Significance of low-frequency local fluctuation motions in the transmembrane B and C a-helices of bacteriorhodopsin, to facilitate efficient proton uptake from the cytoplasmic surface, as revealed by

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• 作者：Atsushi Kira ; Michikazu Tanio ; Satoru Tuzi and Hazime Sait?
• 关键词：Bacteriorhodopsin ; Conformation and dynamics change ; Low ; frequency local fluctuations ; Proton uptake ; Site ; directed solid ; state 13C NMR
• 刊名：European Biophysics Journal
• 出版年：2004
• 出版时间：November 2004
• 年：2004
• 卷：33
• 期：7
• 页码：580-588
• 全文大小：391 KB

文摘

13C NMR spectra of [1-13C]Val- or -Pro-labeled bacteriorhodopsin (bR) and its single or double mutants, including D85N, were recorded at various pH values to reveal conformation and dynamics changes in the transmembrane -helices, in relation to proton release and uptake between bR and the M-like state caused by modified charged states at Asp85 and the Schiff base (SB). It was found that the D85N mutant acquired local fluctuation motion with a frequency of 104 Hz in the transmembrane B -helix, concomitant with deprotonation of SB in the M-like state at pH 10, as manifested from a suppressed 13C NMR signal of the [1-13C]-labeled Val49 residue. Nevertheless, local dynamics at Pro50 neighboring with Val49 turned out to be unchanged, irrespective of the charged state of SB as viewed from the 13C NMR of [1-13C]-labeled Pro50. This means that the transmembrane B -helix is able to acquire the fluctuation motion with a frequency of 104 Hz beyond the kink at Pro50 in the cytoplasmic side. Concomitantly, fluctuation motion at the C helix with frequency in the order of 104 Hz was found to be prominent, due to deprotonation of SB at pH 10, as viewed from the 13C NMR signal of Pro91. Accordingly, we have proposed here a novel mechanism as to proton uptake and transport based on a dynamic aspect that a transient environmental change from a hydrophobic to hydrophilic nature at Asp96 and SB is responsible for the reduced pK_a value which makes proton uptake efficient, as a result of acquisition of the fluctuation motion at the cytoplasmic side of the transmembrane B and C -helices in the M-like state. Further, it is demonstrated that the presence of a van der Waals contact of Val49 with Lys216 at the SB is essential to trigger this sort of dynamic change, as revealed from the 13C NMR data of the D85N/V49A mutant.