Amino acids tRNAs

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. 

Amino add activation. Before binding to the ribosomes, tRNAs are loaded with the correct amino acids by specific ligases (7 see p. 248). It is the amino acid tRNA ligases that carry out the transfer (translation) of the genetic information from the nucleic acid level to the protein level. 

Some 20 different amino acid tRNA ligases in the cytoplasm each bind one type of tRNA (see p. 82) with the corresponding amino acid. This reaction, known as amino acid activation, is endergonic and is therefore coupled to ATP cleavage in two steps. 

Short-term ozone exposures (45 5 pphm, 15 min) of seedlings were followed after 24 hr by non-linear reductions in chlorophyll/g fresh weight and stimulations in fresh weight/organ. The utilization of l C-protein hydrolysate by tissue discs is not only predominantly energy-dependent, but also strictly dependent with respect to inhibition or stimulation upon the time after ozonation. Uptake of labelled amino acids into the soluble pools of tissue discs is sensitive to as little as 15 min of in vivo exposure (45 5 pphm), and incorporation into insoluble protein is sensitive during 15 to 30 min, after which no further reduction is observed for up to 90 min of exposure. The reduction of amino acid influx into the soluble pools is not accountable to a reduction in amino acid tRNA charging, and is probably not due to a reduction in amino acid incorporation. 

Initiation (Figs. 29-10 and 29-11), elongation (Fig. 29-12), and termination are three distinct steps in the synthesis of a protein. A variety of specialized proteins are required for each stage of synthesis. Their sequential interaction with ribosomes can be viewed as a means of ensuring an orderly sequence of steps in the synthesis cycle. The rate of protein formation will depend upon the concentrations of amino acids, tRNAs, protein factors, numbers of ribosomes, and kinetic constants. The formation of specific proteins can also be inhibited by translational repressors, proteins that compete with ribosomes for binding to target mRNAs.287... 

How many high-energy bonds are required to synthesize a protein of 100 amino acid residues Assume that all components are available (amino acids, tRNA, ATP, and GTP). 

T. C. Stadtman, Specific occurrence of selenium in enzymes and amino acid tRNAs, FASEB, 1 (1987), 375-379. 

The initial application of the anchor principle described by Page related to data on the selectivity of amino acid-tRNA synthetases, from which he estimated intrinsic... 

Amino acid-AMP + tRNA amino acid-tRNA + AMP... 

The aminoacyl-tRNA S5mthetases (amino acid tRNA ligases) join amino acids to their appropriate transfer RNA molecules for protein S5mthesis. They have the very important task of selecting both a specific amino acid and a specific tRNA and joining them according to Eq. These reactions repre-... 

For the next cycle, a new acyl amino acid-tRNA is bound to the mRNA, the ribosome tracks along the mRNA and the emptied tRNA is ejected. As the polypeptide is elongated it assumes its secondary and tertiary structure (Figure 4.32). 

A transfer RNA has one binding site for a specific sequence of three nucleotides in mRNA (the anticodon site) and another binding site for the encoded amino acid. tRNAs thus ensure that the genetic code is translated into the correct sequence of... 

Application of the anchor principle to data on the selectivity of amino acid-tRNA synthetases gives estimated intrinsic binding energies for the carboxyl and amino groups of 18 and >28kJmol, respectively. However, since the sidechains, rather than the ionic groups, are the primary determinants of amino acid/tRNA synthetase specificity, these energies are likely to be underestimates. 

Amino acid activation Amino acids tRNAs Aminoacyl-tRNA synthetases ATP, Mg2+... 

This is RNA of M,. -25,000. It serves as an adaptor in polypeptide chain synthesis (Chap. 8). It constitutes 10-20% of the total RNA in a cell, and there is at least one tRNA for each amino acid. tRNAs are unique in that they contain a relatively high proportion of nucleosides of unusual structure such as pseudouridine, inosine, and 2 -0-methylnucleoside. There are also many types of modified bases such as those which are methylated, or acetylated. 

The 23 pairs of chromosomes in a human nucleus contain, together, about 200 000 genes at the other end of the scale, a virus may have as few as ten. [A gene is a strip of DNA that completely specifies a particular RNA, which itself completely specifies one protein (mRNA) or one amino acid (tRNA)]. 

After the discovery of three-letter codons, researchers were anxious to answer the next question Which triplets of bases (codons) code for which amino acids In 1961, Marshall Nirenberg and his coworkers at the National Institutes of Health (NIH) in Bethesda, Maryland, attempted to break the code in a very ingenious way. They made a synthetic molecule of mRNA consisting of uracil bases only. Thus, this mRNA contained only one codon, the triplet UUU. They incubated this synthetic mRNA with ribosomes, amino acids, tRNAs, and the appropriate enzymes for protein synthesis. The exciting result of this experiment was that a polypeptide that consisted only of phenylalanine was synthesized. Thus, the first word of the genetic code had been deciphered UUU phenylalanine. 

The next question is. Which of the 64 triplets codes for which amino acid In 1961, Marshall Nirenberg provided a simple experimental approach to the problem, based on the observation that synthetic polynucleotides direct polypeptide synthesis in much the same manner as do natural mRNAs. Nirenberg found that when ribosomes, amino acids, tRNAs, and appropriate protein synthesizing enzymes were incubated in vitro. 

The process of translation involves the synthesis of a protein from the information encoded in the messenger molecule [50]. The process begins by the incorporation of a chain-initiating N-formylmethionyl-tRNAf molecule into a ribosome under the influence of a number of initiation factors. The ribosome then moves down the messenger, adding amino acid tRNA molecules in the sequence dictated by the mRNA codons. The growing peptide chain remains attached to the ribosome... 

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