Conformational gating of product release sets the catalytic ceiling of a bioluminescent reporter

Luciferases are widely used bioluminescent reporters, yet the molecular determinants of their catalytic efficiency and light-emission stability remain incompletely understood. Here, we reconstruct the complete catalytic pathway of Renilla luciferase by combining steady-state, transient, and temperature-dependent kinetics with crystallography and molecular simulations. We show that the enzyme is substantially undersaturated with oxygen (Km,O2 = 719 M), causing its true turnover number (kcat = 21.9 s-1) to be systematically underestimated. Concurrently, elevated oxygen drives irreversible enzyme inactivation after ~1,500 turnovers, revealing a fundamental trade-off that limits oxygen-affinity engineering. Instead, the genuine bottleneck of the catalytic cycle is the induced-fit conformational opening of the product-bound enzyme. Selective engineering of this transition step through rational loop grafting yielded a variant AncFT-L14 with enhanced catalytic efficiency and glow-type bioluminescence with substantially slower signal decay in cell lysates. Collectively, our results identify conformational dynamics as a primary tunable determinant of luciferase function. More broadly, this work establishes a mechanistically grounded framework for the development of next-generation bioluminescent tools and for engineering enzymes controlled by dynamically gated ligand exchange.