Aminoacyl-AMP

Initially, the amino acid is activated by an ATP-dependent process, producing an aminoacyl-AMP. This may be considered to be nucleophilic attack of... 

The intermediate aminoacyl-AMP can also be seen to be an anhydride, but in this case a mixed anhydride of carboxylic and phosphoric acids (see Box 7.27). This can react with a hydroxyl group in ribose, part of... 

Initially, the amino acid is activated by an ATP-dependent process, producing an aminoacyl-AMP. A hydroxyl group in ribose, part of a terminal adenosine group of tRNA, then reacts with this mixed anhydride. In this way, the amino acid is bound to tRNA via an ester linkage as an aminoacyl-tRNA. The tRNA involved will be specific for the particular amino acid. A detailed mechanism for this process has been considered in 8ection 13.5.1. 

This enzyme [EC 5.1.1.11], also known as phenylalanine racemase (ATP-hydrolyzing), catalyzes the reaction of ATP with L-phenylalanine to produce o-phenylalanine, AMP, and pyrophosphate. In this unusual racemase reaction, a thiol group of an enzyme-bound pantotheine forms a thiolester from an initial aminoacyl-AMP intermediate then, as is typical of acyl thioesters, the a-proton becomes labile, thereby permitting reversible inversion of configuration to produce an equilibrated mixture of thiolester-bound enantiomers. Hydrolysis of the thiolester yields the product. 

This reaction occurs in two steps in the enzyme s active site. In step (Fig. 27-14) an enzyme-bound intermediate, aminoacyl adenylate (aminoacyl-AMP), forms when the carboxyl group of the amino acid reacts with the a-phosphoryl group of ATP to form an anhydride linkage, with displacement of pyrophosphate. In the sec-... 

In addition to proofreading after formation of the aminoacyl-AMP intermediate, most aminoacyl-tRNA synthetases can also hydrolyze the ester linkage between amino acids and tRNAs in the aminoacyl-tRNAs. This hydrolysis is greatly accelerated for incorrectly charged tRNAs, providing yet a third filter to enhance the fidelity of the overall process. The few aminoacyl-tRNA synthetases that activate amino acids with no close structural relatives (Cys-tRNA synthetase, for example) demonstrate little or no proofreading activity in these cases, the active site for aminoacylation can sufficiently discriminate between the proper substrate and any incorrect amino acid. 

The equilibrium constant for this reaction is about 1, so that the energy derived from the cleavage of the phosphate anhydride of ATP is conserved in the mixed anhydride. Aminoacyl-AMP remains tightly bound to the enzyme and, as we will soon show, this fact has allowed researchers to crystallize this important complex and analyze its structure. 

Formation of aminoacyl-tRNA. This is a two-step process involving a single enzyme that links a specific amino acid to a specific tRNA molecule. In the first step (1) the amino acid is activated by the formation of an aminoacyl-AMP complex. This complex then reacts with a tRNA molecule to form an aminoacyl-tRNA complex (2). 

The attachment of an amino acid to a tRNA is catalyzed by an enzyme called aminoacyl-tRNA synthetase. A separate aminoacyl-tRNA synthetase exists for every amino acid, making 20 synthetases in total. The synthesis reaction occurs in two steps. The first step is the reaction of an amino acid and ATP to form an aminoacyl-adenylate (also known as aminoacyl-AMP). 

In the second step, without leaving the enzyme, the aminoacyl group of aminoacyl-AMP is transferred to the 3 end of the tRNA molecule to form aminoacyl-tRNA ... 

The attachment of an amino acid to an appropriate tRNA is accomplished via aminoacyl-tRNA synthetase and the hydrolysis of ATP. There is a separate enzyme specific for each amino acid, and it will recognize all tRNAs for that amino acid. The reaction proceeds in two steps and requires Mg2+ (Fig. 17-8). The first step, amino acid activation, results in the formation of an aminoacyl-AMP-enzyme intermediate. In the second step, the aminoacyl group is transferred to its appropriate (cognate) tRNA, the amino acid being linked to tRNA through an ester bond. It appears that recognition between the synthetase and tRNA is achieved through very precise contact between the... 

The activation reaction is catalyzed by specific aminoacyl-tRNA synthetases, which are also called activating enzymes. The first step is the formation of an aminoacyl adenylate from an amino acid and ATP. This activated species is a mixed anhydride in which the carboxyl group of the amino acid is linked to the phosphoryl group of AMP hence, it is also known as aminoacyl-AMP. 

The activation and transfer steps for a particular amino acid are catalyzed by the same aminoacyl-tRNA synthetase. Indeed, the aminoacyl-AMP intermediate does not dissociate from the synthetase. Rather, it is tightly bound to the active site of the enzyme by noncovalent interactions. Aminoacyl-AMP is normally a transient intermediate in the synthesis of aminoacyl-tRNA, but it is relatively stable and readily isolated if tRNA is absent from the reaction mixture. 

An amino acid first reacts with ATP, forming an enzyme [aminoacyl-AMP] complex and pyrophosphate. Cleavage of pyrophosphate drives this reaction. 

The aminoacyl-AMP then forms an ester with the 2 or 3 hydroxyl of a tRNA spedfic for that amino add produdng an aminoacyl-tRNA and AMP. 

D. Aminoacyl-tRNA synthetases react with ATP and an amino acid to form an enzyme [aminoacyl-AMP] complex. The amino acid is then transferred to the 3 end of tRNA. These enzymes are very specific for both the amino acid and the tRNA. The codon on mRNA for the amino acid is complementary to the anticodon on the tRNA. 

Acylation is accomplished in two steps, both of which are catalyzed by an aminoacyl tRNA synthetase. In the first, or activation, step an aminoacyl AMP is generated in a reaction between an amino acid and ATP ... 

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