Nagendra H G, Sukumar N, Vijayan M
Molecular Biophysics Unit, Indian Institute of Science, Bangalore.
Proteins. 1998 Aug 1;32(2):229-40. doi: 10.1002/(sici)1097-0134(19980801)32:2<229::aid-prot9>3.0.co;2-f.
Earlier studies involving water-mediated transformations in lysozyme and ribonuclease A have shown that the overall movements in the protein molecule consequent to the reduction in the amount of surrounding water are similar to those that occur during enzyme action, thus highlighting the relationship among hydration, plasticity, and action of these enzymes. Monoclinic lysozyme retains its crystallinity even when the level of hydration is reduced further below that necessary for activity (about 0.2 gram of water per gram of protein). In order to gain insights into the role of water in the stability and the plasticity of the protein molecule and the geometrical basis for the loss of activity that accompanies dehydration, the crystal structures of monoclinic lysozyme with solvent contents of 17.6%, 16.9%, and 9.4% were determined and refined. A detailed comparison of these forms with the normally hydrated forms show that the C-terminal segment (residues 88-129) of domain I and the main loop (residues 65-73) in domain II exhibit large deviations in atomic positions when the solvent content is reduced, although the three-dimensional structure is essentially preserved. Many crucial water bridges between different regions of the molecule are conserved in spite of differences in detail, even when the level of hydration is reduced well below that required for activity. The loss of activity that accompany dehydration appears to be caused by the removal of functionally important water molecules from the active-site region and the reduction in the size of the substrate binding cleft.
早期涉及溶菌酶和核糖核酸酶A中水介导转化的研究表明，由于周围水量减少，蛋白质分子中的整体运动与酶作用过程中发生的运动相似，从而突出了这些酶的水合作用、可塑性和作用之间的关系。即使水合水平进一步降低到低于活性所需水平（每克蛋白质约0.2克水），单斜晶型溶菌酶仍保持其结晶性。为了深入了解水在蛋白质分子稳定性和可塑性中的作用以及脱水伴随的活性丧失的几何基础，测定并精修了溶剂含量分别为17.6%、16.9%和9.4%的单斜晶型溶菌酶的晶体结构。将这些形式与正常水合形式进行详细比较表明，当溶剂含量降低时，结构域I的C端片段（残基88 - 129）和结构域II中的主环（残基65 - 73）在原子位置上表现出较大偏差，尽管三维结构基本保留。即使水合水平降低到远低于活性所需水平，分子不同区域之间的许多关键水桥尽管细节上存在差异但仍得以保留。脱水伴随的活性丧失似乎是由于从活性位点区域去除了功能重要的水分子以及底物结合裂隙尺寸减小所致。
