As described in Section 25.5B, acid-base catalysis is believed to be the mechanism by which ribosomes catalyze the formation of peptide bonds in the process of protein translation. Key to this proposal is assistance by the N3 nitrogen (highlighted in the scheme on the next page) of a nearby adenine in the ribosome for the removal of a proton from the a-amino group of the amino acid adding to the growing peptide chain. The ability of this adenine group to remove the proton is, in turn, apparently facilitated by relay of charge made possible by other nearby groups in the ribosome. The constellation of these groups is shown in the scheme. Draw mechanism arrows to show formation of a resonance contributor wherein the adenine group could carry a formal negative charge, thereby facilitating its removal of the a-amino proton of the amino acid. (The true electronic structure of these groups is not accurately represented by any single resonance contributor, of course. A hybrid of the contributing resonance structures weighted according to stability would best reflect the true structure.)

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Ribosome G2102 (2061) O. NH iA2486 (2451) G2482 (2447) .H This nitrogen is believed to be involved in proton transfers during the peptide bond-forming reaction. NH Ribosome N N-H O2 A2485 (2450) он