Page 189 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Encyclopedia of Physical Science and Technology EN013D-617 July 27, 2001 11:42

Protein Synthesis 229

FIGURE 7 The elongation cycle. Aminoacylated tRNAs are transported to the ribosome by EF-Tu, and they are
positioned in the ribosomal A-site upon hydrolysis of EF-Tu-bound GTP. Nucleotide exchange is catalyzed by EF-Ts.
Following peptide bond formation, the GTPase EF-G triggers translocation of the peptidyl–tRNA from the A-site to
the P-site; the empty (deacylated) tRNA exits the ribosome by way of the E-site. The mRNA moves the length of one

codon in the 3 direction, probably pulled through the ribosome by tRNA translocation.

decoding center of the ribosome, located on the small sub- EF-Tu-bound GTP occurs. EF-Tu:GDP is then released
unit. Peptide bond formation occurs on the large ribosomal from the ribosome, and the AA–tRNA occupies the A-
subunit, at the peptidyl transferase center. Thus, this seg- site. While EF-Tu transports all elongator tRNAs amino-
regation of functions parallels the two arms of the tRNA. acylated with natural amino acids to the ribosome, this
The anticodon portion of the tRNA binds to the small sub- factor has negligible affinity for formylated or nonformy-
unit, where the genetic message is read. The acceptor arm lated tRNA fMet . The unpaired first position in the tRNA fMet
of tRNA (with its attached amino acid) contacts the large acceptor stem helix apparently is a negative recognition
subunit, where catalysis occurs. element for EF-Tu:GTP, because this element prevents the
Although the synthesis of a peptide bond is the key initiator tRNA from pairing with internal AUG or GUG
step in translation, this is the easiest part of protein syn- codons.
thesis. Once the amino group of an aminoacyl–tRNA is The elongation factor EF-Ts is a nucleotide exchange
properly positioned close enough to the carbonyl group factor that regenerates active EF-Tu:GTP (from EF-
of a peptidyl–tRNA, peptide bond formation through nu- Tu:GDP) for binding subsequent AA–tRNAs following
cleophilic attack is energetically favorable. The ribosome GTP hydrolysis. Before their functions were known, elon-
can be considered as a single enzyme whose function is gation factors Tu and Ts were named for their observed
to catalyze peptide bond formation. thermal stabilities in vitro—Tu indicates that this protein
is Temperature unstable, while Ts stands for Temperature
stable. In eukaryotes, the two subunits of elongation
B. Elongation Factors
factor EF-1 perform the functions of EF-Tu and EF-Ts.
Addition of each incoming amino acid requires the co- Oncethe A-siteis occupied by the incomingAA–tRNA,
operation of three elongation factors. Elongation factor nucleophilic attack on the peptidyl–tRNA by the AA–
Tu (EF-Tu) is the most abundant protein in E. coli, with tRNA occurs. The condensation reaction produces a new
about 100,000 copies per cell, or 5% of the cell’s protein. peptide bond and lengthens the polypeptide chain by one
This protein is a GTPase, and the EF-Tu:GTP complex amino acid (Fig. 8). As a result, the growing protein is now
specifically binds aminoacyl–tRNAs (AA–tRNAs). For- attached to the A-site tRNA; this addition to and transfer
mation of the ternary complex (EF-Tu:GTP:AA–tRNA) of the polypeptide chain is called transpeptidation.
protects the ester bond (linking the amino acid to its cog- Following formation of the peptide bond, a major re-
nate tRNA) from hydrolysis, and transports the AA–tRNA arrangement of components in the functional center of
to the ribosomal A-site. Once the correct codon–anticodon the ribosome must take place. Because the most recently
interaction is confirmed, ribosome-triggered hydrolysis of entered tRNA has become the peptidyl–tRNA, it must