(60)
10. Definere begrebet proofreading samt beskrive denne process på syntetase enzymet
 Devlin, s. 241
Stryer. s. 820, fig. 29.9; fig. 29.10

 

Proofreading by aminoacyl-tRNA synthetases increases the fidelity of protein synthesis. Proofreading is used when tRNA is supposed to see the difference between two amino acids whose chemical properties are almost the same, so they can’t be separated by a specific jon that binds to only one of them in the catalytic site, but are structurally very similar, fx. one has an extra CH3 – group. 

For that reason, most aminoacyl-tRNA synthetases contain an extra editing site in addition to the amino-acylation site. These two sites act as a double sieve to ensure very high fidelity. The flexible CCA arm of the aminoacyl-tRNA can move the amino acid between the activation site (a. acyl-site) and the editing site.  

Two amino acids that are chemically identical but structurally different, must differ in size. Thus, an incorrect amino acid can either be bigger or smaller then the correct one. 

The acylation site is hydrophobe, and rejects amino acids bigger then the correct one because there is insufficient room for it. So, if the amino acid does not fit in the activation site, it is automatically rejected, due to hydrophobic steric forces.

If the amino acid fits well into the editing site, the amino acid is removed by hydrolysis. The editing site is hydrolytic, and it cleaves amino acids that are smaller then the correct one.

The correct aminoacid would fit perfectly in the acylation site and won't at all fit in the editing site, because it would be too big.

 

Just to sum up, the aminoacyl t-RNA synthetase has several structural domains with different functions:

 

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