Page 259 - Color Atlas of Biochemistry

250 Molecular genetics
Translation I: initiation
Like amino acid activation (see p. 248), pro-
tein biosynthesis (translation) takes place in
the cytoplasm. It is catalyzed by complex
nucleoprotein particles, the ribosomes,and
mainly requires GTP to cover its energy re-
quirements.
A. Structure of the eukaryotic ribosome
Ribosomes consist of two subunits of different
size, made up of ribosomal RNA (rRNA) and
nearly 80 proteins (the number of proteins
applies to rat liver ribosomes). It is customary process is simpler and has been better studied
to give the sedimentation coef cients (see
p. 200) of ribosomes and their components
instead of theirmasses. Forexample,the eu-
karyotic ribosome has a sedimentation coef-
ficient of 80 Svedberg units (80 S), while the
sedimentation coef cients of its subunits are
40 S and 60 S (S values are not additive).
The smaller 40 S subunit consists of one
molecule of 18 S rRNA and 33 protein mole-
cules. The larger 60 S subunit contains three
types of rRNA with sedimentation coef cients
of 5S, 5.8 S, and 28S and 47 proteins. In the
presence of mRNA, the subunits assemble to
form the complete ribosome, with a mass
about 650 times larger than that of a hemo-
globin molecule.
The arrangement of the individual compo-
nents of a ribosome has now been deter-
mined for prokaryotic ribosomes. It is known
that filamentous mRNA passes through a cleft
between the two subunits near the charac-
teristic “horn” on the small subunit. tRNAs
also bind near this site. The illustration shows occupied during this phase of translation.
thesizeof a tRNA molecule forcomparison.
Prokaryotic ribosomes have a similar struc-
ture, but are somewhat smaller than those of
eukaryotes (sedimentation coef cient 70 S
for the complete ribosome, 30 S and 50 S for
the subunits). Mitochondrial and chloroplast
ribosomes are comparable to prokaryotic
ones.
B. Polysomes
In cells that are carrying out intensive protein
synthesis, ribosomes are often found in a lin-
ear arrangement like a string of pearls; these
are known as polysomes. This arrangement
arises because several ribosomes are translat-
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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ing a single mRNA molecule simultaneously.
The ribosome first binds near the start codon
(AUG; see p. 248) at the 5 end of the mRNA
(top). During translation, the ribosome moves
in the direction of the 3 end until it reaches a
stop codon (UAA, UAG, or UGA). At this point,
the newly synthesized chain is released, and
the ribosome dissociates again into its two
subunits.
C. Initiation of translation in E. coli
Protein synthesis in prokaryotes is in princi-
ple the same as in eukaryotes. However, as the
in prokaryotes, the details involved in trans-
lation are discussed here and on p. 252 using
the example of the bacterium Escherichia coli.
Thefirst phaseoftranslation, initiation,in-
volves several steps. First, two proteins, ini-
tiation factors IF–1 and IF–3, bind to the 30 S
subunit (1). Another factor, IF–2, binds as a
complex with GTP (2). This allows the subunit
to associate with the mRNA and makes it
possible for a special tRNA to bind to the start
codon (3). In prokaryotes, this starter tRNA
carries the substituted amino acid N-
formylmethionine (fMet). In eukaryotes, it car-
ries an unsubstituted methionine. Finally, the
50 S subunit binds to the above complex (4).
During steps 3 and 4, the initiation factors are
released again, and the GTP bound to IF–2 is
hydrolyzed to GDP and P i .
In the 70 S initiation complex,formylme-
thionine tRNA is initially located at a binding
site known as the peptidyl site (P).A second
binding site, the acceptor site (A),is not yet
Sometimes, a third tRNA binding site is de-
fined as an exit site (E), from which uncharged
tRNAs leave the ribosome again (see p. 252;
not shown).