Stereoisomeric bio-inversion key to biosynthesis of firefly d-luciferin

The chirality of the luciferin substrate is critical to the luciferin–luciferase reaction producing bioluminescence. In firefly, the biosynthetic pathway of d-luciferin is still unclear, although it can be synthesized in vitro from d-cysteine. Here, we show that the firefly produces both d- and l-luciferin, and that the amount of active d-luciferin increases gradually with maturation stage. Studies of firefly body extracts indicate the possible conversion of l-cysteine via l-luciferin into d-luciferin, suggesting that the biosynthesis is enzymatically regulated by stereoisomeric bio-inversion of l-luciferin. We conclude that the selection of chirality in living organisms is not as rigid as previously thought.