Attachment of amino acid to tRNA

After attachment of amino acids to tRNA, the amino acids are assembled beginning with the amino terminus and proceeding in the direction of the carboxy terminus. The ribosome is the machinery that translates the mRNA into protein. The ribosome is a very complex protein that contains ribosomal RNA as a functional and structural component. The ribosome assembles around the mRNA, and the cap and other signals allow alignment of the mRNA into the correct position. The initial assembly of the mRNA into the ribosome requires association of the small ribosomal subunit with an initiator tRNA (Met or fMet). Small is a misstatement, because the small ribosomal subunit is a large, complex assembly of numerous smaller proteins—it s just smaller than the... 

Although the accuracy of translation (approximately one error per 104 amino acids incorporated) is lower than those of DNA replication and transcription, it is remarkably higher than one would expect of such a complex process. The principal reasons for the accuracy with which amino acids are incorporated into polypeptides include codon-anticodon base pairing and the mechanism by which amino acids are attached to their cognate tRNAs. The attachment of amino acids to tRNAs, considered the first step in protein synthesis, is catalyzed by a group of enzymes called the aminoacyl-tRNA synthetases. The precision with which these enzymes esterify each specific amino acid to the correct tRNA is now believed to be so important for accurate translation that their functioning has been referred to collectively as the second genetic code. 

The attachment of amino acids to tRNA involves the formation of an ester bond between the alpha-carboxyl group of the amino acid and the 3 -hydroxyl group of the terminal adenosine of tRNA. It requires specific enzymes, the aminoacyl-tRNA synthetases. There are 20 different synthetases, each specific for one of the 20 amino acids, and each enzyme recognizes something unique in the structure of its cognate tRNA. The structural determinants which ensure accuracy of this charging reaction vary for different tRNAs. The anticodon may play apart but sometimes even a single base elsewhere is sufficient to determine the specificity of the tRNA-synthetase interaction. The accuracy of... 

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. 

That driving force is hydrolysis of ATP. The attachment of amino acids to their cognate tRNAs is highly specific. 

Protein synthesis involves more than 100 different proteins and more than 30 kinds of RNA molecules. The process begins by the attachment of amino acids to specific tRNA molecules. Subsequent steps take place on the ribosome amino acids are transported to the ribosome on their tRNA carriers, and they do not leave the ribosome until they have become an integral part of a polypeptide chain. 

Now that we have specifically attached each amino acid to its cognate tRNA, we need to make the interface between each aminoacyl-tRNA and the correct triplet codon on mRNA. The aminoacyl moiety has nothing to do with this it is simply a matter of codon-anticodon recognition, which is shown schematically in figure 13.3. 

Formation of the first peptide linkage. The formylmethionine group is transferred from its tRNA at the P site to the amino group of the second aminoacyl-tRNA at the A site of the ribosome. This involves nucleophilic attack by the amino group of the second amino acid on the carboxyl carbon of the methionine. The resulting bond formation attaches both amino acids to the tRNA at the A site. 

Some aminoacyl-tRNA synthetases use the anticodon of the tRNA as a recognition site as they attach the amino acid to the hydroxyl group at the 3 -end of the tRNA (Fig. 15.6). However, other synthetases do not use the anticodon but recognize only bases located at other positions in the tRNA. Nevertheless, insertion of the amino acid into a growing polypeptide chain depends solely on the bases of the anticodon, through complementary base-pairing with the mRNA codon. 

Transfer RNA (tRNA) serves as a carrier of amino acid residues for protein synthesis. Transfer RNA molecules also fold into a characteristic secondary structure (marginal figure). The amino acid is attached as an aminoacyl ester to the 3 -terminus of the tRNA. Aminoacyl-tRNAs are the substrates for protein biosynthesis. The tRNAs are the smallest RNAs (size range—23 to 30 kD) and contain 73 to 94 residues, a substantial number of which are methylated or otherwise unusually modified. Transfer RNA derives its name from its role as the carrier of amino acids during the process of protein synthesis (see Chapters 32 and 33). Each of the 20 amino acids of proteins has at least one unique tRNA species dedicated to chauffeuring its delivery to ribosomes for insertion into growing polypeptide chains, and some amino acids are served by several tRNAs. For example, five different tRNAs act in the transfer of leucine into... 

