Wiegand T W, Janssen R C, Eaton B E
NeXstar Pharmaceuticals Incorporated, Boulder, CO 80301, USA.
Chem Biol. 1997 Sep;4(9):675-83. doi: 10.1016/s1074-5521(97)90223-4.
It is generally accepted that, during evolution, replicating RNA molecules emerged from pools of random polynucleotides. This prebiotic RNA world was followed by an era of RNA-mediated catalysis of amide-bond formation. RNA would thus have provided the machinery responsible for the assembly of peptides and the beginning of the protein world of today. Naturally occurring ribozymes, which catalyze the cleavage or ligation of oligonucleotide phosphodiester bonds, support the idea that RNA could self-replicate. But was RNA constrained to this path and were RNA-acylated carriers required before RNA could catalyze the formation of amide bonds?
We have isolated RNA catalysts that are capable of mediating amide-bond synthesis without the need for specifically designed templates to align the substrates, and we have kinetically characterized these catalysts. The rate enhancement observed for these RNA amide synthases exceeds the noncatalyzed amidation rate by a factor of approximately 10(4). In addition, Cu2+ ions caused a change in the affinity of RNA for the substrate rather than being directly involved in amide-bond formation.
The discovery of these new amide synthases shows how functionally modified nucleic acids can facilitate covalent-bond formation without templating. Previously unforeseen RNA-evolution pathways can, therefore, be considered; for example, to guide amide-bond formation, en route to the protein world, it appears that substrate-binding pockets were formed that are analogous to those of protein enzymes.
人们普遍认为，在进化过程中，复制性RNA分子从随机多核苷酸池中出现。在这个前生物RNA世界之后，是一个由RNA介导的酰胺键形成催化的时代。因此，RNA可能提供了负责肽组装的机制以及当今蛋白质世界的开端。天然存在的核酶可催化寡核苷酸磷酸二酯键的切割或连接，这支持了RNA能够自我复制的观点。但是，RNA是否局限于这条路径，以及在RNA能够催化酰胺键形成之前是否需要RNA酰化载体呢？
我们分离出了能够介导酰胺键合成的RNA催化剂，无需特定设计的模板来排列底物，并且我们对这些催化剂进行了动力学表征。这些RNA酰胺合成酶观察到的速率增强比非催化酰胺化速率高出约10^4倍。此外，Cu2+离子引起了RNA对底物亲和力的变化，而不是直接参与酰胺键的形成。
这些新的酰胺合成酶的发现表明，功能修饰的核酸如何能够在没有模板的情况下促进共价键的形成。因此，可以考虑以前未预见到的RNA进化途径；例如，为了在通往蛋白质世界的途中引导酰胺键的形成，似乎形成了与蛋白质酶类似的底物结合口袋。
