 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. 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.^[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.^[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.