Proteins do not have a monopoly on acting as biological catalysts. Certain RNA molecules are also capable of speeding reactions. The most famous of these molecules was discovered by Tom Cech in the early 1980s. Studying excision of an intron in Tetrahymena, Cech was puzzled at his inability to find any proteins catalyzing the process. Ultimately, the catalysis was recognized as coming from the intron itself. It was a self-splicing RNA and since then, many other examples of catalytic RNAs capable of cutting other RNAs have been found.
Figure 4.12.1: Image showing the diversity of ribozyme structures. From left to right: leadzyme, hammerhead ribozyme, twister ribozyme. Image used with permission from Wikipedia (CC-BY-SA-4.0 and credit: Lucasharr).
Ribozymes, however, are not rarities of nature. The protein- making ribosomes of cells are essentially giant ribozymes. The 23S rRNA of the prokaryotic ribosome and the 28S rRNA of the eukaryotic ribosome catalyze the formation of peptide bonds. Ribozymes are also important in our understanding of the evolution of life on Earth. They have been shown to be capable via selection to evolve self-replication. Indeed, ribozymes actually answer a chicken/egg dilemma - which came first, enzymes that do the work of the cell or nucleic acids that carry the information required to produce the enzymes. As both carriers of genetic information and catalysts, ribozymes are likely both the chicken and the egg in the origin of life.