  2.6.1.B18 | 7-cyano-7-carbaguanine in tRNA + NH3 = 7-carboximidamide-guanine in tRNA |
Asp28 deprotonates Cys21, enabling it to nucleophilically attack the nitrile carbon of preQ0 to produce the covalent thioimide intermediate. The ammonium cation, binding in the pocket defined by Asp28 and the pi-system of Tyr90, is deprotonated by Asp28, allowing the neutral ammonia to attack the thioimide carbon from the Tyr90 side (the face of preQ0 facing the protein core). Proton transfers appear to be facilitated in this process via a tightly bound water molecule observed in the preQ0-bound structure that is H-bonded to His62 and the thioimide nitrogen atom. Collapse of the resulting diaminothioorthoester intermediate via cleavage of the carbon-sulfur bond then provides archaeosine-modified tRNA |
-, 743764 |
