Amino acid-specific tRNA

Amino acid-specific aminoacyl tRNA synthetases couple amino acids (+H3N-CH(R)-COO ) to the 3 -OH of amino acid-specific tRNAs (tRNA-CCA-3 -OH) in the following reactions ... 

A. may consist of either one polypeptide chain or of 2 or 4 homologous or heterologous subunits. Eukaryotic cells contain more than different A., because mitochondria and plastids have their own ami-no-acid-spedfic A., which differ in their specificity toward homologous tRNA from those of the cytoplasm. Some A. are able to load several amino-acid-specific tRNAs, e.g. leucyl-tRNA synthetase of . coli, which can load 5 different spedes of tRNA, . 

Fig. 5.1 Simplified model representation of the activation of an amino acid (ASY) at an amino acid-activating enzyme (i.e., an amino acid-specific aminoacyl-tRNA synthetase)...

The information contained in the DNA (i.e., the order of the nucleotides) is first transcribed into RNA. The messenger RNA thus formed interacts with the amino-acid-charged tRNA molecules at specific cell organelles, the ribosomes. The loading of the tRNA with the necessary amino acids is carried out with the help of aminoacyl-tRNA synthetases (see Sect. 5.3.2). Each separate amino acid has its own tRNA species, i.e., there must be at least 20 different tRNA molecules in the cells. The tRNAs contain a nucleotide triplet (the anticodon), which interacts with the codon of the mRNA in a Watson-Crick manner. It is clear from the genetic code that the different amino acids have different numbers of codons thus, serine, leucine and arginine each have 6 codewords, while methionine and tryptophan are defined by only one single nucleotide triplet. 

In order that tRNA molecules can function as adaptors between amino acids and mRNA, two things must be true. First, there must be a mechanism for attaching each amino acid specifically to its cognate tRNA. Doing so will create molecules known as aminoacyl-tRNAs. Secondly, each aminoacyl-tRNA must recognize the correct triplet code for that amino acid on mRNA. 1 am going to worry about the former issue first. 

Synthases Attach Amino Acids to tRNAs Each Synthase Recognizes a Specific Amino Acid and Specific Regions on Its Cognate tRNA Aminoacyl-tRNA Synthases Can Correct Acylation Errors... 

The CME model for stochastic protein production provides specific predictions the k increase with the level of mRNA in a cell, and 9 is a function of the concentrations of the amino acids charged tRNA, and the size of the protein. 

In addition to the bases oudined above, transfer RNA (tRNA) (involved in amino acid-specific codon recognition in protein synthesis) contains unusual chemically modified bases (e.g. 

Amino acids must be activated for translation to occur. Activation ensures that the correct amino acid will be recognized and fiiat there is sufficient energy for peptide bond formation. Activation is the covalent coupling of amino acids to specific adapter molecules. The adapter molecules are called transfer RNA (tRNA). There is atleast one tRNA for each of the 20 naturally occurring amino acids. The tRNA recognize the codons carried by the mRNA and position them to facilitate peptide bond formation. 

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. 

Before the activated amino acid is ready to be incorporated into the growing protein chain at a definite point, it must first find its specific position on the mRNA template. This requires prior bonding of the activated amino acid onto tRNA. 

Special RNA molecules called transfer RNA (tRNA) actually perform the translation because they contain a three nucleotide sequence at one end, called an anticodon, that is complementary to the codon of the mRNA. At the other end of the tRNA, an amino acid (specific to the anticodon) is attached. The amino acid is covalently joined (via a peptide bond) to the growing polypeptide chain on the ribosome (See Figure 5.20). 

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 ... 

En2yme variations. Apart from the obvious problems caused by varying levels of specific or nonspecific ribonucleases present in various cell preparations, other problems may be inherent in the in vitro amino-acylation reaction. For example, it has been reported that the optimal ATP and divalent cation concentrations are not necessarily constant for acylation with all amino acids [28] further, there are, in at least one case, marked differences in the rate of acylation of a set of isoaccept-ing tRNA species [29]. Thus, a careful evaluation of optimal acylation conditions for each amino acid and tRNA fraction is required. 

Fig. 5. Synthesis of amino-acyl tRNA. Amino acids are first activated by reacting with ATP to form an aminoacyl-adenylate. This reaction is catalyzed by the same amino acid-specific enzyme, amino-acyl tRNA synthase, than the second reaction. In the second reaction, the amino acyl is transferred to the ribose of the 3 -end adenosine of the tRNA. The esterification can proceed on the 3 - (as shown) or 2 - hydroxyl of the ribose, depending on the enzyme. The amino-acyl tRNA is then involved in the elongation of the growing polypeptide chain at the level of the ribosome.

Column 3 tRNA/RNA are arranged alphabetically according to amino acid specificity (for tRNAs) and then source organism. Mixed (unfractioiiated) species are listed after those identified by amino acid. Amino acids are abbreviated by the three letter nomenclature, Ala , Arg , etc. Met f and Met m represent the translational initiator and the ordinary methionine species, respectively. TYMV = turnip yellow mosaic virus. TMV=tobacco mosaic virus. 

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