tRNA Function: Synthetases
Each tRNA is charged with the proper amino acid via
a covalent ester bond at its 3' end by a family of enzymes called aminoacyl-tRNA
synthetases. Each enzyme must recognize both
the tRNA(s) specific for an amino acid and the corresponding amino acid.
This energy-consuming process is ATP-dependent and results in the cleavage
of two high-energy phosphate bonds (one in going from ATP to AMP + PP, and one for the cleavage of pyrophosphate into two inorganic phosphates:
There are 20 different aminoacyl-tRNA synthetases, one for
each amino acid. Despite the fact that they all carry out very similar
tasks, they vary greatly in size (40-100 kDalton).
Since there are 61 amino acid codons, most tRNA synthetases
must be able to recognize more than one type of tRNA (i.e. 6 codons for
Arg). These tRNAs are called cognate tRNAs for that
particular synthetase. This mapping is achieved through so-called
recognition domains on the tRNA. In the figures below (a result using x-ray crystallopraphy - yeah!), tRNA is shown with a red
backbone and yellow bases; tRNA synthetase is shown as a space-filling
model in blue:
The recognition domains include unique sections of the acceptor stem, loops and/or the anticodon (black dots):
Accuracy & Proofreading
The accuracy of charging tRNA with the proper amino
acid is crucial because once charged, only the tRNA anticodon
determines incorporation, not the attached amino acid.
The error rate of charging is very low: 1 error in 10,000 charging reactions.
This is achieved by two means:
amino acid specificity pocket in a specific synthetase will only bind amino
acids similar in size and charge.
the synthetase also has proofreading capability which, once
a wrong aminoacyl-adenylate complex is formed (1st step), will hydrolyze
the complex before it can be covalently attached to the tRNA
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