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Advances in bioluminescence mechanism of calcium-regulated photoproteins

Name
Eugene
Surname
Vysotski
Scientific organization
Institute of Biophysics, Russian Academy of Sciences, Siberian Branch; Siberian Federal University
Academic degree
PhD
Position
Head of laboratory
Scientific discipline
Life Sciences & Medicine
Topic
Advances in bioluminescence mechanism of calcium-regulated photoproteins
Abstract
Photoproteins represent a unique class of protein biochemistry. The spatial structures of several hydromedusan photoproteins, their ligand-dependent conformation states, and some their mutants with altered bioluminescence properties have been determined. Based on spatial structures and mutagenesis studies, a proton-relay mechanism for triggering the bioluminescence by Ca2+ was suggested. Mechanism provides qualitative rationalizations of many aspects of photoprotein bioluminescence as well as the function of residues of the substrate-binding cavity in this process.
Keywords
bioluminescence, photoprotein, coelenterazine, luciferase, calcium
Summary

Bioluminescence is a widely distributed phenomenon among marine dwellers. Many of them generate light by oxidation of coelenterazine, an imidazopyrazinone derivative. Based on biochemistry of bioluminescence reactions, these coelenterazine-dependent luminous systems are divided into two classes. One is the luciferase-luciferin reaction in which enzyme catalyzes an oxidative decarboxylation of coelenterazine by O2 with generation of a product, coelenteramide, in its excited electronic state. Another type is Ca2+-regulated photoproteins, a unique class of protein biochemistry. The best known and studied among those are aequorin, first isolated from the jellyfish Aequorea victoria, and obelin, from the hydroid Obelia longissima. All photoproteins consist of a single polypeptide chain to which the oxygen-activated substrate, peroxy substituted coelenterazine, is stabilized by tight, non-covalent binding. Thus, photoproteins can be regarded as luciferases with a long-lived reaction intermediate. The Ca2+ binding to the protein initiates a final step of the reaction, decarboxylation of 2-hydroperoxycoelenterazine, leading to the generation of a protein-bound coelenteramide in its excited electronic state. Ca2+-regulated photoproteins belong to EF-hand Ca2+-binding proteins, one of the most numerous and extensively studied protein families, because all of them contain three EF-hand Ca2+-binding sites.

Over the past decade, the spatial structures of several hydromedusan photoproteins, their ligand-dependent conformation states, and some their mutants with altered bioluminescence properties have been determined. Based on these spatial structures and comprehensive mutagenesis studies, we proposed a proton-relay mechanism for triggering the bioluminescence reaction by Ca2+. The suggested mechanism provides qualitative rationalizations of many aspects of photoprotein bioluminescence as well as the function of residues constituting the substrate-binding cavity in this process.

Bioluminescence is a widely distributed phenomenon among marine dwellers. Many of them generate light by oxidation of coelenterazine, an imidazopyrazinone derivative. Based on biochemistry of bioluminescence reactions, these coelenterazine-dependent luminous systems are divided into two classes. One is the luciferase-luciferin reaction in which enzyme catalyzes an oxidative decarboxylation of coelenterazine by O2 with generation of a product, coelenteramide, in its excited electronic state. Another type is Ca2+-regulated photoproteins, a unique class of protein biochemistry. The best known and studied among those are aequorin, first isolated from the jellyfish Aequorea victoria, and obelin, from the hydroid Obelia longissima. All photoproteins consist of a single polypeptide chain to which the oxygen-activated substrate, peroxy substituted coelenterazine, is stabilized by tight, non-covalent binding. Thus, photoproteins can be regarded as luciferases with a long-lived reaction intermediate. The Ca2+ binding to the protein initiates a final step of the reaction, decarboxylation of 2-hydroperoxycoelenterazine, leading to the generation of a protein-bound coelenteramide in its excited electronic state. Ca2+-regulated photoproteins belong to EF-hand Ca2+-binding proteins, one of the most numerous and extensively studied protein families, because all of them contain three EF-hand Ca2+-binding sites.

 

Over the past decade, the spatial structures of several hydromedusan photoproteins, their ligand-dependent conformation states, and some their mutants with altered bioluminescence properties have been determined. Based on these spatial structures and comprehensive mutagenesis studies, we proposed a proton-relay mechanism for triggering the bioluminescence reaction by Ca2+ (Fig. 1).

Fig. 1. Proton-relay mechanism of the photoprotein Ca2+ trigger and formation of the primary product excited state.

The suggested mechanism provides qualitative rationalizations of many aspects of photoprotein bioluminescence as well as the function of residues constituting the substrate-binding cavity in this process.