半合成イクオリンの結晶構造解析

抄録

The photoprotein aequorin emits light by an intramolecular reaction in the presence of a trace of Ca²⁺. Semi-synthetic aequorins, produced by replacing the coelenterazine moiety in aequorin with the analogues of coelenterazine, show widely different sensitivities to Ca²⁺. To understand the structural basis of the Ca²⁺-sensitivity, we determined the crystal structures of four semi-synthetic aequorins (cp-, i-, br- and n-aequorins) . In general, the protein structures of these semi-synthetic aequorins are almost identical with that of native aequorin. Of the four EF-hand domains in the molecule, EF-hand II does not bind Ca²⁺, and the loop of EF-hand IV is clearly deformed. It is most likely that the binding of Ca²⁺ with EF-hands I and III triggers luminescence. Although little difference was found in the overall structures of aequorins investigated, some significant differences were found in the interactions between the substituents of coelenterazine moiety and the amino-acid residues in the binding pocket. The coelenterazine moieties in i-, br- and n-aequorins have bulky 2-substitutions, which can interfere with the conformational changes of protein structure that follow the binding of Ca²⁺ to aequorin. In cp-aequorin, the cyclopentylmethyl group that substitutes for the original 8-benzyl group does not interact hydrophobically with the protein part, giving the coelenterazine moiety more conformational freedom to promote the light-emitting reaction. The differences of various semisynthetic aequorins in the Ca²⁺-sensitivity and reaction rate are explained by the capability of the involved groups and structures to undergo conformational changes in response to the Ca²⁺-binding.