation product.

Fig. 6.

Proposed pathway of Odontosyllis luciferin biosynthesis.