Extended Data Fig. 6: Schematic illustration of the proposed mechanism for OspC3-catalyzed arginine ADP-riboxanation.

In the four-step reaction, E326 together with D231 in OspC3 first deprotonates N-ribose 2′-OH of NAD⁺ to promote scission of the glycosidic bond between the Nam and the ADPR. This generates an oxocarbenium cation of the N-ribose. Subsequently, the ADPR rotates its β-phosphate, bringing the oxocarbenium cation close to the D231-deprotonated arginine. D231 occupies the position of arginine N^ω, inducing a crooked conformation of the guanidine and exposing its N^δ for attacking C1 of the N-ribose. Following this initial ADP-ribosylation, D177, now becoming close to N-ribose 2′-OH, not only fixes the flipped N-ribose in a proper position but also acts as a base to activate the 2′-OH to attack the guanidine carbon, thereby completing the catalysis of ADP-riboxanation.