Enzymes self-splicing

Several features of these RNA enzymes, or ribozymes, lead to the realization that their biological efficiency does not challenge that achieved by proteins. First, RNA enzymes often do not fulfill the criterion of catalysis in vivo because they act only once in intramolecular events such as self-splicing. Second, the catalytic rates achieved by RNA enzymes in vivo and in vitro are... 

The discovery of self-splicing introns showed that RNA could catalyse chemical reactions. Yet, unlike proteins, RNA has no functional groups with pKa values and chemical properties similar to those considered to be important in protein-based enzymes. Steitz and Steitz (1993) postulated that two metal ions were essential for catalysis by ribozymes using a mechanism similar to DNA cleavage, in which a free 3 OH is produced. They proposed,... 

The study of posttranscriptional processing of RNA molecules led to one of the most exciting discoveries in modern biochemistry—the existence of RNA enzymes. The best-characterized ribozymes are the self-splicing group I introns, RNase P, and the hammerhead ribozyme (discussed below). Most of the activities of these ribozymes are based on two fundamental reactions transesterification (Fig. 26-13) and phosphodiester bond hydrolysis (cleavage). The substrate for ribozymes is often an RNA molecule, and it may even be part of the ribozyme itself. When its substrate is RNA, an RNA cat-... 

The enzymatic activity of the L-19 IVS ribozyme results from a cycle of transesterification reactions mechanistically similar to self-splicing. Each ribozyme molecule can process about 100 substrate molecules per hour and is not altered in the reaction therefore the intron acts as a catalyst. It follows Michaelis-Menten kinetics, is specific for RNA oligonucleotide substrates, and can be competitively inhibited. The kcat/Km (specificity constant) is 10s m- 1 s lower than that of many enzymes, but the ribozyme accelerates hydrolysis by a factor of 1010 relative to the uncatalyzed reaction. It makes use of substrate orientation, covalent catalysis, and metalion catalysis—strategies used by protein enzymes. 

Were these youT answers They are enzymes that catalyze the cleavage of the DNA double helix.They occur naturally in bacteria and viruses and are used as a means of self-defense. Scientists use restriction enzymes to splice together DNA fragments from different organisms. 

Some RNAs Function Like Enzymes Some RNAs Are Self-Splicing Some Ribonucleases Are RNAs Ribosomal RNA Catalyzes Peptide Bond Formation Catalytic RNA May Have Evolutionary Significance Inhibitors of RNA Metabolism... 

In at least one eukaryote, Tetmhymem, the pre-rRNA molecule contains an intron. Removal of the intron during processing of the pre-rRNA does not require the assistance of any protein Instead, in the presence of guanosine, GMP, GDP or GTP, the intron excises itself, a phenomenon known as selfsplicing. This was the first demonstration of ribozymes, that is, catalytic RNA molecules that catalyze specific reactions. The list of ribozymes is growing. For example, self-splicing introns have been discovered in some eukaryotic mRNAs and even peptidyl transferase, a key enzyme activity in protein synthesis, is now known to be a ribozyme (see Topic H2). 

It was first discovered in 1981 by Thomas Cech and coworkers that a primary transcript for the 26 S rRNA from the protozoan Tetrarhymena could cut, splice, and assemble itself into the mature 26 S rRNA. Subsequently it was shown that a specific rRNA could catalyze the assembly of RNAs other than itself and that this could occur in the absence of any protein. Such enzyme-like RNAs were termed ribozymes. Self-splicing has been found to occur for RNAs from a variety of species. Two main types of self-splicing mechanisms are known ... 

Design of RNA molecules with novel catalytic functions called ribozymes (ribonucleotide enzymes) started out from the reprogramming of naturally occurring molecules to accept unnatural substrates [32, 33] A specific RNA cleaving ribozyme, a class I (self-splicing) intron, was modified through variation and selection until it operated efficiently on DNA. The evolutionary path of such a transformation of catalytic activity has been recorded in molecular detail [34]. The basic problem in the evolutionary design of new catalysts is the availability of appropriate analytical tools for the detec-... 

Figure 28.34. Self-Splicing. A ribosomal RNA precursor from Tetrahymena splices itself in the presence of a guanosine CO- factor (G, shown in green). A 414-nucleotide intron (red) is released in the first splicing reaction. This intron then splices itself twice again to produce a linear RNA that has lost a total of 19 nucleotides. This L19 RNA is catalytically active. [After T. Cech. RNA as an enzyme. Copyright 1986 by Scientific American, Inc. All rights reserved.]...

Figure 28.36. Self-Splicing Mechanism. The catalytic mechanism of the selfsplicing intron from Tetrahymena includes a series of transesterification reactions. [After T. Cech. RNA as an enzyme. Copyright 1986 hy Scientific American, Inc. All rights reserved.]...

Class I introns were originally discovered in ciliated protozoa and subsequently were found in fungi, bacteriophages, and other organisms. The RNA itself in a class I in-tron has catalytic activity and class I introns remove themselves from primary RNA transcripts by a self-splicing reaction. Class I introns are not true enzymes in that they function only once. The nucleotides in the intron that is spliced out are recycled in the cell. 

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