FIG. 5.

Pathways for NAD⁺ and nicotinamide metabolism in S. cerevisiae. In yeast, NAD⁺ can be recycled from nicotinamide via the NAD⁺ salvage pathway or synthesized de novo from tryptophan. Nicotinamide generated by Sir2 is converted into nicotinic acid by the nicotinamidase Pnc1 and subsequently into NaMN by Npt1. Nicotinic acid may also enter the pathway exogenously. The formation of desamido-NAD⁺ (NaAD) is catalyzed by one of two adenylyltransferases encoded by NMA1 and NMA2, and the subsequent formation of NAD⁺ by the NAD⁺ synthetase Qns1. Trptophan taken up from the medium is converted into quinolinic acid by Bna1 to Bna5. A quinolinic acid phosphoribosyltransferase, encoded by BNA6/QPT1, catalyzes the subsequent conversion to NaMN, which feeds into the salvage pathway.