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P. 261
252 Molecular genetics
Translation II: elongation and [3] After the transfer of the growing pep-
termination tide to the A site, the free tRNA at the P site
dissociates and another GTP–containing elon-
After translation has been initiated (see gation factor (EF-G GTP) binds to the ribo-
p. 250), the peptide chain is extended by the some. Hydrolysis of the GTP in this factor
addition of further amino acid residues provides the energy for translocation of the
(elongation) until the ribosome reaches a ribosome. During this process, the ribosome
stop codon on the mRNA and the process is moves three bases along the mRNA in the
interrupted (termination). direction of the 3 end. The tRNA carrying
the peptide chain is stationary relative to
the mRNA and reaches the ribosome’s P site
A. Elongation and termination of protein during translocation, while the next mRNA
biosynthesis in E. coli codon (in this case GUG) appears at the A site.
Elongation can be divided into four phases: [4] The uncharged Val-tRNA then dissoci-
[1] Binding of aminoacyl tRNA. First, the ates from the E site. The ribosome is now
peptidyl site (P) of the ribosome is occupied ready for the next elongation cycle.
by a tRNA that carries at its 3 end the com- When oneofthe threestop codons (UAA,
plete peptide chain formed up to this point UAG, or UGA) appears at the A site, termi-
(top left). A second tRNA, loaded with the nation starts.
next amino acid (Val–tRNA Val in the example [5] There are no complementary tRNAs for
shown), then binds via its complementary the stop codons. Instead, two releasing factors
anticodon (see p. 82) to the mRNA codon ex- bind to the ribosome. One of these factors
posed at the acceptor site (in this case GUG). (RF–1) catalyzes hydrolytic cleavage of the
The tRNA binds as acomplex with aGTP- ester bond between the tRNA and the C–ter-
containing protein, the elongation factor Tu minus of the peptide chain, thereby releasing
(EF–Tu) (1a). It is only after the bound GTP the protein.
has been hydrolyzed to GDP and phosphate [6] Hydrolysis of GTP by factor RF–3 sup-
that EF–Tu dissociates again (1b). As the bind- plies the energy for the dissociation of the
ing of the tRNA to the mRNA is still loose whole complex into its component parts.
before this, GTP hydrolysis acts as a delaying Energy requirements in protein synthesis
factor,makingit possible to check whether are high. Four energy-rich phosphoric acid
the correct tRNA has been bound. A further anhydride bonds are hydrolyzed for each
protein, the elongation factor Ts (EF-Ts), later amino acid residue. Amino acid activation
catalyzesthe exchange of GDPfor GTPand in uses up two energy-rich bonds per amino
this way regenerates the EF–Tu GTP com- acid (ATP AMP + PP; see p. 248), and two
plex. EF-Tu is related to the G proteins in- GTPs are consumed per elongation cycle. In
volved in signal transduction (see p. 384). addition, initiation and termination each re-
[2] Synthesis of the peptide bond takes quire one GTP per chain.
place in the next step. Ribosomal peptidyl-
transferase catalyzes (without consumption Further information
of ATP or GTP) the transfer of the peptide
In eukaryotic cells, the number of initiation
chain from the tRNA at the P site to the NH 2
group of the amino acid residue of the tRNA at factors is larger and initiation is therefore
the A site. The ribosome’s peptidyltransferase more complex than in prokaryotes. The cap
activity is not located in one of the ribosomal at the 5 end of mRNA and the polyA tail (see
proteins, but in the 28 S rRNA. Catalytically p. 246) play important parts in initiation.
active RNAs of this type are known as ribo- However, the elongation and termination
zymes (cf. p. 246). It is thought that the few processes are similar in all organisms. The
surviving ribozymes are remnants of the “RNA individual steps of bacterial translation can
world”—an early phase of evolution in which be inhibited by antibiotics (see p. 254).
proteins were not as important as they are
today.
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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