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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. 6.

Fig. 6

Chemical mechanism for the TGT reaction that is consistent with biochemical and structural data. Two active-site aspartate residues serve critical roles in catalysis by TGT. Aspartate 264 nucleophilically attacks the 1′-ribosyl carbon, resulting in a TGT–RNA covalent linkage and the displacement of the guanine base. It is hypothesized that aspartate 89 might be responsible for protonation of the guanine base and/or deprotonation of the incoming preQ1 (perhaps through the intermediacy of water). While guanine is shown being protonated at N7, protonation could be directly at N9.