= Peptidyl transferase center
The <peptidyl transferase center> is the catalytic region of the large <ribosomal subunit> where a growing peptide is transferred between <transfer RNAs> during <translation (biology)>. An amino group on the incoming aminoacyl-tRNA attacks the ester linking the existing chain to the P-site tRNA. A new <peptide bond> forms, leaving the extended chain attached to the A-site tRNA. Thus the reaction transfers a chain between tRNAs rather than joining two free amino acids in solution.<The structural basis of ribosome activity in peptide bond synthesis by Poul Nissen and others>[Science 289, 920–930: substrate-analogue complexes and location of peptide-bond synthesis]{ref}
For example, if the P-site tRNA carries a three-residue chain and the A-site tRNA carries one amino acid, the product is a four-residue chain on the A-site tRNA plus a deacylated P-site tRNA. Subsequent ribosomal movement places that peptidyl-tRNA for another round. The <peptidyl transferase center> performs the bond-forming step; decoding and translocation are coordinated but distinct functions of the <ribosome>.
A decisive structural experiment placed substrate analogues in crystals of an archaeal large subunit. The analogues occupied a cavity formed by conserved <ribosomal RNA>; in those structures the nearest protein side-chain atoms were about 18 angstroms from the bond-forming position. This strongly supported an RNA-based catalytic center and the description of the <ribosome> as a <ribozyme>.<The structural basis of ribosome activity in peptide bond synthesis by Poul Nissen and others>[Abstract and Figures 4–7, RNA enclosure and protein-distance observation; original scan https://pages.ucsd.edu/~msereno/170/readings/16-RiboCatalysis.pdf]{ref}
The result identifies the catalytic environment, but a static structure alone does not determine every proton-transfer step or every kinetic contribution of neighboring groups. Structural placement, chemical perturbations and reaction-rate measurements answer complementary mechanistic questions.
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