Amino acyl RNA synthetase

This enzyme [EC 6.1.1.12], also known as aspar-tate tRNA ligase, catalyzes the reaction of aspartate with ATP and tRNA P to generate aspartyl-tRNA P, AMP, and diphosphate (or, pyrophosphate). See also Amino-acyl-RNA Synthetases... 

In Vitro Protein Synthesis 107 Amino Acid Activation 107 Amino Acyl RNA Synthetase 108 tRNA General Properties Sequence Configuration... 

In each cell synthesizing proteins, there are at least 20 different amino acyl RNA synthetases, one for each amino acid. The overall reaction catalyzed by the synthetase leads to the formation of an ester between amino acid and tRNA. Thus, the reaction takes place in two major steps a first, already discussed, in which the amino acid is activated and complexed with the enzyme, and a second, in which the activated amino acid is transferred to tRNA. ATP and Mg are required for the first step. In the first step, the enzyme catalyzes the formation of a mixed anhydride between the carboxyl of the amino acid and the 5 -P04 of AMP. Pyrophosphate is released in the course of the reaction. The anhydride never appears as a free intermediate but remains tightly attached to the enzyme. 

In any event, in protein metabolism the amino acyl RNA synthetases are located at the gates of specificity. If they attach the wrong amino acid to the wrong tRNA, the mistake will be carried into the protein structure. 

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. 

The sequence-dictated three-dimensional conformation of an RNA molecule is essential in the functionality of several RNA types The ribosomic RNAs fold into structures that scaffold the ribosomes, and the tRNAs fold into defined structures that allow their recognition by both amino-acyl tRNA synthetase and ribosomes. In both cases, some nucleotide sequences that remain single stranded are involved in the enzymatic processes performed by the molecules. Thus, the sequence of an RNA dictates its structure and allows it to perform its function. Both are strictly connected. One single mutation in a rRNA or a tRNA may completely change the favored three-dimensional structure of the molecule and totally impair the RNA s function (s). 

Protein synthesis is essentially a translational process of messenger RNA which has been transcribed from the DNA template. The first reaction is the activation of amino acids by amino-acyl transfer RNA synthetases making use of ATP as an energy source, and the enzymes also attach the amino acid to transfer RNA to form amino-acyl transfer RNA. We have already seen that protein synthesis in plasmodia can be inhibited much further back along the chain, either at the folate co-factor level or during nucleic acid synthesis. It is obvious that interference with RNA transcription from DNA (or with DNA replication), or interference with the availability of essential amino acids, will also affect plasmodial synthesis of protein. Morphological studies of the effects of antimalarials on plasmodia support the occurrence of both modes of action for chloroquine, quinacrine, and quinine. 

There seems to be one specific acyl amino acid synthetase for each amino acid thus, there are at least 20 such enzymes, and although the molecular weight of most of the enzymes is around 100,000 (except the phenyl RNA synthetase of E. coli and yeast, which has a molecular weight of 180,000), the amino acid composition of enzymes catalyzing identical reactions may vary. Comparisons of the electrophoretic and immunological properties and the amino acid composition of two tryosyl RNA synthetases—one purified from E. coli (mol wt 95,000) and another from B. sub-tilis (mol wt 88,000)—reveal significant differences between the two molecules. One of the most remarkable is the relative content of cystine, 15 residues in E. coli enzymes and 2 residues in B. subtilis enzymes. 

The dependency of bacterial RNA synthesis on protein synthesis which has been called "stringent control" has been demonstrated to take place under conditions such as aminoacid-starvation, nitrogen starvation, phosphate and Mg" starvation, energy deprivation and in strains mutated in amino-acyl-t-RNA synthetases. In all these conditions the synthesis of r-RNA and t-RNA is inhibited as far as the m-RNAs are concerned, some m-RNAs are inhibited, whereas the synthesis of others is enhanced" ". ... 

Building on earlier work of Osawa and co-workers [55], Oliver and Kowal [52] tested the feasibility of introducing a noncoded amino acid at an unassigned codon in M. luteus. DNA templates were prepared which coded for 19-mer polypeptides containing either the unassigned codon AGA(Arg) or the termination codon TAG at position 13 under the control of a T7 RNA polymerase promoter. The corresponding tRNAs, produced as described in Sect. 2, were based on tRNA and acylated with phenylalanine. The tRNA was modified to prevent recognition by the alanine aminoacyl-tRNA synthetase and to increase translational efficiency. 

In addition, several amino acid-adding systems have become known in the amino-acylaciun of viral RNAs (35), proteins (36), muropeptides (37). and polymers, such as tcichoic acids (38). Whereas the first three systems employ specific aminoacyl-tRNAs, the last system structurally resembles acyi-coenzyme A synthetases, and it transfers an activated D-Ala to an acyl earner protein transporter (39). Peptide bond formation occurs in all of these systems by separate transferases. 

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