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A ribozyme is a molecule of ribonucleic acid (RNA) that has the ability to act as a catalyst in a chemical reaction. The term ribozyme is an abbreviation of ribonucleic acid enzyme. Before ribozymes were discovered in the 1980s, it was thought that only proteins could act as enzymes and catalyze reactions. It is now known that ribozymes also participate in many important intracellular functions and may have therapeutic applications due to their ability to cleave RNA. Ribozymes can be artificially synthesized as well as naturally produced.
The main function of ribozymes is to split the phosphodiester bonds of target RNA molecules through hydrolysis. Phosphodiester bonds are covalent bonds that link nucleotides in RNA and DNA together, forming the backbone of the molecule. Hydrolysis involves the addition of a water molecule, which has the effect of dissolving the bonds. Ribozymes use hydrolysis to cleave other RNA molecules or themselves, sometimes also ligating, or connecting, two split RNA molecules together in a process called splicing.
Study of RNA splicing in the unicellular organism Tetrahymena thermophila led to the discovery of ribozymes in the early 1980s. Thomas Cech, a professor at the University of Colorado in the U.S., found that RNA in this organism could both cut and ligate itself in the absence of any catalyzing protein. Cech and Yale University professor Sidney Altman were awarded the 1989 Nobel Prize in Chemistry for their research on ribozymes.
In addition to splicing, ribozymes catalyze the assembly of amino acids into proteins during the process of translation. Translation occurs when genetic information in messenger RNA (mRNA) is decoded into a sequence of amino acids through the activity of ribosomes. Ribosomal RNA (rRNA), a main component of ribosomes, can therefore also be considered a ribozyme.
Many different types of ribozymes exist and have been studied in their naturally occurring state. Two of these, the hairpin ribozyme and the hammerhead ribozyme, are found in satellite RNAs in plant viruses. Hammerhead ribozymes have been modified in the laboratory for use as RNA cleaving agents.
It is believed that modified hammerhead ribozymes may have clinical applications in gene therapy. The ability to cleave RNA may help combat retroviruses, since retroviruses rely on an RNA genome for duplication in the host cell. Researchers have also developed completely synthetic ribozymes for similar medical applications. Modified hammerhead ribozymes and synthetic ribozymes are being researched specifically as therapeutic options for treating human immunodeficiency virus (HIV).