SECTION 25.10
Protein Synthesis
577
m R N A + 3 0 S s u b u n it
+ fM et-tR N A ™ «'
+ G T P + in itia tio n f a c to r s
m R N A
F IG U R E 2 5 -1 1
Early steps in protein synthesis: in prokaryotes: formation of the 30S preinitiation complex and 70S initiation complex.
binding sites on the 50S subunit: the
aminoacyl(A)
site
and the
peptidyl(P) site;
each site consists of
several L proteins and 23S rRNA. The 50S subunit is
positioned in the 70S initiation complex such that the
tRNA™61, which was previously bound to the 30S
preinitiation complex, occupies the P site of the 50S
subunit. Positioning tRNA
™ 61
in the P site fixes the
position of the anticodon of tRNA
™ * 51
so that it can
pair with the initiator codon in the mRNA. in this
way, the reading frame is unambiguously defined.
2.
Binding of a second tRNA to the A site.
The A site of
the 70S initiation complex is available to any tRNA
molecule whose anticodon can pair with the codon
adjacent to the AUG complex initiation codon.
However, entry to the A site by the tRNA requires the
activity of a protein called
elongation factor
EF-Tu
(Figure 25-12). EF-Tu binds GTP and an aminoacyl
tRAA, forming a ternary complex, aminoacyl tRNA
[EF-Tu-GTP], This ternary complex enters the A site
for the codon-dependent placement of the aminoacyl
tRNA at the A site. During this process of placement,
GTP is hydrolyzed to GDP and P;, and EF-Tu is
released. EF-Tu-GTP is regenerated from
EF-Tu-GDP through the inter action of another
protein, EF-Ts, and GTP.
3.
Chain elongation (formation of the first peptide
bond).
After a charged tRNA is positioned in the A
site, a peptide bond between f Met and the adjacent
amino acid forms. The peptide bond is formed by an
enzyme complex called peptidyltransferase, whose
active site resides in 23SrRNA of 50S subunit
as
previously discussed
(Figure 25-13). As the peptide
bond is formed, the Met is cleaved from the tRNAfMet
in the P site by another ribosomal protein, tRNA
deacylase.
4.
Translocation of the ribosome.
After the peptide bond
forms, an uncharged tRNA occupies the P site and a
dipeptidyl tRNA is in the A site. At this point three
events, which together make up the translocation step,
occur: the deacylated tRNA
1“ 61
leaves the P site, the
peptidyl tRNA moves from the A site to the P site,
and the ribosome moves a distance of three bases on
the mRNA to position the next codon at the A site.
The translocation step requires the activity of another
elongation protein, EF-G, and hydrolysis of GTP to
provide the energy to move the ribosome. Thus, the
total amount of energy expended in the synthesis of
one peptide bond comes from the hydrolysis of four
high-energy phosphate bonds (equal to about
30 kcal = 7.5 x 4). Recall that the synthesis of each
molecule of ammoacyl tRNA consumes two
high-energy phosphate bonds (ATP
—>
AMP + PPq
PP; + 2Pi). (In these calculations, the one GTP
molecule expended in the formation of the initiation
complex is not included because of its small
contribution to the overall synthesis of a polypeptide.)
5.
Refilling of the A site.
After translocation has
occurred, the A site is again available for a charged
tRNA molecule with a correct anticodon. When
this occurs, the series of elongation reactions is
repeated.
6
.
Termination.
When a chain termination codon is
reached, no aminoacyl tRNA is available that can fill
the A site, so chain elongation stops. Since the
polypeptide chain is still attached to the tRNA
occupying the P site, release of the protein is
