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
the P-site 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 for the advanced
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 reaction, and forms a peptide bond without release
of an H20 molecule.
Figure ©2010 PJ Russell, iGenetics
3rd ed.; all text material ©2014 by Steven M. Carr