While extant organisms synthesize proteins using approximately 20 kinds of genetically coded amino acids, the earliest protein synthesis system is likely to have been much simpler, utilizing a reduced set of amino acids. However, which types of building blocks were involved in primordial protein synthesis remains unclear. Herein, we reconstructed three convergent sequences of an ancestral nucleoside diphosphate kinase, each comprising a 10 amino acid "alphabet," and found that two of these variants folded into soluble and stable tertiary structures. Therefore, an alphabet consisting of 10 amino acids contains sufficient information for creating stable proteins. Furthermore, re-incorporation of a few more amino acid types into the active site of the 10 amino acid variants improved the catalytic activity, although the specific activity was not as high as that of extant proteins. Collectively, our results provide experimental support for the idea that robust protein scaffolds can be built with a subset of the current 20 amino acids that might have existed abundantly in the prebiotic environment, while the other amino acids, especially those with functional sidechains, evolved to contribute to efficient enzyme catalysis.

中文翻译：

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包含约 13 个氨基酸的字母表的热稳定和催化活性蛋白质的重建和表征。

虽然现存的生物体使用大约 20 种基因编码的氨基酸合成蛋白质，但最早的蛋白质合成系统可能要简单得多，使用的是一组减少的氨基酸。然而，原始蛋白质合成中涉及哪些类型的构建块仍不清楚。在此，我们重建了祖先核苷二磷酸激酶的三个收敛序列，每个序列都包含一个 10 个氨基酸的“字母表”，并发现其中两个变体折叠成可溶性和稳定的三级结构。因此，由 10 个氨基酸组成的字母表包含足够的信息来创建稳定的蛋白质。此外，在 10 种氨基酸变体的活性位点中重新掺入更多的氨基酸类型提高了催化活性，虽然比活性不如现存蛋白质高。总的来说，我们的研究结果为以下观点提供了实验支持，即可以用当前 20 种氨基酸的一个子集构建强大的蛋白质支架，这些氨基酸可能在益生元环境中大量存在，而其他氨基酸，尤其是那些具有功能性侧链的氨基酸，进化为有助于高效的酶催化。