CASE FILES: BIOCHEMISTRY
Initiation requires the smaller ribosomal subunit, an ini-
tiation tRNA (with a 5'-CAU-3' anticodon), mRNA with its initiator
codon (5'-AUG-3'), and several initiation factors, all of which form the
ribosomal initiation complex. The initiation complex is near completion
when a tRNA carrying the amino acid methionine hydrogen bonds to
the AUG codon on the mRNA. When these components are in place, the
larger ribosomal subunit joins the complex in such a way that the initiator
met-tRNAmet is localized in the P- or peptidyl site (Figure 9-1). The chain
initiation phase ends and a second amino acid can be inserted.
Elongation occurs by bringing an aminoacyl-charged
tRNA to the A- or acceptor site, followed by formation of the peptide
bond and the translocation of a tRNA from the A- to the P-site. Proteins
called elongation factors are the workhorses in the process of elongating
the nascent polypeptide chain by one amino acid at a time. Translation
elongation factors are involved in the following three-step cycle:
Codon recognition: A hydrogen bond forms between the mRNA
codon and the anticodon of the next aminoacyl tRNA at the empty
A-site of the ribosome.
Peptide bond formation: An enzyme called peptidyl transferase
embedded in the large ribosomal subunit catalyzes the formation of a
peptide bond between the polypeptide of the peptidyl-tRNA in the
P-site and the newly arrived aminoacyl tRNA in the A-site.
Translocation: The tRNA in the P-site is ejected so that the newly
formed peptidyl-tRNA located in the
-site can shift over to the
freeing up the
-site for the next aminoacyl-charged tRNA.
Elongation comes to an end when one (or more) of
three stop codons (UAA, UAG, UGA) is encountered. A protein called
a release factor binds directly to the termination codon in the A-site. The
newly synthesized protein is hydrolyzed from the tRNA and both the
tRNA and the protein are released from the ribosome.
Figure 9-1. Various sites of antibiotic action in protein synthesis.