Essential Cell Biology 5th edition

From RNA to Protein
251
has a mass of 30,000 daltons. The small ribosomal subunit matches the
tRNAs to the codons of the mRNA, while the large subunit catalyzes
the formation of the peptide bonds that covalently link the amino acids
together into a polypeptide chain. These two subunits come together on
an mRNA molecule near its 5ʹ end to start the synthesis of a protein. The
mRNA is then pulled through the ribosome like a long piece of tape. As
the mRNA inches forward in a 5ʹ-to-3ʹ direction, the ribosome translates
its nucleotide sequence into an amino acid sequence, one codon at a
time, using the tRNAs as adaptors. Each amino acid is thereby added
in the correct sequence to the end of the growing polypeptide chain
(Movie 7.7). When synthesis of the protein is finished, the two subunits
of the ribosome separate. Ribosomes operate with remarkable efficiency:
a eukaryotic ribosome adds about 2 amino acids to a polypeptide chain
each second; a bacterial ribosome operates even faster, adding about 20
amino acids per second.
How does the ribosome choreograph all the movements required for
translation? In addition to a binding site for an mRNA molecule, each
ribosome contains three binding sites for tRNA molecules, called the A
site, the P site, and the E site (Figure 7–36). To add an amino acid to a
growing peptide chain, a charged tRNA enters the A site by base-pairing
with the complementary codon on the mRNA molecule. Its amino acid is
then linked to the growing peptide chain, which is held in place by the
tRNA in the neighboring P site. Next, the large ribosomal subunit shifts
forward, moving the spent tRNA to the E site before ejecting it (Figure
7–37). This cycle of reactions is repeated each time an amino acid is
added to the polypeptide chain, with the new protein growing from its
amino to its carboxyl end until a stop codon in the mRNA is encountered
and the protein is released.
E site P site A site
large
ribosomal
subunit
small
ribosomal
subunit
E P A
mRNAbinding
site
(A)
(B)
Figure 7–36 Each ribosome has a binding site for an mRNA molecule and three
binding sites for tRNAs. The tRNA sites are designated the A, P, and E sites (short
for aminoacyl-tRNA, peptidyl-tRNA, and exit, respectively). (A) Three-dimensional
structure of a bacterial ribosome, as determined by x-ray crystallography, with the
small subunit in dark green and the large subunit in light green. Both the rRNAs and
the ribosomal proteins are shown in ECB5 green. e7.33/7.36 tRNAs are shown bound in the E site
(red), the P site (orange), and the A site (yellow). Although all three of the tRNA sites
shown here are filled, during protein synthesis only two of these sites are occupied
by a tRNA at any one time (see Figure 7–37). (B) Highly schematized representation
of a ribosome, in the same orientation as (A), which is used in subsequent figures.
Note that both the large and small subunits are involved in forming the A, P, and E
sites, while only the small subunit contains the binding site for an mRNA. (A, adapted
from M.M. Yusupov et al., Science 292:883–896, 2001. Courtesy of Albion A. Bausom
and Harry Noller.)