vivo Peptide Bond formation & growth of the
acids in the ribosome are
their respective tRNAs by
ester bond (R - O - R) between the carboxyl terminus and
the amino acceptor stem
(left). During formation of a peptide bond, the ester bond of the amino
acid in the (P)eptidyl site is cleaved, and Peptidyl Transferase catalyzes a reaction (see Note)
between its carboxyl terminus and the amino
terminus of the amino acid in the (A)mino site. This
transfers the P-site
acid to the A-site
amino acid, and the
original amino terminus remains unmodified.
polypeptide thus "grows" from the amino terminus to the
Note: In vitro formation
of the peptide bond is often described as a dehydration reaction
that involves the splitting out of an H20.
However, the in vivo reaction does not.
Note first that the
C terminus of the amino acid in the P site participates in
the ester bond as a carbonyl
radical (-C=O), without a separate -OH group. The peptide bond is formed by a
of the N of the A site amino terminus on this carbonyl C. The -O- of the ester bond
remains attached to the 3'-C
of the tRNA. The
proton on the 2'-C
then shifts to the 3'-C
to restore the -OH
terminus of the uncharged tRNA,
and a proton from the amino terminus of the incoming amino
acid the shifts to the 2'-C.
The in vivo
reaction balances, without production of an H20 molecule.