Zhang D, Mauzerall D
Rockefeller University, New York, New York 10021, USA.
Biophys J. 1996 Jul;71(1):381-8. doi: 10.1016/S0006-3495(96)79235-4.
We have studied the photoinduced volume changes, energetics, and kinetics in the early steps of the bacteriorhodopsin (BR) photocycle with pulsed, time-resolved photoacoustics. Our data show that there are two volume changes. The fast volume change ( < or = 200 ns) is an expansion (2.5 +/- 0.3 A3/molecule) and is observed exclusively in the purple membrane (PM), vanishing in the 3-[(3-cholamidopropyl)-dimethylammonio] -1-propane-sulfonate-sulfonate-solubilized BR sample; the slow change (approximately 1 micros) is a volume contraction (-3.7 +/- 0.3 A3/molecule). The fast expansion is assigned to the restructuring of the aggregated BR in the PM, and the 1-micros contraction to the change in hydrogen bonding of water at Asp 212 (Kandori et al. 1995. J. Am. Chem. Soc. 117:2118-2119). The formation of the K intermediate releases most of the absorbed energy as heat, with delta Hk = -36 +/- 8 kJ/mol. The activation energy of the K --> L step is 49 +/- 6 kJ/mol, but the enthalpy change is small, -4 +/- 10 kJ/mol. On the time scale we studied, the primary photochemical kinetics, enthalpy, and volume changes are not affected by substituting the solvent D2O for H2O. Comparing data on monomeric and aggregated BR, we conclude that the functional unit for the photocycle is the BR monomer, because both the kinetics (rate constant and activation energy) and the enthalpy changes are independent of its aggregation state.
我们利用脉冲时间分辨光声技术研究了细菌视紫红质（BR）光循环早期阶段的光致体积变化、能量学和动力学。我们的数据表明存在两种体积变化。快速体积变化（≤200纳秒）是一种膨胀（2.5±0.3埃³/分子），且仅在紫膜（PM）中观察到，在3-[(3-胆酰胺丙基)-二甲基铵]-1-丙烷磺酸盐溶解的BR样品中消失；缓慢变化（约1微秒）是体积收缩（-3.7±0.3埃³/分子）。快速膨胀归因于PM中聚集的BR的结构重组，而1微秒的收缩归因于天冬氨酸212处水的氢键变化（神取等，1995年。美国化学会志117:2118 - 2119）。K中间体的形成将大部分吸收的能量以热的形式释放，ΔHk = -36±8千焦/摩尔。K→L步骤的活化能为49±6千焦/摩尔，但焓变很小，为-4±10千焦/摩尔。在我们研究的时间尺度上，用重水（D2O）替代溶剂水（H2O）不会影响初级光化学动力学、焓和体积变化。比较单体和聚集BR的数据，我们得出结论，光循环的功能单元是BR单体，因为动力学（速率常数和活化能）和焓变均与其聚集状态无关。
