Essential Cell Biology 5th edition

From RNA to Protein
249
Specific Enzymes Couple tRNAs to the Correct
Amino Acid
For a tRNA molecule to carry out its role as an adaptor, it must be linked—
or charged—with the correct amino acid. How does each tRNA molecule
recognize the one amino acid in 20 that is its proper partner? Recognition
and attachment of the correct amino acid depend on enzymes called
aminoacyl-tRNA synthetases, which covalently couple each amino
acid to the appropriate set of tRNA molecules. In most organisms, there
is a different synthetase enzyme for each amino acid. That means that
there are 20 synthetases in all: one attaches glycine to all tRNAs that recognize
codons for glycine, another attaches phenylalanine to all tRNAs
that recognize codons for phenylalanine, and so on. Each synthetase
enzyme recognizes its designated amino acid, as well as nucleotides in
the anticodon loop and in the amino-acid-accepting arm that are specific
to the correct tRNA (Figure 7−32 and Movie 7.6). The synthetases are
thus equal in importance to the tRNAs in the decoding process, because
it is the combined action of the synthetases and tRNAs that allows each
codon in the mRNA molecule to be correctly matched to its amino acid
(Figure 7–33).
The synthetase-catalyzed reaction that attaches the amino acid to the
3ʹ end of the tRNA is one of many reactions in cells that is coupled to
the energy-releasing hydrolysis of ATP (see Figure 3−32). The reaction
produces a high-energy bond between the charged tRNA and the amino
acid. The energy of this bond is later used to link the amino acid covalently
to the growing polypeptide chain.
The mRNA Message Is Decoded on Ribosomes
The recognition of a codon by the anticodon on a tRNA molecule depends
on the same type of complementary base-pairing used in DNA replication
and transcription. However, accurate and rapid translation of mRNA
into protein requires a molecular machine that can latch onto an mRNA,
capture and position the correct tRNA molecules, and then covalently
link the amino acids that they carry to form a polypeptide chain. In both
tRNA Gln
anticodon
loop
ATP
amino-acidaccepting
arm
glutamine
aminoacyl-tRNA
synthetase
Figure 7–32 Each aminoacyl-tRNA
synthetase makes multiple contacts with
its tRNA molecule. For this tRNA, which
is specific for the amino acid glutamine,
nucleotides in both the anticodon
loop (dark blue) and the amino-acidaccepting
arm (green) are recognized by
the synthetase ECB5 m6.58-7.32 (yellow-green). The ATP
molecule that will be hydrolyzed to provide
the energy needed to attach the amino acid
to the tRNA is shown in red.
amino acid
(tryptophan)
H
H 2 N C C
CH 2
O
OH
tRNA
(tRNA Trp )
H 2 N
H
C
CH 2
C
O
O
high-energy
bond
H 2 N
H
C
CH 2
C
O
O
C
C
C
N CH
H
N CH
H
N CH
H
aminoacyl-tRNA
synthetase
(tryptophanyl-tRNA
synthetase)
A C C
ATP AMP + 2 P
LINKAGE OF AMINO
ACID TO tRNA
A C C
ANTICODON IN tRNA
BINDS TO ITS CODON
IN mRNA
anticodon
in tRNA
3′ A C C 5′
base-pairing
5′
U G G
codon in
3′
mRNA
NET RESULT: AMINO ACID IS
SELECTED BY ITS CODON IN
AN mRNA
Figure 7–33 The genetic code is translated by aminoacyl-tRNA synthetases and tRNAs. Each synthetase couples a particular amino
acid to its corresponding tRNAs, a process called charging. The anticodon on the charged tRNA molecule then forms base pairs with
the appropriate codon on the mRNA. An error in either the charging step or the binding of the charged tRNA to its codon will cause
the wrong amino acid to be incorporated into a polypeptide chain. In the sequence of events shown, the amino acid tryptophan (Trp) is
specified by the codon UGG on the mRNA.