Amino acid activation

A polymer-supported sulfonamide, prepared from an amino acid activated ester and a polystyrene-sulfonamide, is stable to acidic hydrolysis (CF3COOH HBr/ HOAc). It is cleaved by the safety-catch method shown below. ... 

A polymer-supported sulfonamide, prepared from an amino acid activated ester... 

Amino acids activated at the amino group by a benzotriazolide moiety react with amino acids under elimination of benzotriazole and C02 to give peptides. Reaction is achieved by warming up equimolar amounts of the components in anhydrous acetonitrile or aqueous acetone.[45] The benzotriazolylcarbonylamino acids are prepared from benzo-triazolyl-1-carboxylic acid chloride and amino acids.[46]... 

As we have noted, the outcome of a virus infection is the synthesis of viral nucleic acid and viral protein coats. In effect, the virus takes over the biosynthetic machinery of the host and uses it for its own synthesis. A few enzymes needed for virus replication may be present in the virus particle and may be introduced into the cell during the infection process, but the host supplies everything else energy-generating system, ribosomes, amino-acid activating enzymes, transfer RNA (with a few exceptions), and all soluble factors. The virus genome codes for all new proteins. Such proteins would include the coat protein subunits (of which there are generally more than one kind) plus any new virus-specific enzymes. 

A further important group of derivatives is that of amino acids activated by phosphoric acid or its esters. In nature, phosphorylation processes play an important activating role in peptide and protein synthesis. 

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)...

It was earlier considered that all the amino acid-activating synthetases were derived from a single primeval synthetase , so that all synthetases would have similar structures. Surprisingly, however, this is not the case. When the primary sequences, and in part the secondary and tertiary structures, of all the synthetases had been determined, clear differences in their construction became obvious. The aminoacyl-tRNA synthetases consist either of one single polypeptide chain (a) or of two or four identical polypeptides (ot2 or 04). In addition, there are heterogeneously constructed species with two sets of two identical polypeptide chains (OC2P2). This nomenclature indicates that, for each synthetase, a or P refers to a primary structure. The number of amino acids can vary from 334 to more than 1,000. 

The amino acid, activated by uptake of SO3, reacts with a second molecule of amino acid to form the dipeptide, which can in turn react further to form a tripeptide (and so on). This peptide synthesis model, which is supported by experimental evidence, appeals because of its simplicity it may well correspond much more closely to conditions on the primeval Earth than do some other models (Chen and Yang, 2007). 

The active site of an enzyme is generally a pocket or cleft that is specialized to recognize specific substrates and catalyze chemical transformations. It is formed in the three-dimensional structure by a collection of different amino acids (active-site residues) that may or may not be adjacent in the primary sequence. The interactions between the active site and the substrate occur via the same forces that stabilize protein structure hydrophobic interactions, electrostatic interactions (charge-charge), hydrogen bonding, and van der Waals interactions. Enzyme active sites do not simply bind substrates they also provide catalytic groups to facilitate the chemistry and provide specific interactions that stabilize the formation of the transition state for the chemical reaction. 

Paracentrotus lividus, embryos 100 Reductions in various amino acid activity levels during exposure for 40 h 15... 

M Goodman, KC Stueben. Amino acid active esters. III. Base-catalyzed racemization of peptide active esters. J Org Chem 27, 3409, 1962. 

Goodman, KC Stueben. Peptide synthesis via amino acid active esters. J Am Chem Soc 81, 3980, 1959. 

AMINO ACID ACTIVATION AND CODON TRANSLATION BY tRNAs... 

Amino acid activation Aminoacyl-tRNA synthetase two high-energy bonds (ATP) to link amino acid to tRNA j ... 

Peptide bond forms (two high-energy bonds from amino acid activation) Peptide bond forms (two high-energy bonds from amino acid activation)... 

Many examples of catalytic nucleic acids obtained by in vitro selection demonstrate that reactions catalyzed by ribozymes are not restricted to phosphodiester chemistry. Some of these ribozymes have activities that are highly relevant for theories of the origin of life. Hager et al. have outlined five roles for RNA to be verified experimentally to show that this transition could have occurred during evolution [127]. Four of these RNA functionalities have already been proven Its ability to specifically complex amino acids [128-132], its ability to catalyze RNA aminoacylation [106, 123, 133], acyl-transfer reactions [76, 86], amide-bond formation [76,77], and peptidyl transfer [65,66]. The remaining reaction, amino acid activation has not been demonstrated so far. 

Both molecules are folded in such a way that the 3 end and the 5 end are close together. As in DNA, most of the bases are located in the inside of the structures, while the much more polar backbone is turned outwards. An exception to this is seen in the three bases of the anticodon of the tRNA (pink), which have to interact with mRNA and therefore lie on the surface of the molecule. The bases of the conserved CCA triplet at the 3 end (red) also jut outward. During amino acid activation (see p.248), they are recognized and bound by the ligases. 

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. 

Like amino acid activation (see p. 248), protein biosynthesis (translation) takes piace in the cytopiasm. it is cataiyzed by compiex nucieoprotein particies, the ribosomes, and mainiy requires GTP to cover its energy requirements. 

Energy requirements in protein synthesis are high. Four energy-rich phosphoric acid anhydride bonds are hydrolyzed for each amino acid residue. Amino acid activation uses up two energy-rich bonds per amino acid (ATP AMP + PP see p. 248), and two GTPs are consumed per elongation cycle. In addition, initiation and termination each require one GTP per chain. 

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