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. Author manuscript; available in PMC: 2010 Jan 5.
Published in final edited form as: Bioorg Chem. 2005 Feb 23;33(3):229–251. doi: 10.1016/j.bioorg.2005.01.001

Fig. 4.

Fig. 4

Two potential chemical mechanisms for the TGT reaction involving two aspartates. In the “associative mechanism,” an aspartate acts as a nucleophile to displace guanine. In the “dissociative mechanism,” an aspartate stabilizes an oxocarbenium ion intermediate. In both mechanisms, a general acid (either enzymic or water) serves as a proton donor to facilitate the departure of guanine.