sentences:  Mono-ADP ribosyltransferases commonly catalyze the addition of ADP-ribose to arginine side chains using a highly conserved R-S-EXE motif of the enzyme.The reaction proceeds by breaking the bond between nicotinamide and ribose to form an oxonium ion.Next, the arginine side chain of the target protein then acts a nucleophile, attacking the electrophilic carbon adjacent to the oxonium ion.