Chemistry Reference
In-Depth Information
FIGURE 3.33
The adaptor hypothesis. (Adapted from
Voet & Voet, 2004
.)
mRNA by base pairing between the codon and a three-base anticodon on the tRNA molecule. The key step in
determining the specificity of protein biosynthesis is the loading of the amino acids onto their corresponding tRNA
by enzymes called aminoacyl tRNA synthetases. This is a two-step process involving the formation of an enzyme-
bound aminoacyl-adenylate between the amino acid and a molecule of ATP, followed by the transfer of the amino
acid to the terminal 2
0
-or3
0
-hydroxyl of its tRNA to form the aminoacyl-tRNA (
Figure 3.34
)
. The importance of
FIGURE 3.34
Structures of an aminoacyl-adenylate and of an aminoacyl-tRNA.
this reaction is underlined by the classic experiment in which cysteine, loaded on tRNA
Cys
, was reductively
converted to alanine using Raney nickel, and it was shown that Cys residues in rabbit haemoglobin synthesised
using this Ala-tRNA
Cys
were systematically replaced by Ala, confirming that the coding properties of this hybrid
tRNA are determined by the tRNA, not by the amino acid which is bound (Chapeville et al., 1962).
An aminoacyl-tRNA synthetase exists for each of the 20 amino acids, and is highly specific for its amino
acid
10
5
reactions. This is,
in part, due to the presence of two physically distinct domains within many synthetases (
Figure 3.35
a
). The
catalytic domain (acylation site) carries out the initial recognition of the amino acid, and transfers it to its cognate
tRNA, whereas the editing domain eliminates the wrong amino acid by hydrolysing either the aminoacyl-
adenylate or the aminoacyl-tRNA. We can illustrate this by the case of threonyl-tRNA synthetase, which must
distinguish between Thr, Val (with a methyl group in place of a hydroxyl), and Ser (with a hydroxyl group but
lacking the methyl group). The catalytic site avoids Val by using a zinc ion, bound to the enzyme by two His and
one Cys residue. The zinc ion can also coordinate Thr through its side-chain hydroxyl group and its amino group
(
Figure 3.35
b): the hydroxyl group also forms hydrogen bonds to an adjacent Asp residue. Val cannot bind in this
way through its methyl group and is therefore not adenylated and transferred to tRNA
Thr
. Ser can, however, be
the wrong amino acid is introduced into a protein on average only once in every 10
4
e
e
