Ribozymes are RNA molecules that have catalytic properties.

In addition to carrying genetic information, RNAs can act as catalysts. The first observation of this was made in 1981. The large precursor RNA that gives rise to one of the ribosomal RNAs of Tetrahymena (a protozoan) is cleaved in a splicing reaction. It was found that this could happen without any protein being present since the intron in the RNA can carry out a self-splicing reaction. In this process, guanosine is used as one substrate. The RNA folds by base pairing to bring the two splice sites into juxtaposition. Guanosine is added to the intron part of the RNA which leads to cleavage. The new 3' end of the RNA chain then attacks the other side of the intron to complete the reaction. Since the folding of the RNA is dependent on base-pairing, the cleavage sites are specific to a given RNA.

This intron from the Tetrahymena pre-ribosomal RNA can be isolated and it folds up, again using base pairing so that it can bind guanosine and another RNA that will base pair to the intron RNA. Again, the RNA is cleaved and the guanosine is added at the 5' end of one of the cleavage products. But not any RNA is cleaved. Only a specific RNA can be bound at a specific site by complementary base-pairing and this determines the site in the RNA that will be cleaved. In this case, the same synthetic version of the Tetrahymena intron can cut many molecules of the RNA and thus is a true catalyst. These ribozyme reactions are found in may single cell eucaryotes and in mitochondria and chloroplasts The reactions are transesterifcation reactions.

Many tRNA are also catalytic. tRNAs, like most other RNAs, are normally synthesized as a large precursors. They are then spliced at specific sites in vivo by an enzyme (RNase P) that contains both protein and RNA. It is the RNA of the RNA-protein complex that plays a major catalytic role, the function of the protein being to increase the rate of cleavage by the RNA. In most cases of RNA catalysis, the substrate is another RNA but ribozymes are not quite this restricted. In the ribosome, RNAs may play a major catalytic role in protein synthesis with the ribosomal proteins playing a subservient role. It has been found, for example, that the large RNA of the eucaryotic ribosome will catalyze the formation of peptide bonds and is a peptidyl transferase.

Since certain RNAs can catalyze RNA cleavage at very specific sites, it is not surprising that this has been exploited by the pharmaceutical industry. These synthetic ribozymes are designed to cut a viral RNA at a specific site, thereby making it uninfectious.