For most of the twentieth century, biologists thought that all enzymes were proteins. After all, they reasoned, only proteins have the diversity of three-dimensional structures and functional groups to bind to substrates and catalyze their transformations (review Key Concept 3.2). Then it was discovered that some RNA molecules can act as enzymes by catalyzing changes in substrates, in most cases other RNA molecules. You will learn about RNA splicing in Key Concept 14.4. Briefly, this involves a cell making a larger RNA that gets cut and spliced to remove internal region(s). The splicing reaction is catalyzed by a ribozyme. Another important ribozyme occurs at the ribosome, where an RNA molecule catalyzes the formation of a peptide bond between amino acids (see Key Concept 14.5).

In retrospect, it is not too surprising that some RNA molecules can act as biological catalysts (ribozymes). As you saw in Figure 4.3, most RNAs are single strands of nucleotides that can fold back on themselves and form a three-dimensional structure by hydrogen bonding. A substrate can fit into the RNA structure, and functional groups on the RNA (such as —NH₂ and C ═ O) can be involved in catalysis.