Fig. 4.

NAD⁺ biosynthesis and homeostasis in yeast. (A) The model of NAD⁺ biosynthesis pathways in yeast. NAD⁺ is produced via de novo synthesis from tryptophan or the salvage of NAD⁺ metabolites from NAD⁺-consuming reactions and from environment. See text for detail steps. (B) A schematic model of extended NAD⁺ pool. Yeast cells constitutively produce, release, and retrieve nicotinamide riboside (NmR). This extended cycle encompasses intracellular and extracellular compartments and might functions as a flexible pyridine nucleotide reservoir.