In the cytoplasm, the mRNA attaches to a ribosome and acts as a template for the construction of a protein with the proper amino acid sequence (a process known as translation ). Single amino acids are brought to the ribosome by transfer RNA molecules (tRNA) and added to the growing amino acid chain in the order instructed by the mRNA. Each time a nucleotide is added to the growing RNA strand, one molecule of ATP is broken down to ADP. Each time a tRNA binds an amino acid and each time the amino acid is added to the protein, additional ATP is broken down to ADP. Because proteins can contain many hundreds of amino acids, the cell must expend the energy in 1,000 or more ATP molecules to build each protein molecule. 

The now deacylated tRNA is attached by its anticodon to the P site at one end and by the open GGA tail to an exit (E) site on the large ribosomal subunit (Figure 38-8). At this point, elongation factor 2 (EE2) binds to and displaces the peptidyl tRNA from the A site to the P site. In turn, the deacylated tRNA is on the E site, from which it leaves the ribosome. The EF2-GTP complex is hydrolyzed to EF2-GDP, effectively moving the mRNA forward by one codon and leaving the A site open for occupancy by another ternary complex of amino acid tRNA-EFlA-GTP and another cycle of elongation. 

Step One is the specific attachment of an amino acid to its cognate tRNA... 

A tRNA molecule is specific for a particular amino acid, though there may be several different forms for each amino acid. Although relatively small, the polynucleotide chain may show several loops or arms because of base pairing along the chain. One arm always ends in the sequence cytosine-cytosine-adenosine. The 3 -hydroxyl of this terminal adenosine unit is used to attach the amino acid via an ester linkage. However, it is now a section of the nucleotide sequence that identifies the tRNA-amino acid combination, and not the amino acid itself. A loop in the RNA molecule contains a specific sequence of bases, termed an anticodon, and this sequence allows the tRNA to bind to a complementary sequence of bases, a codon, on mRNA. The synthesis of a protein from the message carried in mRNA is called translation, and a simplified representation of the process as characterized in the bacterium Escherichia coli is shown below. 

The answer is b. (Hardman, p 1131.) Chloramphenicol inhibits protein synthesis in bacteria and, to a lesser extent, in eukaryotic cells. The drug binds reversibly to the SOS ribosomal subunit and prevents attachment of aminoacyl-transfer RNA (tRNA) to its binding site. The amino acid substrate is unavailable for peptidyl transferase and peptide bond formation. 

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. 

The polypeptide remains attached to the tRNA of the most recent amino acid to be inserted. This association maintains the functional connection between the information in the mRNA and its decoded polypeptide output. At the same time, the ester linkage between this tRNA and the carboxyl terminus of the growing polypeptide activates the terminal carboxyl group for nucleophilic attack by the incoming amino acid to form a new peptide bond (Fig. 27-24). As the existing ester linkage between the polypeptide and tRNA is broken during... 

Transfer RNAs (tRNAs), the smallest of the three major species of RNA molecules (4S), have between 74 and 95 nucleotide residues. There is at least one specific type of tRNA molecule for each of the twenty amino acids commonly found in proteins. Together, tRNAs make up about fifteen percent of the total RNA in the cell. The tRNA molecules contain unusual bases (for example, pseudouracil, see Figure 22.2, p. 290) and have extensive intrachain base-pairing (Figure 30.3). Each tRNA serves as an "adaptor molecule that carries its specific amino acid—covalently attached to its 3-end—to the site of protein synthesis. There it recognizes the genetic code word on an mRNA, which specifies the addition of its amino acid to the growing peptide chain (see p. 429). 

Attachment of a specific amino acid to its corresponding tRNA by aminoacyl-tRNA synthetase (E). 

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