Peptide Bond Formation
Elongation of the polypeptide chain occurs on the ribosome. The growing chain attached to a tRNA occupies the peptidyl, P, site of the ribosome (Fig. 7.14). Alongside is another charged tRNA, also with its anticodon base paired to a codon on the mRNA. This second site is termed the acceptor, or A site since the amino acid here will act as an acceptor as the peptide chain is transferred to it. This transfer is catalyzed by the peptidyl transferase activity of the large subunit. Although this activity is stimulated by ribosomal proteins, in vitro it can be catalyzed by the ribosomal RNA itself. This step does not require an external source of energy like GTP.