The NAMPT enzyme employs a switch that directly senses AMP/ATP and regulates cellular responses to energy stress.

Nicotinamide adenine dinucleotide (NAD) is a crucial compound in energy metabolism and cell signaling. Nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme responsible for NAD biosynthesis from nicotinamide (NAM). Here, we report that NAMPT activity is inhibited by adenosine monophosphate (AMP) in response to energy stress. Our global metabolite-protein interaction mapping reveals that NAMPT differentially interacts with AMP from fasted mouse livers. Crystal structures of NAMPT-AMP show that AMP binds similarly to the NAMPT reaction product, nicotinamide mononucleotide (NMN). The inhibition of NAMPT by AMP can be relieved by NAMPT activators or adenosine triphosphate (ATP), likely in a competitive manner. Based on these findings, we further investigated upstream factors contributing to AMP accumulation and found that activation of purine synthesis unexpectedly promotes the rise of AMP during fasting. Notably, an increased AMP/ATP ratio correlates with NAD decline in ischemic stroke models, in which NAMPT activators can otherwise confer protection.