Aminoacyl-tRNA synthetase

Aminoacyl-tRNA synthetases (AaRS) are enzymes that occur in the cells of all living things and are necessary for translation in protein synthesis , as they catalyze the binding of a proteinogenic amino acid to its tRNA and thus the formation of an aminoacyl-tRNA .

These synthetases are ligases and are needed to load tRNA molecules, depending on their structure - in particular the anticodon sequence - with the corresponding amino acid , which creates an aminoacyl-tRNA. A synthetase is specific for one of the proteinogenic amino acids, for the twenty canonical ones there are usually 20 different aminoacyl-tRNA synthetases, the genes of which are household genes . Eukaryotes also have an additional set of mitochondrial aminoacyl-tRNA synthetases, and plants have another set in plastids . These differ from the main cytoplasmic enzymes as well as from each other and preferentially load the tRNA of the respective organelles.

The reaction steps catalyzed by an aminoacyl-tRNA synthetase ,

• the activation of an amino acid (→ aminoacyl adenylate)
• and then their binding to tRNA (→ Aminoacyl-tRNA),

are summarized in general terms :

${\displaystyle \mathrm {{\mbox{Aminos}}{\ddot {a}}{\mbox{ure}}\;\!+tRNA\;\!+ATP\;\longrightarrow {\mbox{Aminoacyl}}{\mbox{-}}\;\!tRNA\;\!+AMP+{\mbox{Pyrophosphat}}} }$

Enzyme structure

Aminoacyl-tRNA synthetases are large proteins with multiple domains . Each of the synthetases has a catalytic domain, in the folds of which ATP, an amino acid and the acceptor stem of a tRNA are bound. This is where the actual synthetic reaction steps take place, which activate an amino acid molecule and bind to the last nucleotide at the 3 'end of the acceptor stem of a tRNA. The amino acid is bound via its carboxyl group to the oxygen atom of one of the hydroxyl groups (-OH) of the ribose in the terminal nucleoside of the tRNA, an adenosine (often in position 76: A76).

β-sheet structure motifs - represented by arrows - in the TGS domain of the human cytoplasmic threonyl- tRNA synthetase (ThrRS), a homodimeric class II aminoacyl-tRNA synthetase

In addition, there are other domains with which the right amino acid can be selected, the correct tRNA recognized or a wrong connection broken again. The AaRS families differ considerably in terms of both structural and functional features. The individual enzymes are named after their specific amino acid and differentiated with regard to organism and cell compartment (e.g. human alanyl- tRNA synthetase 2, mitochondrial; short: AlaRS2 human; gene name A ARS2 ).

With regard to their structure and the motifs of functional domains, roughly two classes of synthetases can be distinguished:

• I - in the predominantly monomeric class I enzymes, the core domain in the secondary structure has parallel β-sheets in a Rossmann fold and two motifs for ATP binding;

these include synthetases specific for Arg , Cys , Glu , Gln , Ile , Leu , Lys , Met , Trp , Tyr , Val

• II - the mostly dimeric or multimeric class II enzymes, on the other hand, have mixed β-sheets in the core domain and three other motifs that are involved in ATP binding;

these include synthetases specific for Ala , Asn , Asp , Gly , His , Lys , Phe , Pro , Ser , Pyl , Thr

While in all those of class I the amino acid is first esterified with 2'-OH and then transesterified in 3'- O -aminoacyl , almost all of the class II aminoacylates in the 3 'position (with one exception, PheRS). Both classes are further subdivided into subclasses (a, b and c) according to the way the AaRS binds to the tRNA molecule.

Sequence of tRNA loading

In order for a tRNA to be loaded with the corresponding amino acid at all , it must first be activated by the aminoacyl-tRNA synthetase. This happens through the formation of a carboxylic acid-phosphoric acid-anhydride bond between the selected amino acid and ATP , whereby the aminoacyl adenylate and pyrophosphate are formed. Only now can the amino acid activated in this way be transferred to the 3 'end on the acceptor stem of the tRNA and bound with ester formation , in which case an AMP molecule is then split off. The respective aminoacyl-tRNA is thus synthesized as a specifically charged tRNA species, which is then available for the translation process on the ribosome .

The specificity and accurate control of this aminoacylation of the tRNAs is at least as important for the accuracy of protein synthesis as the anticodon-codon interaction between tRNA and mRNA on the ribosome. If one of the tRNA molecules were loaded with the wrong amino acid and used on the ribosome, this wrong amino acid would be incorporated with correct tRNA-mRNA interaction during protein biosynthesis.

Some aminoacyl-tRNA synthetases recognize the appropriate tRNAs mainly on the basis of the anticodon . However, other structural motifs also play a role in the identification of the tRNA molecule by the (complex) protein molecule of the specific AaR synthetase. For an alanyl-tRNA synthetase (AlaRS), mutagenesis experiments showed that it does not recognize the corresponding tRNA from the anticodon, but from the acceptor stem and a hairpin loop . In addition, some aminoacyl-tRNA synthetases show the ability to recognize defective loads and in this case to break the bond between an amino acid and a tRNA again.

There is not an aminoacyl-tRNA synthetase for each of the possible codons or their anticodons, but one of these enzymes is specific for one of the proteinogenic amino acids. In the cytosol of a cell or in a mitochondrion or plastid there is usually a set of around 20 different types of aminoacyl-tRNA synthetases.

Alternative synthetic routes are known for two of the aminoacyl-tRNAs. Archaea , cyanobacteria , as well as mitochondria and plastids , can produce a tRNA loaded (with Gln ) by conversion (via Glu-tRNA ^Gln amidotransferase) from a differently loaded tRNA (with Glu ). In the case of halobacteria , this applies analogously to another ( Asn -tRNA ^Asn from Asp -tRNA ^Asn ); these organisms can therefore get by with a family of 18 aminoacyl-tRNA synthetases.

pathology

In humans cause mutations in the GARS - gene (for glycyl tRNA synthetase) to a form of Charcot-Marie-Tooth disease .

literature

• Woese CR, Olsen GJ, Ibba M, Söll D: Aminoacyl-tRNA synthetases, the genetic code, and the evolutionary process . In: Microbiol. Mol. Biol. Rev. . 64, No. 1, March 2000, pp. 202-36. PMID 10704480 . PMC 98992 (free full text).
• Curnow AW, Hong K, Yuan R, et al. : Glu-tRNAGln amidotransferase: a novel heterotrimeric enzyme required for correct decoding of glutamine codons during translation . In: Proc. Natl. Acad. Sci. USA . 94, No. 22, October 1997, pp. 11819-26. PMID 9342321 . PMC 23611 (free full text).

Individual evidence

1. ↑ UniProt search result human aminoacyl-tRNA synthetases
2. ↑ Burbaum JJ, Schimmel P : Structural relationships and the classification of aminoacyl-tRNA synthetases . In: J. Biol. Chem. . 266, No. 26, September 1991, pp. 16965-8. PMID 1894595 .
3. ↑ InterPro: IPR006195 Aminoacyl-tRNA synthetase, class II, conserved domain
4. ↑ EC  6.1.1.24
5. ↑ RajBhandary UL: Once there were twenty . In: Proc. Natl. Acad. Sci. USA . 94, No. 22, October 1997, pp. 11761-3. PMID 9342308 . PMC 33776 (free full text).
6. ↑ UniProt P41250