(48)
18. Angive at RNA kan have katalytisk aktivitet: selv-splicing og hydrolytisk
aktivitet (hammerheads, Rnase P, groupe I og groupe II introns)
Devlin, s.86-88, fig. 2.64, fig. 2.65
Stryer, s.804-5
RNA can be catalytic. Enzymes whose RNA subunits carry out catalytic reactions are called ribozymes. There are five types of ribozymes. The first three carry out self-processing reaction, while the last two act as true catalysts on separate substrates:
Group I self-splicing introns
– the self-splicing reaction requires an added guanosine nucleotide, that is
necessary as a cofactor. The chemistry of the self-splicing is two
transesterification reactions. This RNA has been found in bacteria and
bacteriophage.
Group II
self-splicing introns
– the attacking moiety here is the 2’-OH group of an adenylate residue in
the intron. It is found in the mtRNA precursors in prokaryotes.
Hammer head ribozymes
are self-cleaving viral RNAs. These RNAs self-cleave, during generation of
single genomic RNA from large multimeric precursors. The catalytic activity of
this ribozymes is due to the fact that it can adopt complex structures, and
the substrate RNA molecule adapts to the structure by making base pair. One
part of the substrate can not make base pairs and becomes “cracked” and is
cleaved in the process.
Ribonuclease P
– contains both a protein and a RNA component. It acts as a true enzyme in
cells, cleaving tRNA precursors. Ribonuclease P recognizes a constant
structure in the tRNA, fx. its 3’-CCA end.
The ribosome itself in the translation process! The active site of the ribosome is on the larger subunit. There are no protein functional groups close enough to catalyse peptide bond formation, leading to the conclusion that some part of the RNA chain serves as the catalyst for the peptide bond formation.
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