RNA ribozymes

In addition to the catalytic action served by the snRNAs in the formation of mRNA, several other enzymatic functions have been attributed to RNA. Ribozymes are RNA molecules with catalytic activity. These generally involve transesterification reactions, and most are concerned with RNA metabofism (spfic-ing and endoribonuclease). Recently, a ribosomal RNA component was noted to hydrolyze an aminoacyl ester and thus to play a central role in peptide bond function (peptidyl transferases see Chapter 38). These observations, made in organelles from plants, yeast, viruses, and higher eukaryotic cells, show that RNA can act as an enzyme. This has revolutionized thinking about enzyme action and the origin of life itself. 

Three pieces of evidence are cited in support of an RNA World. Firstly, some 17 RNA ribozyme catalysts have been discovered that produce a diverse array of organic molecules, including peptide bond formation. Second, the ability to form the peptide bond and build proteins may lead to a complex evolution favoured by the proximity of proto-proteins, producing enhanced reaction efficiency. Finally, RNA is the intermediate in the biosynthesis of DNA, suggesting that it must have preceded DNA in the evolutionary process. 

The intron group I ribozymes feature common secondary structure and reaction pathways. Active sites capable of catalyzing consecutive phosphodi-ester reactions produce properly spliced and circular RNAs. Ribozymes fold into a globular conformation and have solvent-inaccessible cores as quantified by Fe(II)-EDTA-induced free-radical cleavage experiments. The Tetrahy-mem group I intron ribozyme catalyzes phosphoryl transfer between guanosine and a substrate RNA strand—the exon. This ribozyme also has been proposed to use metal ions to assist in proper folding, to activate the nucleophile, and to stabilize the transition state. ... 

In the following year, Scott and co-workers solved the X-ray crystallographic structure of an all-RNA hammerhead ribozyme with a 2 -OCH3 group incorporated at the active site cytosine (Cn) to prevent cleavage (PDB IMME). This structure differed from that of IHMH in several important ways (1) it was an all-RNA ribozyme rather than an RNA-DNA hybrid (2) the connectivity of the ribozyme backbone strands was different (for instance... 

The whole process originated from RNA RNA- Ribozymes Proteic enzymes DNA... 

There are other ways in which nucleic-acid-related compounds could be exploited as therapeutics. A new, emerging area concerns the application of RNA as a dmg. The discovery of catalytic RNA (ribozymes) by Cech and Altman was a fundamental advance in nucleic acid chemistry. According to traditional double helix dogma, RNA was a passive information-transmitting molecule. The identification of ribozymes enabled the conceptual advance that RNA can also act as a catalyst for the following biochemical processes ... 

The chemical nature of enzyme was controversial for a long time, until Buchner succeeded in isolating an enzyme system (zymase) from yeast in a cell-free extract in 1897.2) Urease was then crystallized by Sumner in 1926,3) followed by crystallization of several proteolytic enzymes by Northlop and his colleagues. At present the chemical nature of enzyme is defined as a protein with catalytic activity based on the specific activaiton of its substrate. However, this definition has been somewhat open to debate since a catalytic RNA, ribozyme, was discoved in 1982. 

In this section is an overview of some historical aspects of enzyme studies with special emphasis on new methods of purification, structure determination, and research on the reaction mechanism of an enzyme. Enzyme application for medical and industrial use and development of novel enzymes such as catalytic RNA (ribozyme) and catalytic antibody (abzymes) are also briefly described. 

In cells expressed RNA ribozyme-aptamer repeated in tandem ensure high intracellular aptamer dosage. 

The question whether toxins can be used to translocate nucleic acids into cells is very interesting, but this has not been tested seriously up to now. Toxins are interesting candidates for delivery of anti-sense RNA, ribozymes and genes for transformation and gene therapy. So far there is not convincing evidence that toxins have been effective for such purposes. From what is now known about the translocation of diphtheria toxin, it is likely that the nucleic acid would have to be linked end-to-end to the N-terminus of the A-fragment of this... 

Of all the functions of proteins, catalysis is probably the most important. In the absence of catalysis, most reactions in biological systems would take place far too slowly to provide products at an adequate pace for a metabolizing organism. The catalysts that serve this function in organisms are called enzymes. With the exception of some RNAs (ribozymes) that have catalytic activity (described in Sections 11.7 and 12.4), aU other enzymes are globular proteins (section 4.3). Enzymes are the most efficient catalysts known they can increase the rate of a reaction by a factor of up to 10 ° over uncatalyzed reactions. Non-enzymatic catalysts, in contrast, typically enhance the rate of reaction by factors of 102 to 104... 

There was a time when proteins were considered the only biological macromolecules capable of catalysis. The discovery of the catalytic activity of RNA has thus had a profound impact on the way biochemists think. A few enzymes with RNA components had been discovered, such as telomerase (Chapter 10) and RNase P, an enzyme that cleaves extra nucleotides off the 5 ends of tRNA precursors. It was later shown that the RNA portion of RNase P has the catalytic activity. The field of catalytic RNA (ribozymes) was launched in earnest by the discovery of RNA that catalyzes its own selfsplicing. It is easy to see a connection between this process and the splicing... 

Most enzymes are proteins, but some catalytic RNAs (ribozymes) are known. 

Not really. A system rich in amino acids would have at least some peptides. And there are many processes that are easily catalyzed by simple proteins but have never been demonstrated using RNA ribozymes. An example would be the sort of electron transfer mediated by iron sulfur clusters. Cellular synthesis of purines and pyrimidines must be very ancient, but it would seem likely that these are merely representatives of many other processes involving amino acids and peptides. 

Thousands of different reactions occur in living systems, and most of these are intrinsically slow. Life as we know it has only been made possible by catalysts known as enzymes. Enzymes are remarkable for the diversity of their selectivity, specificity, and catalytic power. Nearly all enzymes are proteins, but the importance of catalytically active RNA (ribozymes) is increasingly being recognized. 

Catalytic RNA ( Ribozymes ) and Conjectures about a Prebiotic RNA World ... 

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