Tyr72 and Tyr80 are Involved in the Formation of an Active Site of a Luciferase of Copepod Metridia longa.

Luciferase of copepod Metridia longa (MLuc) is a naturally secreted enzyme catalyzing the oxidative decarboxylation of coelenterazine with the emission of light. To date, three nonallelic isoforms of different lengths (17-24 kDa) for M. longa luciferase have been cloned. All the isoforms are single-chain proteins consisting of a 17-residue signal peptide for secretion, variable N-terminal part and conservative C-terminus responsible for luciferase activity. In contrast to other bioluminescent proteins containing a lot of aromatic residues which are frequently involved in light emission reaction, the C-terminal part of MLuc contains only four Phe, two Tyr, one Trp and two His residues. To figure out whether Tyr residues influence bioluminescence, we constructed the mutants with substitution of Tyr to Phe (Y72F and Y80F). Tyrosine substitutions do not eliminate the ability of luciferase to bioluminescence albeit significantly reduce relative specific activity and change bioluminescence kinetics. In addition, the Tyr replacements have no effect on bioluminescence spectrum, thereby indicating that tyrosines are not involved in the emitter formation. However, as it was found that the intrinsic fluorescence caused by Tyr residues is quenched by a reaction substrate, coelenterazine, in concentration-dependent manner, we infer that both tyrosine residues are located in the luciferase substrate-binding cavity.