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NanoLuc, a superior β-barrel fold luciferase, was engineered 10 years ago but the nature of its catalysis remains puzzling. Here experimental and computational techniques were combined, revealing that imidazopyrazinone luciferins bind to an intra-barrel catalytic site but also to an allosteric site shaped on the enzyme surface. Binding to the allosteric site prevents simultaneous binding to the catalytic site, and vice versa, through concerted conformational changes. We demonstrate that restructuration of the allosteric site can dramatically boost the luminescent reaction in the remote active site. Mechanistically, an intra-barrel arginine coordinates the imidazopyrazinone component of luciferin to attack O2 via a radical charge-transfer mechanism, as well as it protonates the excited amide product to secure high emission intensity. Concomitantly, an aspartate, supported by two tyrosines, fine-tune the electronic state of amide product, promoting the formation of the blue color emitter. This information is critical to engineering the next-generation of light-producing biosystems.

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 bioluminescence 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 unprecedented snapshots of coelenterazine-to-coelenteramide oxidation. Bound coelenterazine adopts a Y-shaped conformation, enabling the deprotonated imidazopyrazinone component to attack O2 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.

Insertion-deletion mutations are sources of major functional innovations in naturally evolved proteins, but directed evolution methods rely primarily on substitutions. Here, we report a powerful strategy for engineering backbone dynamics based on InDel mutagenesis of a stable and evolvable template, and its validation in application to a thermostable ancestor of haloalkane dehalogenase and Renilla luciferase. First, extensive multidisciplinary analysis linked the conformational flexibility of a loop-helix fragment to binding of the bulky substrate coelenterazine. The fragment’s key role in extant Renilla luciferase was confirmed by transplanting it into the ancestor. This increased its catalytic efficiency 7,000-fold, and fragment-containing mutants showed highly stable glow-type bioluminescence with 100-fold longer half-lives than the flash-type Renilla luciferase RLuc8, thereby addressing a limitation of a popular molecular probe. Thus, our three-step approach: (i) constructing a robust template, (ii) mapping functional regions by backbone mutagenesis, and (iii) transplantation of a dynamic feature, provides a potent strategy for discovering protein modifications with globally disruptive but functionally innovative effects.

Protein dynamics are often invoked in explanations of enzyme catalysis, but their design has proven elusive. Here we track the role of dynamics in evolution, starting from the evolvable and thermostable ancestral protein AncHLD-RLuc which catalyses both dehalogenase and luciferase reactions. Insertion-deletion (InDel) backbone mutagenesis of AncHLD-RLuc challenged the scaffold dynamics. Screening for both activities reveals InDel mutations localized in three distinct regions that lead to altered protein dynamics (based on crystallographic B-factors, hydrogen exchange, and molecular dynamics simulations). An anisotropic network model highlights the importance of the conformational flexibility of a loop-helix fragment of Renilla luciferases for ligand binding. Transplantation of this dynamic fragment leads to lower product inhibition and highly stable glow-type bioluminescence. The success of our approach suggests that a strategy comprising (i) constructing a stable and evolvable template, (ii) mapping functional regions by backbone mutagenesis, and (iii) transplantation of dynamic features, can lead to functionally innovative proteins., Directed evolution commonly relies on point mutations but InDels frequently occur in evolution. Here the authors report a protein-engineering framework based on InDel mutagenesis and fragment transplantation resulting in greater catalysis and longer glow-type bioluminescence of the ancestral luciferase.

To obtain structural insights into the emergence of biological functions from catalytically promiscuous enzymes, we reconstructed an ancestor of catalytically distinct, but evolutionarily related, haloalkane dehalogenases (EC 3.8.1.5) and Renilla luciferase (EC 1.13.12.5). This ancestor has both hydrolase and monooxygenase activities. Its crystal structure solved to 1.39 Å resolution revealed the presence of a catalytic pentad conserved in both dehalogenase and luciferase descendants and a molecular oxygen bound in between two residues typically stabilizing a halogen anion. The differences in the conformational dynamics of the specificity-determining cap domains between the ancestral and descendant enzymes were accessed by molecular dynamics and hydrogen−deuterium exchange mass spectrometry. Stopped-flow analysis revealed that the alkyl enzyme intermediate formed in the luciferase-catalyzed reaction is trapped by blockage of a hydrolytic reaction step. A single-point mutation (Ala54Pro) adjacent to one of the catalytic residues bestowed hydrolase activity on the modern luciferase by enabling cleavage of this intermediate. Thus, a single substitution next to the catalytic pentad may enable the emergence of promiscuous activity at the enzyme class level, and ancestral reconstruction has a clear potential for obtaining multifunctional catalysts.