Protein aminoacyl-tRNA synthetases

The inventory of phylogenetically relevant components offered by ribosomes comprises no less than three RNA molecules and about fifty proteins. Aminoacyl-tRNA synthetases and protein factors assisting the initiation, elongation and termination reactions of polypeptide synthesis constitute further probes of potential usefulness to delineate the unfolding of the early lineages. 

The regions of the tRNA molecule teferred to in Chapter 35 (and illustrated in Figure 35-11) now become important. The thymidine-pseudouridine-cyti-dine (T PC) arm is involved in binding of the amino-acyl-tRNA to the ribosomal surface at the site of protein synthesis. The D arm is one of the sites important for the proper recognition of a given tRNA species by its proper aminoacyl-tRNA synthetase. The acceptor arm, located at the 3 -hydroxyl adenosyl terminal, is the site of attachment of the specific amino acid. 

YAMAGUCHI M and suGiMOTO E (2000) Stimulatory effect of genistein and daidzein on protein synthesis in osteoblastic MC3T3-E1 cells Activation of aminoacyl-tRNA synthetase. Mol Cell Biochem 214, 97-102. 

Although aminoacyl-tRNA synthetases are necessary for protein synthesis in all tissues, their importance in chemical carcinogenesis is difficult to assess. Mutation induction by this pathway has been studied extensively (123), yet metabolic activation in a carcinogen-target tissue has not been demonstrated. The only exception is hepatic prolyl-tRNA synthetase activation of N-hydroxy-Trp-P-2 however, hepatic O-acetylation of this substrate also occurs to an appreciable extent (12). Further investigations involving the use of specific enzyme inhibitors would be helpful in addressing this problem. 

The tRNA synthetases may provide a check to make sure that the correct amino acid has been attached to the correct tRNA. If an incorrect amino acid is attached to the tRNA, it will be incorporated into the protein at the position specified by the identity of the tRNA. At least some of the aminoacyl tRNA synthetases have a proofreading function that hydrolyzes any incorrect aminoacyl tRNAs (for example, a Val residue attached to an lie tRNA). 

Aminoacyl-tRNA synthetases Degradation of proteins, peptides, and glycopeptides Nucleoproteins Protein modification... 

Aside from these relatively direct applications of site-directed mutagenesis, combination of recombinant DNA techniques with other experimental strategies will no doubt prove to be of increasing importance. If the gene of interest can be expressed with sufficient efficiency in auxotrophs, then proteins in which selected amino acids are isotopically enriched [12] may be produced to increase the sensitivity and selectivity of magnetic resonance techniques. Alternatively, amino acid analogues that are recognized as substrates by aminoacyl tRNA synthetases may be incorporated randomly in place of the true substrate amino... 

CoRTi, O., et al.. The p38 subunit of the aminoacyl-tRNA synthetase complex is a Parkin substrate linking protein biosynthesis and neurodegeneration. Hum Mol Genet, 2003, 12(12), 1427-37. 

Aminoacyl-tRNA synthetases charge the appropriate tRNA with the correct amino acid, which is important in maintaining the fidelity of protein translation. To genetically encode an unnatural amino acid, the substrate specificity of the orthogonal synthetase needs to be altered to charge the orthogonal tRNA with only the desired unnatural amino acid and none of the common 20 amino acids. A general scheme was developed for... 

There is a family of enzymes that catalyze the attachment of amino acids to then-cognate tRNAs, aminoacyl-tRNA synthetases. There is one or more of these enzymes for each of the 20 amino acids that occur commonly in proteins. Each of these enzymes recognizes (a) a specific amino acid and (b) its cognate tRNA. Imagine a soup of 20 amino acids and 20 tRNAs, one for each amino acid. For example, the aminoacyl-tRNA synthetase for, saline would specifically pick valine out of the soup and catalyze its attachment to the tRNA for valine, tRNA . Simply, we can write the product of the reaction as val-tRNA . This is a lovely example of the role of molecular recognition in a critical life process. 

Figure 20.25 Regulation of the activities of the aminoacyl-tRNA synthetases by the concentrations of free tRNAs (i.e. uncharged tRNA). Changes in the concentrations of free tRNAs provide the mechanism for communication between control via the initiation factor (Figure 20.20) and ribosomal protein kinase (steps 6 and 7) and the flux-generating step.

