Crystal Structure of a Ca2+-discharged Photoprotein (original) (raw)
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Journal of Biological Chemistry, 2004
Ca 2؉-regulated photoproteins are members of the EFhand calcium-binding protein family. The addition of Ca 2؉ produces a blue bioluminescence by triggering a decarboxylation reaction of protein-bound hydroperoxycoelenterazine to form the product, coelenteramide, in an excited state. Based on the spatial structures of aequorin and several obelins, we have postulated mechanisms for the Ca 2؉ trigger and for generation of the different excited states that are the origin of the different colors of bioluminescence. Here we report the crystal structure of the Ca 2؉-discharged photoprotein obelin at 1.96-Å resolution. The results lend support to the proposed mechanisms and provide new structural insight into details of these processes. Global conformational changes caused by Ca 2؉ association are typical of the class of calcium signal modulators within the EF-hand protein superfamily. Accommodation of the Ca 2؉ ions into the loops of the EF-hands is seen to propagate into the active site of the protein now occupied by the coelenteramide where there is a significant repositioning and flipping of the His-175 imidazole ring as crucially required in the trigger hypothesis. Also the H-bonding between His-22 and the coelenterazine found in the active photoprotein is preserved at the equivalent position of coelenteramide, confirming the proposed rapid excited state proton transfer that would lead to the excited state of the phenolate ion pair, which is responsible for the blue emission of bioluminescence.
Proceedings of the National Academy of Sciences, 2006
The crystal structure at 1.93-Å resolution is determined for the Ca 2؉ -discharged obelin containing three bound calcium ions as well as the product of the bioluminescence reaction, coelenteramide. This finding extends the series of available spatial structures of the ligand-dependent conformations of the protein to four, the obelin itself, and those after the bioluminescence reaction with or without bound Ca 2؉ and͞or coelenteramide. Among these structures, global conformational changes are small, typical of the class of ''calcium signal modulators'' within the EF-hand protein superfamily. Nevertheless, in the active site there are significant repositions of two residues. The His-175 imidazole ring flips becoming almost perpendicular to the original orientation corroborating the crucial importance of this residue for triggering bioluminescence. Tyr-138 hydrogen bonded to the coelenterazine N1-atom in unreacted obelin is moved away from the binding cavity after reaction. However, this Tyr is displaced by a water molecule from within the cavity, which now forms a hydrogen bond to the same atom, the amide N of coelenteramide. From this observation, a reaction scheme is proposed that would result in the neutral coelenteramide as the primary excited state product in photoprotein bioluminescence. From such a higher energy state it is now energetically feasible to account for the shorter wavelength bioluminescence spectra obtained from some photoprotein mutants or to populate the lower energy state of the phenolate anion to yield the blue bioluminescence ordinarily observed from native photoproteins.
Protein Science, 2005
The crystal structures of calcium-loaded apo-aequorin and apo-obelin have been determined at resolutions 1.7 Å and 2.2 Å, respectively. A calcium ion is observed in each of the three EF-hand loops that have the canonical calcium-binding sequence, and each is coordinated in the characteristic pentagonal bipyramidal configuration. The calcium-loaded apo-proteins retain the same compact scaffold and overall fold as the unreacted photoproteins containing the bound substrate, 2-hydroperoxycoelenterazine, and also the same as the Ca 2+-discharged obelin bound with the product, coelenteramide. Nevertheless, there are easily discerned shifts in both helix and loop regions, and the shifts are not the same between the two proteins. It is suggested that these subtle shifts are the basis of the ability of these photoproteins to sense Ca 2+ concentration transients and to produce their bioluminescence response on the millisecond timescale. A mechanism of intrastructural transmission of the calcium signal is proposed.
Preparation and X-ray crystallographic analysis of the Ca 2+ -discharged photoprotein obelin
Acta Crystallographica Section D Biological Crystallography, 2004
Ca(2+)-regulated photoproteins belong to the EF-hand Ca(2+)-binding protein family. The addition of calcium ions initiates bright blue bioluminescence of the photoproteins, a result of the oxidative breakdown of coelenterazine peroxide to coelenteramide. Crystals of the Ca(2+)-discharged W92F mutant of obelin from Obelia longissima have been grown, representing the first crystallization of a photoprotein after the Ca(2+)-triggered bioluminescence. A green fluorescence observed from the crystals clearly demonstrates that coelenteramide, the bioluminescence product of coelenterazine peroxide, is bound within the protein. The diffraction pattern exhibits tetragonal Laue symmetry. Systematic absences indicate that the space group is either P4(3)2(1)2 or P4(1)2(1)2. The unit-cell parameters are a = b = 53.4, c = 144.0 A. The crystals diffract to 1.9 A resolution.
