James L C, Tawfik D S
Centre for Protein Engineering, Cambridge CB2 2HQ, United Kingdom.
Protein Sci. 2001 Dec;10(12):2600-7. doi: 10.1110/ps.14601.
It is generally accepted that enzymes evolved via gene duplication of existing proteins. But duplicated genes can serve as a starting point for the evolution of a new function only if the protein they encode happens to exhibit some activity towards this new function. Although the importance of such catalytic promiscuity in enzyme evolution has been proposed, little is actually known regarding how common promiscuous catalytic activities are in proteins or their origins, magnitudes, and potential contribution to the survival of an organism. Here we describe a pattern of promiscuous activities in two completely unrelated proteins-serum albumins and a catalytic antibody (aldolase antibody 38C2). Despite considerable structural dissimilarities-in the shape of the cavities and the position of catalytic lysine residues-both active sites are able to catalyze the Kemp elimination, a model reaction for proton transfer from carbon. We also show that these different active sites can bind promiscuously an array of hydrophobic negatively charged ligands. We suggest that the basic active-site features of an apolar pocket and a lysine residue can act as a primitive active site allowing these promiscuous activities to take place. We also describe, by modelling product formation at different substrate concentrations, how promiscuous activities of this kind- inefficient and rudimentary as they are-can provide a considerable selective advantage and a starting point for the evolution of new functions.
人们普遍认为，酶是通过现有蛋白质的基因复制进化而来的。但是，只有当复制基因所编码的蛋白质碰巧对这种新功能表现出某种活性时，这些复制基因才能成为新功能进化的起点。尽管已经有人提出这种催化多效性在酶进化中的重要性，但实际上对于蛋白质中多效性催化活性的普遍程度、其起源、大小以及对生物体生存的潜在贡献，我们知之甚少。在这里，我们描述了两种完全不相关的蛋白质——血清白蛋白和一种催化抗体（醛缩酶抗体38C2）中的多效性活性模式。尽管在腔的形状和催化赖氨酸残基的位置上存在相当大的结构差异，但两个活性位点都能够催化肯普消除反应，这是一个碳上质子转移的模型反应。我们还表明，这些不同的活性位点可以杂乱地结合一系列疏水带负电荷的配体。我们认为，一个非极性口袋和一个赖氨酸残基的基本活性位点特征可以作为一个原始活性位点，使这些多效性活性得以发生。我们还通过模拟不同底物浓度下产物的形成，描述了这种多效性活性——尽管效率低下且原始——如何能够提供相当大的选择优势，并成为新功能进化的起点。