AMINO ACID RACEMASE Amino adds, peptides proteins, BIOCHEMICAL NOMENCLATURE AMINO ACID TURNOVER KINETICS AMINOACYLASE AMINOACYL-tRNA HYDROLASE Aminoacyl-tRNA synthetase,... 

The translation of the mRNA into proteins is the final step in the biological flow of information (see Fig. 6.1). Similar to other macromolecular polymerizations, protein synthesis can be divided into initiation, chain elongation, and termination. Critical players in this process are the aminoacyl transfer RNAs (tRNAs). These molecules form the interface between the mRNA and the growing polypeptide. Activation of tRNA involves the addition of an amino acid to its acceptor stem, a reaction catalyzed by an aminoacyl-tRNA synthetase. Each aminoacyl-tRNA synthetase is highly specific for one amino acid and its corresponding tRNA molecule. The anticodon loop of each aminoacyl-tRNA interacts... 

During the first stage of protein synthesis, taking place in the cytosol, aminoacyl-tRNA synthetases esterify the 20 amino acids to their corresponding tRNAs. Each enzyme is specific for one amino acid and one or more corresponding tRNAs. Most organisms have one aminoacyl-tRNA synthetase for each amino acid. For amino acids with two or more corresponding tRNAs, the same enzyme usually aminoacylates all of them. 

Proofreading by Aminoacyl-tRNA Synthetases The amino-acylation of tRNA accomplishes two ends (1) activation of an amino acid for peptide bond formation and (2) attachment of the amino acid to an adaptor tRNA that ensures appropriate placement of the amino acid in a growing polypeptide. The identity of the amino acid attached to a tRNA is not checked on the ribosome, so attachment of the correct amino acid to the tRNA is essential to the fidelity of protein synthesis. 

Amino acids are activated by specific aminoacyl-tRNA synthetases in the cytosol. These enzymes catalyze the formation of aminoacyl-tRNAs, with simultaneous cleavage of ATP to AMP and PPj. The fidelity of protein synthesis depends on the accuracy of this reaction, and some of these enzymes carry out proofreading steps at separate active sites. In bacteria, the initiating aminoacyl-tRNA in all proteins is A-formylmethionyl-tRNAfMet. 

Requirements include all the amino acids that eventually appear in the finished protein, at least one specific type of tRNA for each amino acid, one aminoacyl-tRNA synthetase for each amino acid, the mRNA coding for the protein to be synthesized, fully competent ribosomes, protein factors needed for initiation, elongation, and termination of protein synthesis, and ATP and GTP as energy sources. 

Among the many proteins that bind to RNA molecules437 39 are the aminoacyl-tRNA synthetases, a variety of other well known enzymes,440 the ribosomal proteins discussed in Chapter 29, and various proteins with dual functions of catalysis and regulation of... 

The aminoacyl-tRNA synthetases join amino acids to their appropriate tRNA molecules for protein synthesis. They have the very important task of selecting both a specific amino acid and a specific tRNA and joining them. The enzymes differ in size and other properties. However, they all appear to function by a common basic chemistry that makes use of cleavage of ATP at Pa (Eq. 12-48) via an intermediate aminoacyl adenylate and that is outlined also in Eq. 17-36. These enzymes are discussed in Chapter 29. ... 

While peptide antibiotics are synthesized according to enzyme-controlled polymerization patterns, both proteins and nucleic acids are made by template mechanisms. Tire sequence of their monomer emits is determined by genetically encoded information. A key reaction in the formation of proteins is the transfer of activated aminoacyl groups to molecules of tRNA (Eq. 17-36). Tire tRNAs act as carriers or adapters as explained in detail in Chapter 29. Each aminoacyl-tRNA synthetase must recognize the correct tRNA and attach the correct amino acid to it. The tRNA then carries the activated amino acid to a ribosome, where it is placed, at the correct moment, in the active site. Peptidyltransferase, using a transacylation reaction, in an insertion mechanism transfers the C terminus of the growing peptide chain onto the amino group of... 

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