Baase W A, Eriksson A E, Zhang X J, Heinz D W, Sauer U, Blaber M, Baldwin E P, Wozniak J A, Matthews B W
Howard Hughes Medical Institute, University of Oregon, Eugene 97403.
Faraday Discuss. 1992(93):173-81. doi: 10.1039/fd9929300173.
The lysozyme from bacteriophage T4 is being used as a model system to determine the roles of individual amino acids in the folding and stability of a typical globular protein. One general finding is that the protein is very adaptable, being able to accommodate many potentially destabilizing replacements. In order to determine the importance of 'alpha-helix propensity' in protein stability, different replacements have been made within alpha-helical segments of T4 lysozyme. Several such substitutions of the form Xaa-->Ala increase the stability of the protein, supporting the idea that alanine is a strongly helix-favouring amino acid. It is possible to engineer a protein that has up to ten alanines in succession, yet still folds and has normal activity. This illustrates the redundancy that is present in the amino acid sequence. A number of 'cavity-creating' mutants of the form Leu-->Ala have been constructed to understand better the nature of hydrophobic stabilization. The structural consequences of these mutations differ from site to site. In some cases the protein structure hardly changes at all; in other cases removal of the wild-type side-chain allows surrounding atoms to move in and occupy the vacated space, although a cavity always remains. The destabilization of the protein associated with these cavity-creating mutations also varies from case to case. The results suggest how to reconcile recent conflicting reports concerning the strength of the hydrophobic effect in proteins.
来自噬菌体T4的溶菌酶正被用作一个模型系统，以确定单个氨基酸在典型球状蛋白折叠和稳定性中的作用。一个普遍的发现是，该蛋白具有很强的适应性，能够容纳许多潜在的使稳定性降低的替代氨基酸。为了确定“α-螺旋倾向”在蛋白质稳定性中的重要性，人们在T4溶菌酶的α-螺旋片段内进行了不同的替代。几种形式为Xaa→Ala的此类替代增加了蛋白质的稳定性，支持了丙氨酸是一种强烈倾向于形成螺旋的氨基酸这一观点。有可能设计出一种连续含有多达十个丙氨酸的蛋白质，但其仍能折叠并具有正常活性。这说明了氨基酸序列中存在的冗余性。已经构建了一些形式为Leu→Ala的“产生空洞”的突变体，以更好地理解疏水稳定作用的本质。这些突变的结构后果因位点而异。在某些情况下，蛋白质结构几乎没有变化；在其他情况下，去除野生型侧链会使周围原子移动并占据空出的空间，尽管总是会留下一个空洞。与这些产生空洞的突变相关的蛋白质不稳定情况也因情况而异。这些结果表明了如何调和最近关于蛋白质中疏水作用强度的相互矛盾的报道。
