Figure 9.

Important amino acids in the cADPR metabolism in CD38 (adapted from Ref. 86). CD38 catalyzes the formation of cADPR from NAD⁺ and hydrolysis of cADPR to ADPR.⁸⁰⁾ Lys-129 of CD38 is the cADPR binding site and ATP competes with cADPR for the binding site, resulting in the inhibition of the hydrolysis of cADPR.⁸⁶⁾ [Enzyme-cADPR*] is suggested to be an enzyme-stabilized ADP-ribosyl oxocarbonium ion intermediate. The intermediate is thought to be attacked by H₂O to form ADPR. Cys-119 and Cys-201 are essential for the hydrolysis reaction.⁸²⁾ Glu-226 is essential for ADP-ribosyl cyclase.²⁸⁹⁾