= Large ribosomal subunit
The <large ribosomal subunit> is the <ribosomal subunit> containing the <peptidyl transferase center>, where <peptide bonds> are formed during <biological translation>. It contains <ribosomal RNA> and <ribosomal proteins>, and provides a <ribosomal exit tunnel> through which the growing <polypeptide> passes. The <small ribosomal subunit> organizes decoding of <messenger RNA>; synthesis requires coordination between both parts of the <ribosome>.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[From RNA to Protein, functions of the two ribosomal subunits]{ref}
For a reaction example, let one <peptidyl-tRNA> carry a chain of four <amino acid residues> and an incoming <aminoacyl-tRNA> carry one more. <Peptidyl transfer> produces a five-residue chain attached to the incoming <transfer RNA>. The existing chain is transferred and extended; the reaction does not join five free <amino acids> all at once. Subsequent movement of the <ribosome> prepares the complex for another elongation cycle.
A 2000 structural experiment examined an archaeal <large ribosomal subunit> and complexes containing two <substrate analogues>. The analogues contacted conserved <23S ribosomal RNA> in the catalytic region, with no <protein> side-chain atoms within about 18 angstroms of the bond-forming position. This supported the identification of the <ribosome> as a <ribozyme>. The same study found that both <RNA> and <proteins> contribute to the <ribosomal exit tunnel>.<The structural basis of ribosome activity in peptide bond synthesis by Poul Nissen and others>[Abstract: Haloarcula marismortui structures, analogue contacts and exit-tunnel composition]{ref} The structural evidence identifies the reaction environment; it does not make every chemical step or proton-transfer mechanism directly visible.
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