Low-barrier hydrogen bond in photoactive yellow protein

光受容タンパク質(photoactive yellow protein)中の低障壁水素結合

山口 繁生*; 上久保 裕生*; 栗原 和男; 黒木 良太; 新村 信雄*; 清水 伸隆*; 山崎 洋一*; 片岡 幹雄*

Yamaguchi, Shigeo*; Kamikubo, Hironari*; Kurihara, Kazuo; Kuroki, Ryota; Niimura, Nobuo*; Shimizu, Nobutaka*; Yamazaki, Yoichi*; Kataoka, Mikio*

Low barrier hydrogen bonds (LBHBs) have been proposed to play roles in protein functions, including enzymatic catalysis and proton transfer. Transient formation of LBHBs is expected to stabilize specific reaction intermediates. However, based on experimental results as well as on theoretical considerations, arguments against the importance of LBHB in proteins have been raised. The discrepancy is due to the absence of direct identification of the hydrogen atom position. Here, we show by high-resolution neutron crystallography of photoactive yellow protein (PYP) that a LBHB exists in a protein, even in the ground state. We identified 87% (819/942) of the hydrogen positions in PYP, and demonstrated that the hydrogen bond between the chromophore and E46 is a LBHB. This LBHB stabilizes an isolated electric charge buried in the hydrophobic environment of the protein interior. We propose that in the excited state, the fast relaxation of the LBHB into a normal hydrogen bond is the trigger for photo-signal propagation to the protein moiety. These results give insights into the novel roles of LBHBs, as well as the mechanism of the formation of LBHBs.