The pyridine nucleotides, NAD + , NADH, NADP + , and NADPH have long-established and well-characterised roles as redox factors in processes such as oxidative phos- phorylation, the TCA cycle, and as electron acceptors in photosynthesis. Recent years have seen an increase in the number of signalling and gene regulatory processes where NAD + or NADP + are metabolised. Cyclic ADP-ribose (cADPR) and nico- tinic acid adenine dinucleotide phosphate (NAADP) are metabolites of NAD + and NADP + , respectively, and now have widely accepted roles as potent intracellular calcium releasing agents in animals, but are less well characterised in plants. NAD kinases catalyse the transfer of a phosphate group from ATP to NAD to form NADP and are well characterised in plants in their requirement for the calcium binding protein calmodulin, thereby putatively linking their regulation to stress-induced intracellular calcium release. A second group of proteins unrelated to those above, the sirtuins (Sir2) and poly ADP-ribose polymerases (PARPs), cleave NAD and transfer the ADP-ribose group to acetyl groups and proteins, respectively. These have roles in transcriptional control and DNA repair in eukaryotes.
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