iGen 06-18smc

In vivo Peptide Bond formation & growth of the polypeptide chain

    The amino acids in the ribosome are attached to their respective tRNAs by an 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 unmodifiedThe polypeptide thus "grows" from the amino terminus to the carboxyl terminus.

    Note for the advanced student: 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 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