Ca2+ binding and conformational switch of the photoprotein mnemiopsin

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 apo-mnemiopsin. method: 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. © 2017 bentham science publishers.