A ribozyme is an RNA molecule whose folded structure enables it to accelerate a chemical reaction. Unlike most enzymes, which are composed of protein, ribozymes use RNA as their catalytic framework. Their activities include RNA cleavage, RNA joining, RNA processing, and peptide-bond formation within the ribosome. The discovery of RNA catalysis established that nucleic acids can perform chemical work as well as carry biological information. Some ribozymes function independently under suitable laboratory conditions; others operate within complexes containing essential supporting proteins. (nobelprize.org)
Discovery and historical development
Before the early 1980s, biological catalysis was generally attributed to proteins. In 1982, Thomas Cech and colleagues demonstrated that a precursor of ribosomal RNA from the single-celled organism Tetrahymena thermophila could undergo RNA splicing after proteins had been removed. An intervening sequence, or intron, excised itself, and the surrounding exons were joined. The reaction required appropriate salts and a guanosine factor, but no protein enzyme. This showed that the catalytic activity was intrinsic to the RNA. (pubmed.ncbi.nlm.nih.gov)
In 1983, Sidney Altman and colleagues showed that the RNA component of bacterial ribonuclease P, an enzyme involved in transfer RNA maturation, could catalyze its processing reaction without its protein component under suitable experimental conditions. Cech and Altman shared the 1989 Nobel Prize in Chemistry for discovering the catalytic properties of RNA. These findings expanded the concept of an enzyme beyond protein-based catalysts. (pubmed.ncbi.nlm.nih.gov)
Structure and catalytic mechanisms
RNA is a chain of nucleotides, but catalytic activity depends on how that chain folds. Base pairing produces helices, while loops and long-range contacts organize a three-dimensional active site. Studies of the hammerhead ribozyme showed that contacts distant from the cleavage site can strongly enhance catalysis by positioning essential groups correctly. Consequently, a shortened RNA construct may behave differently from the corresponding natural molecule. (pubmed.ncbi.nlm.nih.gov)
Ribozymes accelerate reactions through substrate positioning, proton transfer, and interactions with metal ions. In acid–base catalysis, RNA functional groups help activate a reacting group or facilitate departure of a leaving group. Hydrogen bonds and other contacts organize these groups within the active site. Magnesium and other cations can stabilize RNA folding and participate in chemistry, although their precise contribution differs among ribozymes. Divalent metal ions are not an absolute requirement for every RNA-catalyzed reaction under every experimental condition. (pubmed.ncbi.nlm.nih.gov)
The chemistry of RNA cleavage also varies. Ribonuclease P performs hydrolysis, whereas hepatitis delta virus ribozymes cleave through attack by an adjacent hydroxyl group on the RNA backbone, producing a 2′,3′-cyclic phosphate and a 5′-hydroxyl terminus. Thus, superficially similar cleavage reactions need not share the same mechanism or products. (pubmed.ncbi.nlm.nih.gov)
Major biological examples
Self-splicing introns. Group I introns include the original Tetrahymena example. Their reactions remove an internal RNA segment and reconnect the flanking sequences. Group II introns constitute a different structural class; their branching pathway produces an intron lariat containing an unusual 2′–5′ linkage. Structural studies reveal similarities between their catalytic organization and that of the spliceosome. (pubmed.ncbi.nlm.nih.gov)
Ribonuclease P. Its RNA-based forms process precursor tRNAs, including removal of their 5′ leader sequences. The bacterial RNA component can perform catalysis independently in vitro, providing a clear example of an RNA enzyme acting on separate substrate molecules rather than simply modifying itself. (pubmed.ncbi.nlm.nih.gov)
Self-cleaving RNAs. Hammerhead and hepatitis delta virus ribozymes contain compact catalytic structures that cut particular backbone positions. Hepatitis delta virus RNA fragments were experimentally shown to possess intrinsic cleavage activity, demonstrating that catalytic RNA also occurs in a viral genome. (pubmed.ncbi.nlm.nih.gov)
The ribosome. The ribosome catalyzes peptide-bond formation during translation. Structural analysis established that conserved ribosomal RNA forms the catalytic center of its large subunit. The ribosome is therefore a ribozyme-containing RNA–protein machine, not a protein-free enzyme. Its activity connects RNA catalysis directly to protein synthesis. (pubmed.ncbi.nlm.nih.gov)
The spliceosome. The spliceosome removes introns from precursor messenger RNA. Experiments demonstrated that U6 small nuclear RNA positions catalytic metals involved in both splicing reactions. Although numerous proteins support assembly and rearrangement, RNA directly participates in the catalytic chemistry. (pmc.ncbi.nlm.nih.gov)
Experimental study and engineering
Ribozymes are investigated by measuring reaction rates, altering RNA sequences or chemical groups, and determining structures through methods such as X-ray crystallography. Combining structural and biochemical evidence is important: a folded structure alone does not establish which groups perform catalysis, and changes outside the active site can influence activity indirectly. Full-length hammerhead studies illustrate how these approaches can resolve apparently conflicting observations. (pubmed.ncbi.nlm.nih.gov)
Laboratory directed evolution has produced RNA polymerase ribozymes that extend primers along RNA templates. A 2016 study demonstrated synthesis of functional RNAs and substantial amplification of short templates. Other experiments developed ribozymes that assemble RNA using trinucleotide building blocks. These systems broaden the experimentally demonstrated capabilities of RNA, but their measured performance remains dependent on substrates, sequence, and reaction conditions. (pubmed.ncbi.nlm.nih.gov)
Evolutionary significance
Ribozymes provide experimental support for the plausibility of the RNA world hypothesis, which proposes an early stage of evolution in which RNA combined hereditary and catalytic functions before the modern division of labor between DNA and proteins. Laboratory RNA synthesis and amplification show that parts of such a system are chemically achievable. They do not establish that a particular RNA world existed historically; the emergence of sufficiently effective replication under prebiotic conditions remains a separate question. (nobelprize.org)