Catalytic mechanism for Renilla-type luciferases

The widely used coelenterazine-powered Renilla luciferase was discovered over 40 years ago, but the oxidative mechanism by which it generates blue photons remains unclear. Here we decipher Renilla -type catalysis through crystallographic, spectroscopic and computational experiments. Structures of ancestral and extant luciferases complexed with the substrate-like analogue azacoelenterazine or a reaction product were obtained, providing molecular snapshots of coelenterazine-to-coelenteramide oxidation. Bound coelenterazine adopts a Y-shaped conformation, enabling the deprotonated imidazopyrazinone component to attack O 2 via a radical charge-transfer mechanism. A high emission intensity is secured by an aspartate from a conserved proton-relay system, which protonates the excited coelenteramide product. Another aspartate on the rim of the catalytic pocket fine-tunes the electronic state of coelenteramide and promotes the formation of the blue light-emitting phenolate anion. The results obtained also reveal structural features distinguishing flash-type from glow-type bioluminescence, providing insights that will guide the engineering of next-generation luciferase‒luciferin pairs for ultrasensitive optical bioassays. Renilla luciferase is a popular bioluminescent enzyme, but the molecular details of its mechanism of action on luciferins such as coelenterazine remained elusive. Now, protein crystal structures and biochemical analyses provide an atomistic description of its catalytic mechanism.