Everything about Ribozyme totally explained
A
ribozyme (from
ribonucleic acid en
zyme, also called
RNA enzyme or
catalytic RNA) is an RNA
molecule that catalyzes a
chemical reaction. Many natural ribozymes catalyze either the
hydrolysis of one of their own
phosphodiester bonds, or the hydrolysis of bonds in other RNAs, but they've also been found to catalyze the
aminotransferase activity of the
ribosome.
Investigators studying the
origin of life have produced ribozymes in the laboratory that are capable of
catalyzing their own synthesis under very specific conditions, such as an
RNA polymerase ribozyme. Mutagenesis and selection has been performed resulting in isolation of improved variants of the "Round-18" polymerase ribozyme from 2001. "B6.61" is able to add up to 20
nucleotides to a primer template in 24 hours, until it decomposes by hydrolysis of its phosphodiester bonds.
Some ribozymes may play an important role as therapeutic agents, as enzymes which tailor defined RNA sequences, as
biosensors, and for applications in
functional genomics and gene discovery.
Discovery
Before the discovery of ribozymes,
enzymes, which are defined as catalytic
proteins, were the only known biological
catalysts. In 1967,
Carl Woese,
Francis Crick, and
Leslie Orgel were the first to suggest that RNA could act as a catalyst. This idea was based upon the discovery that RNA can form complex
secondary structures. The first ribozymes were discovered in the 1980s by
Thomas R. Cech, who was studying RNA
splicing in the
ciliated
protozoan Tetrahymena thermophila and
Sidney Altman, who was working on the bacterial
RNase P complex. These ribozymes were found in the
intron of an RNA transcript, which removed itself from the transcript, as well as in the RNA component of the RNase P complex, which is involved in the maturation of pre-
tRNAs. In 1989,
Thomas R. Cech and
Sidney Altman won the
Nobel Prize in
chemistry for their "discovery of catalytic properties of RNA." The term
ribozyme was first introduced by Kelly Kruger
et al. in 1982 in a paper published in
Cell.
It had been a firmly established belief in biology that catalysis was reserved for proteins. However in 1989 the Nobel Prize was presented to Sidney Altman and Tomas Cech for discovering that RNA can catalyze a reaction. In retrospect, catalytic RNA makes a lot of sense. This is based on the old question regarding the origin of life: Which comes first, enzymes that do the work of the cell or nucleic acids that carry the information required to produce the enzymes? Nucleic acids as catalysts circumvents this problem.
In the 1970s Thomas Cech, at the
University of Colorado at Boulder, was studying the excision of introns in a ribosomal RNA gene in
Tetrahymena thermophila. While trying to purify the enzyme responsible for splicing reaction, he found that intron could be spliced out in the absence of any added cell extract. Much as they tried, Cech and his colleagues couldn't identify any protein associated with the splicing reaction. After much work, Cech proposed that the intron sequence portion of the RNA could break and reform phosphodiester bonds. At about the same time, Sidney Altman, who is a Professor at
Yale University, was studying the way tRNA molecules are processed in the cell when he and his colleagues isolated a enzymes called RNase-P, which is responsible for conversion of a precursor
tRNA into the active tRNA. Much to their surprise, they found that RNase-P contained RNA in addition to protein and that RNA was an essential component of the active enzyme. This was such a foreign idea that they'd difficulty publishing their findings. The following year, Altman demonstrated that RNA can act as a catalyst by showing that the RNase-P RNA submit could catalyze the cleavage of precursor tRNA into active tRNA in the absence of any protein component.
Since Cech's and Altman's discovery, other investigators have discovered other examples of self-cleaving RNA or catalytic RNA molecules. Many ribozymes have either a hairpin – or hammerhead – shaped active center and a unique secondary structure that allows them to cleave other RNA molecules at specific sequences. It is now possible to make ribozymes that will specifically cleave my RNA molecule. These RNA catalysts may have pharmaceutical applications. For example, a ribozyme has been designed to cleave the RNA of HIV. If such a ribozyme was made by a cell, all incoming virus particles would have their RNA genome cleaved by the ribozyme, which would prevent infection.
Activity
Although most ribozymes are quite rare in the cell, their roles are sometimes essential to life. For example, the functional part of the
ribosome, the
molecular machine that
translates RNA into proteins, is fundamentally a ribozyme. Ribozymes often have divalent metal ions such as Mg
2+ as
cofactors.
RNA can also act as a hereditary molecule, which encouraged
Walter Gilbert to propose that in the past, the
cell used RNA as both the genetic material and the structural and catalytic molecule, rather than dividing these functions between
DNA and
protein as they're today. This hypothesis became known as the "
RNA world hypothesis" of the
origin of life.
If ribozymes were the first molecular machines used by early life, then today's remaining ribozymes -- such as the ribosome machinery -- could be considered
living fossils of a life based primarily on nucleic acids.
A recent test-tube study of
prion folding suggests that an RNA may catalyze the pathological
protein conformation in the manner of a
chaperone enzyme.
Known ribozymes
Naturally occurring ribozymes include:
Artificial ribozymes
Since the discovery of ribozymes that exist in living organisms, there has been interest in the study of new synthetic ribozymes made in the laboratory. For example, artificially-produced self-cleaving RNAs that have good enzymatic activity have been produced. Tang and Breaker isolated self-cleaving RNAs by in vitro selection of RNAs originating from random-sequence RNAs. Some of the synthetic ribozymes that were produced had novel structures, while some were similar to the naturally occurring hammerhead ribozyme.
The techniques used to discover artificial ribozymes involve Darwinian evolution. This approach takes advantage of RNA's dual nature as both a catalyst and an informational polymer, making it easy for an investigator to produce vast populations of RNA catalysts using
polymerase enzymes. The ribozymes are mutated by reverse transcribing them with
reverse transcriptase into various
cDNA and amplified with
mutagenic PCR. The selection parameters in these experiments often differ. One approach for selecting a
ligase ribozyme involves using
biotin tags, which are covalently linked to the substrate. If a molecule possesses the desired ligase activity, a
streptavidin matrix can be used to recover the active molecules.
Further Information
Get more info on 'Ribozyme'.
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