The spatial structure of the Ca2+-regulated photoprotein obelin has been solved to resolution of 1.1 Angstrom. Two oxygen atoms are revealed substituted at the C2-position of the coelenterazine in contrast to the obelin structure at 1.73 Angstrom resolution where one oxygen atom only was disclosed. The electron density of the second oxygen atom was very weak but after exposing the crystals to a trace of Ca2+, the electron densities of both oxygen atoms became equally intense. In addition, one Ca2+ was found bound in the loop of the first EF-hand motif. Four of the ligands were provided by protein residues Asp30, Asn32, Asn34, and the main chain oxygen of Lys36. The other two were from water molecules. From a comparison of B-factors for the residues constituting the active site, it is suggested that the variable electron densities observed in various photoprotein structures could be attributed to different mobilities of the peroxy oxygen atoms. (C) 2003 Elsevier Inc. All rights reserved.