Amino acid-tRNA synthetase

The initial application of the anchor principle described by Page related to data on the selectivity of amino acid-tRNA synthetases, from which he estimated intrinsic... 

Application of the anchor principle to data on the selectivity of amino acid-tRNA synthetases gives estimated intrinsic binding energies for the carboxyl and amino groups of 18 and >28kJmol, respectively. However, since the sidechains, rather than the ionic groups, are the primary determinants of amino acid/tRNA synthetase specificity, these energies are likely to be underestimates. 

It was earlier considered that all the amino acid-activating synthetases were derived from a single primeval synthetase , so that all synthetases would have similar structures. Surprisingly, however, this is not the case. When the primary sequences, and in part the secondary and tertiary structures, of all the synthetases had been determined, clear differences in their construction became obvious. The aminoacyl-tRNA synthetases consist either of one single polypeptide chain (a) or of two or four identical polypeptides (ot2 or 04). In addition, there are heterogeneously constructed species with two sets of two identical polypeptide chains (OC2P2). This nomenclature indicates that, for each synthetase, a or P refers to a primary structure. The number of amino acids can vary from 334 to more than 1,000. 

Remarkably, incorporation of fluorinated amino acids into proteins can also be accomplished in vivo. This supposes that the fluorinated amino acid analogs are recognized by the appropriate amino acyl-tRNA synthetase enzyme with efficiency similar to that of the natural amino acid. The proliferase response elicited by a fluorinated analog (a trifluoroisoleucine derivative) of murine interleukin-2 produced in an appropriate Escherichia coli strain was nearly as high as that of the authentic cytokine, indicating folding into an authentic, native structure [84],... 

By observing changes in nucleotides that alter substrate specificity, researchers have identified nucleotide positions that are involved in discrimination by the amino-acyl-tRNA synthetases. These nucleotide positions seem to be concentrated in the amino acid arm and the anticodon arm, including the nucleotides of the anticodon itself, but are also located in other parts of the tRNA molecule. Determination of the crystal structures of aminoacyl-tRNA synthetases complexed with their cognate tRNAs and ATP has added a great deal to our understanding of these interactions (Fig. 27-17). 

Suitable N-substituted sulfamoyladenosines (1) inhibit amino acyl tRNA synthetases (aaTRS s) by mimicking the enzyme bound reaction intermediate 2 (Scheme 1)/ Thus the Structure Activity Relationship (SAR) obtained by varying the R group in structure 1 should probe the key interactions for amino acid selectivity in the aaTRS under investigation. 

A given amino acid can match more than one kind of codon. However, a given codon can normally match only one amino acid. There are thus 61 codons to match only 20 amino acids. There are also 3 terminal codons that specify a stop signal to protein synthesis. Amino acyl-tRNA synthetase is an enzyme that attaches amino acids to their corresponding transfer RNA. 

Amino acyl tRNA synthetases are enzymes that catalyze the covalent joining of an amino acid to its specific tRNA molecule. E. coli has 20 aminoacyl-tRNA synthetases, each of which recognizes one particular amino acid and one or more tRNAs. The two general classes of aminoacyl-tRNA synthetases (I and II) differ in amino acid sequence, the ways in which they bind their cognate tRNAs, and in their quaternary structures. 

Ammo acid -I- tRNA -I- ATP — amino acyl-tRNA -H AMP -I- PPi amino acyl-tRNA synthetase... 

There is a specific amino acyl-tRNA synthetase for each amino acid these enzymes have two specific binding sites, one for the amino acid and the... 

Amino acids are attached to their tRNAs by highly specific enzymes known as aminoacyl-tRNA synthetases. Twenty different synthetases exist, one for each amino acid. Each synthetase recognizes a particular amino acid and all of the tRNAs that carry that amino acid. 

Amino acid activation Amino acids tRNAs Aminoacyl-tRNA synthetases ATP, Mg2+... 

Ans. Pickup of amino acids by tRNAs is controlled by enzymes referred to as aminoacyl-iRNA synthetases. There is at least one aminoacyl-tRNA synthetase for each amino acid. The synthetase recognizes and allows only a particular amino acid to become bonded to a particular tRNA for transport to the ribosome. The recognition is a consequence of the three-dimensional structures of synthetase, tRNA, and amino acid. The amino acid is bonded to the acceptor stem (Fig. 21-6) at the 3 end of tRNA, specifically by formation of an ester linkage between the carboxyl group of the amino acid and the HO group on C-3 (or in some cases the C-2 ) of ribose ... 

In the second stage, the mRNA joins with the ribosomes to form polysomes (Figure 30-1), which are templates for forming polypeptide chains. But the a-amino acids required for the synthesis are not directly linked onto the polypeptides by the polysomes. The amino acids are first esterified via their carboxyl groups to the 3 ends of tRNA to form amino acyl-tRNA with the aid of amino acyl-tRNA synthetases. A specific tRNA is required for each type of a-amino acid. 

Eigneg E.A. and Loftfield R.B. (1974) Kinetic techniques for the investigation of amino acid tRNA ligases (aminoacyl-tRNA synthetases, amino acid activating enzymes). Methods EnzymoL, 29, 601-619. 

Amino acids bind to activating enzymes (amino acyl-tRNA synthetases), which recognize both the amino acid and the appropriate tRNA molecule. The first step is reaction between the amino acid and ATP, to form amino acyl AMP, releasing pyrophosphate. The amino acyl AMP then reacts with the -CCA tail of tRNA to form amino acyl-tRNA, releasing AMP... 

The specificity of these enzymes is critically important to the process of translation. Each enzyme recognizes only one amino acid but will bind and react with all the various tRNA species that carry an anticodon for that amino acid. Mistakes are extremely rare. The easiest possible mistake would be the attachment of valine to the tRNA for isoleucine, or vice versa, because of the close similarity between the structures of these two amino acids (see Figure 4.18). However, it is only about once in every 3,000 times that this mistake occurs. Amino acyl-tRNA synthetases have a second active site that checks that the correct amino acid has been attached to the tRNA and, if this is found not to be the case, hydrolyses the newly formed bond, releasing tRNA and the amino acid. 

---