TRNA synthase

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... 

In E. coli, the products of four genes (selA, selB, selC, and selD) are required for the incorporation of selenocysteine. The product of the selC gene is tRNASer (a suppressor tRNA the anticodon of which is UCA). The first step in the incorporation of selenocysteine is catalyzed by seryl-tRNA synthase. The products of selA and selD function in the subsequent conversion of Ser-tRNASer to selenocysteyl-tRNASer. The probable pathway is... 

A unique class of enzymes, called aminoacyl-tRNA synthases, attach amino acids to their cognate tRNAs. This attachment serves two functions (1) The linkage between... 

The second reaction catalyzed by the aminoacyl-tRNA synthases results in the attachment of the amino acid through an ester linkage to the 3 terminal ribose of tRNA ... 

Our understanding of synthase reactions and the types of the active sites involved in these reactions was advanced substantially by the crystallization and structural solution of tyrosyl-tRNA synthase complexed with the reaction intermediate tyrosyl-adenylate (fig. 29.10). The reaction intermediate is bound in a deep cleft in the enzyme and interacts with it through 11 hydrogen bonds. Six of these bonds are with the AMP moiety, and five are with the tyrosyl moiety of the intermediate. The amino acid selectivity of tyrosyl-tRNA synthase is thus determined primarily by the formation of specific hydrogen bonds with the amino acid. 

The crystal structure of glutaminyl-tRNA synthase, complexed with tRNA and ATP, has been determined by Tom Steitz and his colleagues (fig. 29.12). This accomplishment provided the first structure of a tRNA-protein complex and thus offers important insight into the general nature of the recognition of tRNAs by proteins. 

Aminoacyl-tRNA Synthases Can Correct Acylation Errors... 

Many aminoacyl-tRNA synthases appear to contain a second catalytic site that serves to correct errors by hydrolyzing incorrectly matched amino acids and tRNAs. These proof-reading hydrolysis reactions are best understood in the case... 

Identity elements in four tRNAs. Each circle represents one nucleotide. Filled circles indicate nucleotides that serve as recognition elements to the appropriate aminoacyl-tRNA synthase. It is possible that other identity elements occur in these structures that are still to be discovered. (From L. H. Schulman and J. Abelson, Recent excitement in understanding transfer RNA identity, Science 240 1591, June 17, 1988. Copyright 1988 by the AAAS. Reprinted by permission.)... 

In archaea and eukaryotes, incorporation of Sec into polypeptides requires three steps involving three different enzymes. In the first step, SerRS mischarges tRNA with serine to form Ser-tRNA . Subsequently, a kinase, called O-phosphoseryl-tRNA kinase (PSTK), phosphorylates serine to generate 0-phosphoseryl-tRNA (Sep-tRNA ). Finally, a pyridoxal phosphate-dependent enzyme called Sep-tRNA Sec-tRNA synthase (SepSecS) (9) or Sec synthase (SecS) (13), converts Sep-tRNAS to Sec-tRNAS . 

Many methanogenic archaeabacteria lack cysteinyl-tRNA synthetase (CysRS). Interestingly, a Class II enzyme called O-phosphoseryl-tRNA synthetase (SepRS) acylates tRNA with O-phosphoserine (Sep) to form Sep-tRNA , which is then converted to Cys-tRNA by the enzyme Sep-tRNA Cys-tRNA synthase (SepCysS). It has been proposed that this indirect pathway may be the sole route for cysteine biosynthesis in these organisms (9). The crystal structure of SepRS was recently... 

In archaea (42) and eukarya (48), selenocysteine also starts with the serylation of tRNA by seryl-tRNA synthetase. In the presence of Mg + and ATP, phosphoseryl-tRNA kinase specifically phosphorylates the seryl moiety of Ser-tRNA to produce 0-phosphoseryl-tRNA (49). The conversion of 0-phosphoserine (Sep) to selenocysteine proceeds by the action of the PFP-dependent enzyme Sep-tRNA Sec-tRNA synthase (SepSecS) using selenophosphate as a selenium donor to produce Sec-tRNA (50). As the phosphate of Sep provides a better leaving group than the water of serine, Sep to Sec conversion is more chemically favorable than Ser to Sec conversion. This favorableness and the greater stability of the Sep-tRNA as compared with Ser-tRNA (49) would enhance the production of Sec-tRNA in archaea and eukarya, which have more extended selenoproteomes than bacteria (50). 

All tRNA s are similar in structure (Fig. 12.5). The TDC arm participates in binding of the charged tRNA to a site on the ribosome where protein synthesis occurs. The DHU (or D) arm is necessary for recognition by the proper aminoacyl tRNA synthase (the enzyme). The acceptor end is at the 3 terminus and ends in the sequence CAA. The anticodon arm consists of seven nucleotides, the sequence of which is read 3 to 5 (opposite convention to the usual 5 to 3 ). The anticodon sequence is 3 variable base modified purine-X-Y-Z-Py-Py 5. The central bases, X, Y, Z comprise the anticodon, codon 5 ... 

The amino is linked via the 5 position to the ribose on the ATP, liberating PPi. Notice the amino-AMP complex remains bound to the enzyme, aminoacyl tRNA synthase. 

Gln-tRNA synthase/tRNAGln San Diego MWPC 242.8 93.6 115.7 C222t 2.8 Rould, Perona, Soil and... 

In an important experiments Nirenberg and Matthaei, in 1961, isolated ribosomes from E. coli and mixed them with crude extracts of soluble materials, also from E. coli cells. The extracts included tRNA molecules and aminocyl-tRNA synthases. The 20 amino acids, ATP, and an ATP-generating system (PEP + pyruvate kinase) were added. Nirenberg showed that under such conditions protein was synthesized by ribosomes in response to the presence of added RNA. For example, RNA from tobacco mosaic virus (Chapter... 

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