Protein synthesis aminoacyl-tRNA synthetases

Keywords hepatic resection, protein synthesis, aminoacyl-tRNA-synthetases, silatranes, germatranes... 

CHAPTER NINETEEN Protein Synthesis Aminoacyl-tRNA synthetases must also recognize and bind the correct (cognate) tRNA molecules. For some enzymes (e.g., glutaminyl-tRNA synthetase), anticodon structure is an important feature of the recognition process. However, several enzymes appear to recognize other tRNA structural elements, for example, the acceptor stem, in addition to or instead of the anticodon. 

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 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. 

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. ... 

Not all aminoacyl-tRNA synthetases have editing sites. The cysteinyl- and tyrosyl-tRNA synthetases bind the correct substrates so much more tightly than their competitors that they do not need to edit.13,14 Similarly, since the accuracy of transcription of DNA by RNA polymerase is better than the overall observed error rate in protein synthesis at about 1 part in 104, RNA polymerases do not need to edit.15 The same should be true for codon-anticodon interactions on the ribosome. However, it is possible that accuracy has been sacrificed to achieve higher rates in this case, which is analogous to a change from Michaelis-Menten to Briggs-Haldane kinetics, and so an editing step is required.16... 

Protein synthesis is an energy-intensive process. High-energy phosphate bonds are expended for each peptide bond formed. One high-energy bond is consumed when an amino acid is activated by its aminoacyl-tRNA synthetase. Delivery of aminoacyl-tRNA by EF-Tu consumes one GTP per amino acid, and the translocation reaction consumes another. 

Aminoacyl-tRNA synthetases (aaRSs) compose a family of essential enzymes that attach amino acids covalently to tRNA molecules during protein synthesis. Some aaRSs possess a hydrolytic amino acid editing function to ensure the fidelity of protein synthesis. In addition, aminoacylation can occur by indirect pathways that rely on mischarged tRNA intermediates and enzymes other than aaRSs. Throughout evolution, structural and functional divergence of aaRSs has yielded diverse secondary roles. 

Aminoacyl-tRNA synthetases (aaRSs) are critical components of the translation machinery for protein synthesis in every living cell (1). Each aaRS enzyme in this family links a single amino acid covalently to one or more tRNA isoacceptors to form charged tRNAs. Identity elements within the tRNAs serve as molecular determinants or antideterminants that aid in selection by cognate aaRSs (2). Some aaRSs also have an amino acid editing mechanism to clear their mistakes (3). The canonical aaRSs and aaRS-like proteins have functionally diverged to perform many other important roles in the cell (4, 5). Their versatility and adaptability have provided unique opportunities to develop biotechnology tools and to advance medical research. 

Transfer RNA molecules (tRNAs), messenger RNA, and many proteins participate in protein synthesis along with ribosomes. The link between amino acids and nucleic acids is first made by enzymes called aminoacyl-tRNA synthetases. By specifically linking a particular amino acid to each tRNA, these enzymes implement the genetic code. This chapter focuses primarily on protein synthesis in prokaryotes because it illustrates many general principles and is relatively well understood. Some distinctive features of protein synthesis in eukaryotes also are presented. 

Proofreading by Aminoacyl-tRNA Synthetases Increases the Fidelity of Protein Synthesis... 

How do synthetases choose their tRNA partners This enormously important step is the point at which "translation" takes place—at which the correlation between the amino acid and the nucleic acid worlds is made. In a sense, aminoacyl-tRNA synthetases are the only molecules in biology that "know" the genetic code. Their precise recognition of tRNAs is as important for high-fidelity protein synthesis as is the accurate selection of amino acids. 

Protein synthesis is called translation because information present as a nucleic acid sequence is translated into a different language, the sequence of amino acids in a protein. This complex process is mediated by the coordinated interplay of more than a hundred macromolecules, including mRNA, rRNAs, tRNAs, aminoacyl-tRNA synthetases, and protein factors. Given that proteins typically comprise from 100 to 1000 amino acids, the frequency at vchich an incorrect amino acid is incorporated in the course of protein synthesis must be less than 10 4. Transfer RNAs are the adaptors that make the link betvceen a nucleic acid and an amino acid. These molecules, single chains of about 80 nucleotides, have an L-shaped structure. 

The liver also plays an essential role in dietary amino acid metabolism. The liver absorbs the majority of amino acids, leaving some in the blood for peripheral tissues. The priority use of amino acids is for protein synthesis rather than catabolism. By what means are amino acids directed to protein synthesis in preference to use as a fuel The K jyj value for the aminoacyl-tRNA synthetases is lower than that of the enzymes taking part in amino acid catabolism. Thus, amino acids are used to synthesize aminoacyl-tRNAs before they are catabolized. When catabolism does take place, the first step is the removal of nitrogen, which is subsequently processed to urea. The liver secretes from 20 to 30 g of urea a day. The a-ketoacids are then used for gluconeogenesis or fatty acid synthesis. Interestingly, the liver cannot remove nitrogen from the branch-chain amino acids (leucine, isoleucine, and valine). Transamination takes place in the muscle. 

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. 

Protein synthesis is called Lranslalioii because information present as a nucleic acid sequence is translated into a different language, the sequence of amino acids in a protein. This complex process is mediated by the coordinated interplay of inore than a hundred macro molecules, including mRNA, rRNAs, tRNAs, aminoacyl-tRNA synthetases,... 

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