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Peptide bond formation on the ribosome requires that aminoacyl-tRNAs and peptidyl-tRNAs are properly positioned on the A site and the P site of the peptidyl transferase center (PTC) so that nucleophilic attack can occur. Here we analyse some constraints associated with the induced-fit mechanism of the PTC, that promotes this positioning through a compaction around the aminoacyl ester orchestrated by U2506. The physical basis of PTC decompaction, that allows the elongated peptidyl-tRNA to free itself from that state and move to the P site of the PTC, is still unclear. From thermodynamics considerations and an analysis of published ribosome structures, the present work highlights the rational of this mechanism, in which the free-energy released by the new peptide bond is used to kick U2506 away from the reaction center. Furthermore, we show the evidence that decompaction is impaired when the nascent peptide is not yet anchored inside the exit tunnel, which may contribute to explain why the first rounds of elongation are inefficient, an issue that has attracted much interest for about two decades. Results in this field are examined in the light of the present analysis and a physico-chemical correlation in the genetic code, which suggest that elementary constraints associated with the size of the side-chain of the amino acids penalize early elongation events.

Peptidyl transferase center decompaction and structural constraints during early protein elongation on the ribosome