NAD+ Metabolism in Humans and Yeast
A revised scheme for NAD+ metabolism in which double arrows depict steps common to yeast and humans (with yeast gene names). Single arrows depict steps unique to humans (PBEF, nicotinamide phosphoribosyltransferase) and yeast (Pnc1, nicotinamidase). Additional gene products are responsible for some of the steps. For example, there are multiple Sir2-related lysine deacetylases in humans and yeast and additional NAD+ glycohydrolases in humans along the arrow marked Sir2, and there are two Nrk enzymes in humans along the arrow marked Nrk1. The scheme remains incomplete in that there are enzyme and E.C. names for some additional possible steps in NAD+ biosynthesis, particularly for breakdown of NAD+ to nicotinamide riboside (Magni et al., 2004), that have not been genetically validated.