Modern organisms commonly use the same set of 20 genetically coded amino acids for protein synthesis with very few exceptions. However, earlier protein synthesis was plausibly much simpler than modern one and utilized only a limited set of amino acids. Nevertheless, few experimental tests of this issue with arbitrarily chosen amino acid sets had been reported prior to this report. Herein we comprehensively and systematically reduced the size of the amino acid set constituting an ancestral nucleoside kinase that was reconstructed in our previous study. We eventually found that two convergent sequences, each comprised of a 13-amino acid alphabet, folded into soluble, stable and catalytically active structures, even though their stabilities and activities were not as high as those of the parent protein. Notably, many but not all of the reduced-set amino acids coincide with those plausibly abundant in primitive Earth. The inconsistent amino acids appeared to be important for catalytic activity but not for stability. Therefore, our findings suggest that the prebiotically abundant amino acids were used for creating stable protein structures and other amino acids with functional side chains were recruited to achieve efficient catalysis.

中文翻译：

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蛋白质中氨基酸组的全面还原表明益生元氨基酸对于稳定蛋白质的重要​​性。

现代生物通常使用相同的20种遗传编码氨基酸进行蛋白质合成，只有少数例外。然而，早期的蛋白质合成似乎比现代的蛋白质合成简单得多，并且仅利用了有限的一组氨基酸。但是，在此报告之前，几乎没有关于使用任意选择的氨基酸组进行此问题的实验测试的报道。在本文中，我们全面系统地减少了构成祖先核苷激酶的氨基酸集的大小，该氨基酸集在我们先前的研究中得以重建。我们最终发现两个收敛序列，每个序列包含一个13个氨基酸的字母，折叠成可溶性，稳定和催化活性的结构，即使它们的稳定性和活性不如亲本蛋白高。尤其，许多但并非全部的还原定氨基酸与原始地球中似乎丰富的氨基酸相吻合。不一致的氨基酸似乎对催化活性很重要，但对稳定性却不重要。因此，我们的发现表明，益生元丰富的氨基酸可用于创建稳定的蛋白质结构，而其他具有功能性侧链的氨基酸则可用于实现有效的催化作用。