Saga Yoshitaka, Ishikawa Tomoko, Watanabe Tadashi
Department of Bioscience and Biotechnology, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan.
Bioelectrochemistry. 2002 Jul;57(1):17-22. doi: 10.1016/s1567-5394(01)00173-6.
The transient photocurrent response from bacteriorhodopsin (bR) on tin oxide electrodes was strongly influenced by metal ions bound to bR molecules. The photocurrent polarity reversal pH, which corresponded to the pH value for the reversal of the proton release/uptake sequence in the bR photocycle, of cation-substituted purple membrane (PM) was shifted to lower pH with the increase in the cation affinities to carboxyl groups and a close correlation was noted between the two values. This suggests that the metal ion present in the extracellular region of a bR molecule modulates the pK(a) of proton release groups of bR by stabilizing the ionized state of the proton-releasing glutamic acids. The behavior of photocurrents at light-off in alkaline media, reflecting the proton uptake by bR, was unchanged by binding monovalent (Na(+) and K(+)) or divalent cations (Mg(2+) and Ca(2+)), but was drastically changed by binding La(3+) ions. This can be explained by invoking a substantial slowing of the proton uptake process in the presence of La(3+).
氧化锡电极上细菌视紫红质（bR）的瞬态光电流响应受到与bR分子结合的金属离子的强烈影响。阳离子取代的紫膜（PM）的光电流极性反转pH值（对应于bR光循环中质子释放/摄取序列反转的pH值）随着阳离子对羧基亲和力的增加而向较低pH值偏移，并且在这两个值之间发现了密切的相关性。这表明bR分子细胞外区域中存在的金属离子通过稳定质子释放谷氨酸的离子化状态来调节bR质子释放基团的pK(a)。碱性介质中熄灯时反映bR质子摄取的光电流行为，在结合单价（Na(+)和K(+)）或二价阳离子（Mg(2+)和Ca(2+)）时不变，但在结合La(3+)离子时发生了剧烈变化。这可以通过在存在La(3+)的情况下质子摄取过程的显著减慢来解释。
