Name the enzyme responsible for the synthesis of RNA in prokaryotes
Enzyme-tRNA cocrystal structures have also provided clues to the catalytic mechanisms used by AARSs. In the case of the Class I glutaminyl-tRNA synthetase (GlnRS) complexed with its cognate tRNAGln, the structure of the complex identified variations in the tRNA architecture compared with uncomplexed tRNA. GlnRS makes essential contacts with nucleotides in the tRNAGln anticodon, and in the cocrystal structure these nucleotides were indeed splayed out from their normal positions to interact closely with amino acids of the enzyme. Furthermore, the acceptor end of the tRNA showed significant distortion compared to the expected helical structure. The first base pair of the acceptor helix was broken, and the 3′-end of the tRNA bent back in a hairpin conformation toward the rest of the helix (Fig. 3). Such a folded-back orientation of the acceptor end of tRNAGln is necessary for the 3′-terminus to reach the glutaminyl adenylate in the enzyme active site. Furthermore, amino acid residues in the enzyme’s active site are inserted into the acceptor stem nucleotides to break the first base pair and facilitate the distortion of the end of the helix. Binding of the cognate anticodon may trigger this distortion, which is necessary for efficient transfer of the activated amino acid to the 2′-OH of the bound tRNA. Because the anticodon binding site is located a significant distance from the enzyme active site, tRNA recognition in this case has elements of a signal transduction mechanism.
Which enzyme is responsible for rna synthesis : …
Binding interactions alone can not completely discriminate against all noncognate amino acids. Some enzymes misactivate noncognate amino acids at a significant frequency and require an additional accuracy mechanism. The most well-characterized example is isoleucyl-tRNA synthetase (IleRS), responsible for activating isoleucine and attaching it to tRNAIle at the exclusion of valine, which is smaller than isoleucine by a single methylene group. IleRS is more successful in discriminating against larger or bulkier amino acids, but the smaller valine simply fits more loosely in the enzyme active site.
Coli, RNA polymerase comprises six polypeptide subunits, five of which compose the polymerase core enzyme responsible for adding RNA nucleotides to a growing strand.
Chapter 27 - Columbia University
Covalent attachment of an amino acid to its cognate tRNA occurs in a two-step mechanism catalyzed by an AARS (E). In the first step, energy is consumed in the form of ATP to convert the enzyme-bound amino acid to its high-energy aminoacyl adenylate (AA-AMP). In the second step the activated amino acid is transferred to the 3′ end of the cognate tRNA, where an ester bond is formed between the carboxyl group of the amino acid and either the 2′- or 3′-hydroxyl on the terminal adenosine’s ribose sugar to yield AA-tRNA. While for most AARSs the first step is tRNA-independent, glutaminyl-, glutamyl-, and arginyl-tRNA synthetases require cognate tRNA binding for aminoacyl adenylate formation.
The Synthesis and Degradation of Nucleotides
In some cases binding of cognate tRNA influences amino acid selection. Glutaminyl-tRNA synthetase (GlnRS) is one of three AARSs that require the presence of cognate tRNA for aminoacyl adenylate formation. Biochemical and genetic studies demonstrated that conserved residues of GlnRS that contact the 3′-terminus of tRNAGln are required to properly form the glutamine binding site. The presence of noncognate tRNAs decreased the enzyme’s affinity for glutamine because only tRNAGln was properly oriented to allow formation of the glutamine binding pocket.