Lozier R H, Xie A, Hofrichter J, Clore G M
Laboratory of Chemical Physics, National Institutes of Diabetes, Digestive, and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892.
Proc Natl Acad Sci U S A. 1992 Apr 15;89(8):3610-4. doi: 10.1073/pnas.89.8.3610.
The absorbance changes that accompany the light-driven proton-pumping cycle of bacteriorhodopsin measured over a broad range of times, wavelengths, temperatures, and pH values have been globally fitted to the kinetic model K in equilibrium with L in equilibrium with X in equilibrium with M in equilibrium with N in equilibrium with O----bR. A remarkably good fit to the data was obtained by optimizing the rate constants at 20 degrees C and the corresponding activation energies at each pH value, together with the extinction coefficients for each intermediate, which were assumed to be independent of both pH and temperature. Back-reactions are included for all but the last step of the cycle and are found to be essential for fitting the data. The rates of these reactions are large, and the analogous irreversible model produced significantly worse fits to the data. Small systematic differences between the fit and the experimental data associated with the X, M, and O intermediates, together with the inability of the model to produce spectra for the X and M intermediates consistent with their assignment as molecular species, indicate that this model must be an incomplete description of the photocycle. We suggest that these problems arise from the presence of additional occupied states that are difficult to distinguish on the basis of their visible absorption spectra alone.
在广泛的时间、波长、温度和pH值范围内测量的伴随细菌视紫红质光驱动质子泵循环的吸光度变化，已通过动力学模型K与L平衡、L与X平衡、X与M平衡、M与N平衡、N与O平衡----bR进行了全局拟合。通过在20℃下优化速率常数以及在每个pH值下相应的活化能，连同每个中间体的消光系数（假定其与pH值和温度均无关），获得了与数据非常好的拟合。除循环的最后一步外，所有步骤都包含了逆向反应，并且发现这些逆向反应对于拟合数据至关重要。这些反应的速率很大，而类似的不可逆模型对数据的拟合明显更差。与X、M和O中间体相关的拟合与实验数据之间的微小系统差异，以及该模型无法产生与将X和M中间体指定为分子物种相一致的光谱，表明该模型必定是对光循环的不完整描述。我们认为这些问题源于存在仅基于其可见吸收光谱难以区分的额外占据态。
