Cyclic Adenosine Diphosphate Ribose (cADPR)

Cyclic ADP Ribose is a cyclic adenine nucleotide (like cAMP) with two phosphate groups present on 5' OH of the adenosine (like ADP), further connected to another ribose at the 5' position, which, in turn, closes the cycle by glycosidic bonding to the nitrogen 1 (N¹) of the same adenine base (whose position N⁹ has the glycosidic bond to the other ribose). The N¹-glycosidic bond to adenine is what distinguishes cADPR from ADP-ribose (ADPR), the non-cyclic analog. cADPR is produced from nicotinamide adenine dinucleotide (NAD⁺) by ADP-ribosyl cyclases (EC 3.2.2.5) as part of a second messenger system.

cADPR is a cellular messenger for calcium signaling. It is the physiological allosteric modulator of the ryanodine receptor (RyR), which stimulates calcium-induced calcium release at lower cytosolic concentrations of Ca²⁺.