Structure of the Ca2+-regulated photoprotein obelin at 1.7 angstrom resolution determined directly from its sulfur substructure

The crystal structure of the photoprotein obelin (22.2 kDa) from Obelia lon gissima has been determined and refined to 1.7 Angstrom resolution. Contrar y to the prediction of a peroxide, the noncovalently bound substrate, coele nterazine, has only a single oxygen atom bound at the C2-position. The prot ein-coelenterazine 2-oxy complex observed in the crystals is photo-active b ecause, in the presence of calcium ion, bioluminescence emission within the crystal is observed. This structure represents only the second de novo pro tein structure determined using the anomalous scattering signal of the sulf ur substructure in the crystal. The method used here is theoretically diffe rent from that used for crambin in 1981 (4.72 kDa) and represents a signifi cant advancement in protein crystal structure determination.