Mechanistic insights into evolution of bioluminescence from luciferyl-adenylate chemiluminescence

Luciferyl-adenylate (LH₂AMP) is the key chemiluminescent intermediate in the bioluminescence reaction catalyzed by beetle luciferases. Whereas the luciferase-catalyzed bioluminescence reactions usually proceed with high quantum yields in the green-yellow region, the chemiluminescence of LH₂AMP is almost always in the red region. To understand how much basal LH₂AMP chemiluminescence is catalytically incremented by luciferases, and how green bioluminescence arises, we further investigated its chemiluminescence in solvents in the presence of different bases, serum albumins and, for the first time, determined its chemiluminescence quantum yields. The higher stability and chemiluminescence of LH₂AMP in solvents like DMSO and acetonitrile indicate that luciferases provide aprotic environments which stabilize LH₂AMP, favoring the oxidation route. Very weak bases, like N´methyl-acetamide also catalyze the chemiluminescence, corroborating the potential catalytic role of peptide bonds in the in luciferase-like enzymes and BSA. The quantum yield, the luminescence production efficiency, of LH₂AMP chemiluminescence in solvents was much lower (0.01) than the quantum yield in the presence of luciferases (0.2-0.3), confirming that luciferases play a major role to increase the quantum yields of bioluminescence. Weak green chemiluminescence could be generated exclusively in aprotic solvents above saturating concentrations of potassium tert-butoxide, providing strong evidences that green bioluminescence emerges by a salting out-like process of excited oxyluciferin in the active site of beetle luciferases.