Editing Ricin (section)

=== Depurination reaction ===
Within the active site of RTA, there exist several invariant amino acid residues involved in the [[depurination]] of ribosomal RNA.<ref name=pmid14579547/> Although the exact mechanism of the event is unknown, key amino acid residues identified include [[tyrosine]] at positions 80 and 123, [[glutamic acid]] at position 177, and [[arginine]] at position 180. In particular, Arg180 and Glu177 have been shown to be involved in the [[catalytic]] mechanism, and not substrate binding, with [[enzyme kinetics|enzyme kinetic]] studies involving RTA mutants. The model proposed by Mozingo and Robertus,<ref name=pmid7990130/> based on X-ray structures, is as follows:
# Sarcin-ricin loop substrate binds RTA active site with target adenine stacking against tyr80 and tyr123.
# Arg180 is positioned such that it can [[Protonation|protonate]] ''N''-3 of adenine and break the bond between ''N''-9 of the adenine ring and ''C''-1' of the [[ribose]].
# [[Bond cleavage]] results in an [[oxycarbonium]] ion on the ribose, stabilized by Glu177.
# ''N''-3 protonation of adenine by Arg180 allows [[deprotonation]] of a nearby water molecule.
# Resulting [[hydroxyl]] attacks ribose [[carbonium ion]].
# Depurination of adenine results in a neutral ribose on an intact phosphodiester RNA backbone.