Imidazole-assisted catalysis of luminescence reaction in blue fluorescent protein from the photoprotein aequorin

Blue fluorescent protein from the calcium-binding photoprotein aequorin (BFP-aq) is a dissociable complex of Ca2+-bound apoaequorin and coelenteramide, and is identified as a luciferase that catalyzes the oxidation of coelenterazine by molecular oxygen to emit light. Based on the chemical luminescence of coelenterazine oxidation by an acid–base mechanism, we found that the luminescence activity of BFP-aq was stimulated by imidazole at concentrations of 30–300 mM with coelenterazine and its analogues. The kinetic analyses indicate that imidazole has no effect on the binding affinity of coelenterazine to BFP-aq and may act as a catalytic base, accepting a proton from the –NH– group of coelenterazine and stimulating luminescence activity.