Overview

The term ribozyme is used for RNA that can act as an enzyme. Ribozymes are mainly found in selected viruses, bacteria, plant organelles, and lower eukaryotes. Ribozymes were first discovered in 1982 when Tom Cech’s laboratory observed Group I introns acting as enzymes. This was shortly followed by the discovery of another ribozyme, Ribonulcease P, by Sid Altman’s laboratory. Both Cech and Altman received the Nobel Prize in chemistry in 1989 for their work on ribozymes.

Ribozymes can be categorized into two groups depending upon their size – large and small. Large ribozymes can vary in size from a few hundred to several thousand nucleotides. The type I and II introns and bacterial Ribonuclease P are large ribozymes. Small ribozymes are 30 to 150 nucleotides long. They are found in many pathogenic plant viruses and the hepatitis delta virus (HDV), a human pathogen. Hammerhead, hairpin, HDV and Varkud satellite are common types of small ribozymes. Most large ribozymes need metal ions, especially Mg²⁺, for their activity, but metal ions are not necessary for most of the small ribozymes. The glmS ribozyme, in glmS mRNA, is a unique a-ribozyme as it also acts as a riboswitch when glucosamine 6-phosphate is present at high concentrations.

Most naturally occurring ribozymes catalyze self-cleavage breaking phosphodiester bonds present in their own RNA. Unlike a typical protein enzyme, most ribozymes perform a single-turnover reaction because, after self-cleavage, they are no longer active. However, two ribozymes - Ribonulcease P and the 23S RNA in the 50S ribosomal subunit perform different reactions. Bacterial Ribonuclease P is an RNA-protein complex that has endonuclease activity and requires Mg²⁺ ions. Its RNA component acts on the 5' end of premature tRNA to produce the mature 5' end. The 23S RNA present in the ribosome is different from all other known natural ribozymes as instead of phosphoryl transfer reactions, it carries out peptide-bond formation reactions during translation.

As RNA can act as a carrier of genetic information as well as enzymes, it is hypothesized that an “RNA world” may have existed in the past where RNA played an important role in the development of the early life forms.  However, with the evolution of complex life forms, proteins with twenty amino acids might have started acting as enzymes and took over the many reactions carried out by the ribozymes. This theory gets support from the in-vitro developed artificial ribozymes that can carry out a myriad of reactions such as amide bond formation, glycosidic bond formation, carbon-carbon bond formation, and oxidation-reduction reactions.

Procedure

Ribozymes are special types of RNA that can act as enzymes.   

The substrates for most naturally occurring ribozymes are RNA phosphodiester bonds. The only known exception to this is the 23S ribosomal RNA in the bacterial ribosome which catalyzes peptide bond formation.

Ribozymes have been found in all types of organisms and are similar to protein-based enzymes, as they both increase the rate of reactions. Many ribozymes need metal ions, like magnesium, as co-factors for catalyzing reactions.

Some classes of introns can act as ribozymes. Introns are categorized into five distinct classes – nuclear mRNA, nuclear tRNA, archaeal, group one, and group two introns.

Group one and two introns are large ribozymes that are several hundred nucleotides long and are found in fungal and plant mitochondria, chloroplasts, bacteriophages, and eukaryotic viruses. 

They can self-splice without the help of any proteins, whereas nuclear introns are spliced by the spliceosome, an RNA and protein-containing enzyme complex.  

Small ribozymes are usually 50 to 150 nucleotides long and are self-cleaving nucleotide sequence motifs. These can be found in many RNA plant viruses, as well as the hepatitis delta virus, a human pathogen. 

The replication process in these viruses produce long RNA carrying multiple units of the viral genome where each unit carries a small ribozymes like hammerhead. These regularly spaced hammerhead enzymes undergo self-cleavage resulting in breakage of the long RNA into individual genome segments.

Most ribozymes cleave their own nucleotide sequence; however, Ribonuclease P can cleave other RNA molecules.  Ribonuclease P is found in some bacteria and processes precursor tRNA to generate a mature 5’ end.

The naturally available ribozymes are known to catalyze a narrow range of reactions such as phosphoryl transfer in nucleic acids and peptide bond formation in proteins. Scientists have synthesized artificial ribozymes in the lab which can perform a wide range of reactions like carbon-carbon bond formation and oxidation-reduction reactions.