Three Known Biosynthetic Routes to NAD+ in S. cerevisiae
NAD+ is synthesized from the de novo kynurenine pathway that originates with tryptophan using the Bna1-Bna6 gene products, an import pathway that originates with nicotinic acid, and a salvage pathway that utilizes nicotinamide produced as a function of Sir2-related lysine deacetylases. According to this scheme, nicotinic acid mononucleotide (NaMN) is common to all three pathways and Qns1, the glutamine-dependent NAD+ synthetase that converts nicotinic acid adenine dinucleotide (NaAD) to nicotinamide adenine dinucleotide (NAD+), is required for all pathways (Panozzo et al. 2002, Sandmeier et al. 2002, Bitterman et al. 2002, Anderson et al. 2003, Gallo et al. 2004). Note that Nma1 and Nma2 are schematized as NaMN adenylyltransferases, though they were initially characterized as nicotinamide mononucleotide (NMN) adenylyltransferases (Kornberg 1950, Emanuelli et al. 1999, Emanuelli et al. 2003).