Evolution of Biological Catalysis: Ribozyme to RNP Enzyme

The enzymes that perform biological catalysis in contemporary organisms are usually proteins, occasionally ribonucleoprotein (RNP) complexes, and in rare instances pure RNA (ribozymes). Because RNA can serve as both an informational molecule and a biocatalyst, it has been attractive to consider a primordial RNA World in which RNA enzymes catalyzed the replication of RNA genomes and an array of other metabolic steps, before the advent of protein enzymes and DNA genomes. By what pathways, then, did the RNA World evolve to the present state? Here, I describe plausible pathways for the evolution of biological catalysis, with special emphasis on the origin of RNP enzymes. Recent findings support the argument that RNP enzymes are not undergoing extinction, but instead, they are continuing to evolve and to elaborate new functions.