The main function for many proteins in a cell is to catalyse chemical reactions that are vital for the cell's survival. These proteins are well-known as enzymes. Till the recent times it was considered that proteins were the only biological molecules able to carry out catalysis. Near the beginning of 1980's research groups guided by Sidney Altman and Thomas Cech independently discovered that RNAs can also function as catalysts for chemical reactions. These sets of catalytic RNA's are known as ribozymes and the discovery gained them the 1989 Nobel Prize in chemistry. 'Ribozyme', an RNA molecule that function as enzymes, regularly found to catalyse the cleavage of its own or other RNA's .That is RNA's that act as enzymes are known as Ribozymes. The ribozyme extracted by the Cech group known as Tetrahymenia ribozyme. In Tetrahymenia which is a ciliated protozoan, a ribosomal RNA precursor splices itself in the existence of a cofactor 'G'. A four hundred fourteen nucleotide long intron is liberated in the first splicing reaction, this intron then splices itself two more times to create a linear RNA that has lost nineteen nucleotides and is catalytically active. These significant experiments established that RNA molecules are able splice themselves in the lack of proteins. In fact the RNA on its own is catalytic and in certain conditions, thus it is called a ribozyme. More than 1500 alike introns have since been discovered in species as broadly distributed as bacteria and eukaryotes .All together they are known as Group I introns.
Naturally found Ribozymes include:
- Peptidyl transferase 23S rRNA - The process of formation of peptide bond is a thermodynamically spontaneous reaction catalysed by a region on the 23S rRNA of the larger sub-unit (50S) called the peptidyl transferase centre. The peptidyl transferase activity is not interrupted by any proteins of the ribosome but only by the rRNA that is Ribozyme.
- Group I and Group II introns - Both the Group I and Group II introns are self-splicing Ribozymes. The difference is that Group I self-splicing is mediated by a guanosine cofactor as in the case of Tetrahymena but the affecting moity in the Group II splicing is the 2 prime OH group of a specific adenylate of the intron.
- RNase P - Ribonuclease P is the catalytic ribonucleo-protein that acts on the 5 prime leader sequence (the sequence at the 5 prime end of an mRNA) of the precursor tRNA. Latest discoveries have revealed that human nuclear Ribonuclease P is necessary for the usual and effective transcription of various tiny non coding RNA genes which are transcribed by RNA Pol III.
- Hairpin Ribozyme - It is found in satellite RNAs of plant viruses. It catalyses the RNA processing reactions necessary for replication of the satellite RNA molecules in which it is rooted.
- Hammerhead Ribozyme - It was the third ribosome to be revealed after the discovery of Group I intron and RNase P. George Bruening found this self-cleaving motif in the perspective of the satellite RNA of tobacco ringspot virus, which replicates by the process of rolling-circle method. They are always seen in being with satellite RNAs of viruses.
- The mammalian CPEB3 ribozyme - A self-splicing non-coding RNA situated in the 2nd intron of CPEB3 gene that is a member of gene family that regulate polyadenylation (The production of a poly a tail at the end of an RNA molecule) of mRNA. These ribozyme are largely preserved and found in mammals only.
- glmS'' ribozyme - A new natural ribozyme has been revealed in the 5' noncoding region of the glmS messengerRNA. glmSribozyme's activity is mediated by a cellular metabolite. The glmS ribozyme thus denotes the first instance of an novel regulatory mechanism in bacteria.
- Hepatitis delta virus (HDV) ribozyme - It is a non-coding RNA that is essential for replication of viruses and is noted to be the only catalytic RNA recognized having the capability of a human pathogen.
- The Varkud satellite (VS) ribozyme - An RNA enzyme that carries out the splicing of a phosphodiester bond (any molecule in which two parts are joined through a phosphate group).
- The GIR1 branching ribozyme - A one hundred seventy nine nucleotide long ribozyme which has a structural similarities as group I ribozymes.
- Leadzyme - Although originally produced ''in vitro'', natural examples have been discovered. it is able to splice RNA in the presence of lead.
RNA world hypothesis - The breakthrough of ribozymes upholded a hypothesis well-known as the RNA world hypothesis. That is the prior forms of life may have depended solely on RNA to preserve genetic information and to catalyse the chemical reactions. This idea was independently put forward by Carl Woese, F Crick and L orgel during the nineteen sixties , that is years before the finding of ribozymes. Corresponding to the RNA world hypothesis life soon after evolved to use DNA and proteins due to RNA's virtual instability and less significant catalytic properties and slowly ribozymes became increasingly superseded.
Since the finding of ribozymes that subsist in living organisms, there has been affords in the study of new synthetic ribozymes prepared in the laboratory. For instance, artificially-prepared self-splicing RNAs that has fine enzymatic activity have been synthesized. Applications of ribozymes is that they may play prominent roles as therapeutic agents, as enzymes which tailor defined RNA sequences, as biosensors, and for utilization in functional genomics and discover of genes.
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