In vivo Peptide Bond formation
& growth of the polypeptide chain
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 in the (P)eptidyl site is
cleaved, and Peptidyl
Transferase catalyzes a condensation reaction
between its carboxyl terminus and the amino
terminus of the amino acid in the (A)mino site. This transfers
amino 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 a peptide bond is a dehydration
reaction that splits out of an H20.
However, the in vivo reaction is a condensation reaction. Because the C-terminus
of the amino acid in the P site is joined to the 3'
terminus of the tRNA by an ester bond, it
participates in peptide bond formation as a carbonyl
radical (-C=O) without an -OH radical.
When the C-terminus joins with the NH2
terminus of the amino acid in the A site, the end result is the shift
of a proton (-H) from the amino terminus to the
uncharged tRNA molecule. This balances the rection,
and forms a peptide bond without release of an H20 molecule.
Figure © 2010 PJ Russell, iGenetics 3rd ed.; all text material © 2013 by Steven M. Carr