Relocation of Water Molecules between the Schiff Base and the Thr46-Asp96 Region during Light-Driven Unidirectional Proton Transport by Bacteriorhodopsin: An FTIR Study of the N Intermediate

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• 作者：Akio Maeda ; Robert B. Gennis ; Sergei P. Balashov ; and Thomas G. Ebrey
• 刊名：Biochemistry
• 出版年：2005
• 出版时间：April 26, 2005
• 年：2005
• 卷：44
• 期：16
• 页码：5960 - 5968
• 全文大小：197K
• 年卷期：v.44,no.16(April 26, 2005)
• ISSN：1520-4995

文摘

A key event in light-driven proton pumping by bacteriorhodopsin is the formation of the Lintermediate, whose transition to M is accompanied by the first proton transfer step, from the Schiff baseto Asp85 on the extracellular side. Subsequent reprotonation of the Schiff base from the other side of themembrane to form the N intermediate is crucial for unidirectional proton transport. Previous FTIR studieshave suggested that the intense water O-D stretching vibration bands which appear in L at 2589, 2605,and 2621 cm^-1 are due to a cluster of polarized water molecules connecting the Schiff base to the Thr46-Asp96 region closer to the cytoplasmic surface. In the present study the difference spectrum was obtainedof the N intermediate with its photoproduct N', formed after irradiating N at 80 K. The water O-Dstretching vibrations of N appear as a broad feature in a similar frequency region with a similar intensityto those of L. This feature is also affected by T46V like in L. However, the intensities of these watervibrations of N nearly returned to the initial unphotolyzed state upon formation of N', unlike those of Lwhich are preserved in L'. An exception was V49A, which preserved the intense water vibrations of N inN'. The results suggest that both L and N have a water cluster extending from the Schiff base to Thr46.The surrounding protein moiety stabilizes the water cluster in L, but in N it is stabilized mostly by interactionwith the Schiff base.