title: Ca2+ Binding and Conformational Switch of the Photoprotein Mnemiopsin.
abstract: BACKGROUND
  Bioluminescence in Ca2+-binding photoproteins is an intramolecular reaction triggered by the addition of Ca2+. A comparative study has been done on Ca2+-depleted and Ca2+-loaded apo-mnemiopsin to understand the structural transition of the photoprotein by Ca2+ binding. Ca2+ is removed by TCA (trichloroacetic acid) precipitation to obtain Ca2+-depleted apomnemiopsin.
  
  
  METHODS
  UV-visible, CD and fluorescence spectroscopic studies demonstrate that the addition of Ca2+ is brought about by the overall structure of apo-mnemiopsin becomes more open in a concentration- dependent manner without significantly influencing the secondary structure and indicate that the Ca2+-depleted form of apo-mnemiopsin, in contrast to most other EF-hand calcium binding proteins, adopt a closed conformation when compared to the Ca2+-loaded form. On the other hand, dynamic quenching and limited proteolysis analysis revealed that Ca2+-loaded apo-mnemiopsin became much more flexible than Ca2+ free apo-mnemiopsin.
  
  
  RESULTS
  It seems that increased flexibility of the protein, which occurs due to calcium binding, is a critical factor in oxidative decarboxylation reaction on coelenterazine and consequently light emission.
authors.affiliation: Department of Biology, Faculty of Sciences, University of Guilan, Rasht, Iran.
authors.forename: Shima
authors.initials: S
authors.lastname: Tarahomi
authors.affiliation: Department of Biochemistry, Faculty of Biological Sciences, Tarbiat modares University, Tehran, Iran.
authors.forename: Reza H
authors.initials: RH
authors.lastname: Sajedi
authors.affiliation: Department of Biochemistry, Faculty of Biological Sciences, Tarbiat modares University, Tehran, Iran.
authors.forename: Hossein
authors.initials: H
authors.lastname: Rahmani
authors.affiliation: Department of Biophysics, Faculty of Biological Sciences, Tarbiat modares University, Tehran, Iran.
authors.forename: Bijan
authors.initials: B
authors.lastname: Ranjbar
authors.affiliation: Department of Biophysics, Faculty of Biological Sciences, Tarbiat modares University, Tehran, Iran.
authors.forename: Majid
authors.initials: M
authors.lastname: Taghdir
mesh_terms: D002118:Calcium / Q000737:chemistry*; D004926:Escherichia coli; D015870:Gene Expression; D006863:Hydrogen-Ion Concentration; D008164:Luminescent Proteins / Q000737:chemistry*; D011485:Protein Binding; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence
chemical_list: D008164:Luminescent Proteins; C010245:mnemiopsin; D002118:Calcium
keywords: EF-hand calcium-binding proteins; conformational switch; flexibility; mnemiopsin; photoproteins; proteolysis