Structures of the Ca²⁺-regulated photoprotein obelin Y138F mutant before and after bioluminescence support the catalytic function of a water molecule in the reaction

Crystal structures of the Y138F obelin mutant before and after bioluminescence have been determined at 1.72 and 1.30 Å resolution, respectively. Their spatial structures support the catalytic function of a water molecule found in an internal cavity in the light-emission reaction of Ca²⁺-regulated photoproteins.