Protein Science, 2005
The crystal structures of calcium-loaded apo-aequorin and apo-obelin have been determined at resolutions 1.7 Å and 2.2 Å, respectively. A calcium ion is observed in each of the three EF-hand loops that have the canonical calcium-binding sequence, and each is coordinated in the characteristic pentagonal bipyramidal configuration. The calcium-loaded apo-proteins retain the same compact scaffold and overall fold as the unreacted photoproteins containing the bound substrate, 2-hydroperoxycoelenterazine, and also the same as the Ca 2+-discharged obelin bound with the product, coelenteramide. Nevertheless, there are easily discerned shifts in both helix and loop regions, and the shifts are not the same between the two proteins. It is suggested that these subtle shifts are the basis of the ability of these photoproteins to sense Ca 2+ concentration transients and to produce their bioluminescence response on the millisecond timescale. A mechanism of intrastructural transmission of the calcium signal is proposed.
Ca 2+ -Regulated Photoproteins: Structural Insight into the Bioluminescence Mechanism
Accounts of Chemical Research, 2004
The bioluminescent jellyfish has contributed two famous proteins to modern science: green fluorescent protein or GFP, which finds wide use as a probe in cell biology studies, and aequorin, which has been used for intracellular calcium measurement for more than 30 years. More recently, obelin, a protein from the bioluminescent hydroid and also in the family of what are called "Ca 2+ -regulated photoproteins", has been shown to have very attractive properties both in general applications and for basic structural biology investigations. This review will survey the new information into their molecular mechanism of bioluminescence action. † Russian Academy of Sciences, Siberian Branch.
Protein Science, 2000
The crystal structure of the photoprotein obelin~22.2 kDa! from Obelia longissima has been determined and refined to 1.7 Å resolution. Contrary to the prediction of a peroxide, the noncovalently bound substrate, coelenterazine, has only a single oxygen atom bound at the C2-position. The protein-coelenterazine 2-oxy complex observed in the crystals is photo-active because, in the presence of calcium ion, bioluminescence emission within the crystal is observed. This structure represents only the second de novo protein structure determined using the anomalous scattering signal of the sulfur substructure in the crystal. The method used here is theoretically different from that used for crambin in 1981 4.72 kDa! and represents a significant advancement in protein crystal structure determination.
Protein Science, 2000
The crystal structure of the photoprotein obelin (22.2 kDa) from Obelia longissima has been determined and refined to 1.7 Å resolution. Contrary to the prediction of a peroxide, the noncovalently bound substrate, coelenterazine, has only a single oxygen atom bound at the C2-position. The protein-coelenterazine 2-oxy complex observed in the crystals is photo-active because, in the presence of calcium ion, bioluminescence emission within the crystal is observed. This structure represents only the second de novo protein structure determined using the anomalous scattering signal of the sulfur substructure in the crystal. The method used here is theoretically different from that used for crambin in 1981 (4.72 kDa) and represents a significant advancement in protein crystal structure determination.
FEBS Journal, 2014
Ca 2+-regulated photoproteins are responsible for the bioluminescence of a variety of marine coelenterates. All hydromedusan photoproteins are a single-chain polypeptide to which 2-hydroperoxycoelenterazine is tightly but non-covalently bound. Bioluminescence results from oxidative decarboxylation of 2-hydroperoxycoelenterazine, generating protein-bound coelenteramide in an excited state. The bioluminescence spectral maxima of recombinant photoproteins vary in the range 462-495 nm, despite a high degree of identity of amino acid sequences and spatial structures of these photoproteins. Based on studies of obelin and aequorin mutants with substitution of Phe to Tyr and Tyr to Phe, respectively [Stepanyuk GA et al. (2005) FEBS Lett 579, 1008-1014], it was suggested that the spectral differences may be accounted for by an additional hydrogen bond between the hydroxyl group of a Tyr residue and an oxygen atom of the 6-(p-hydroxyphenyl) substituent of coelenterazine. Here, we report the crystal structures of two conformation states of the F88Y obelin mutant that has bioluminescence and product fluorescence spectra resembling those of aequorin. Comparison of spatial structures of the F88Y obelin conformation states with those of wild-type obelin clearly shows that substitution of Phe to Tyr does not affect the overall structures of either F88Y obelin or its product following Ca 2+ discharge, compared to the conformation states of wild-type obelin. The hydrogen bond network in F88Y obelin being due to the Tyr substitution clearly supports the suggestion that different hydrogen bond patterns near the oxygen of the 6-(p-hydroxyphenyl) substituent are the basis for spectral modifications between hydromedusan photoproteins.