ENPP5

Ectonucleotide pyrophosphatase/phosphodiesterase family member 5 (NPP5), the least characterized ENPP family member. Single-pass type I transmembrane ectoenzyme with an N-terminal signal peptide and a C-terminal transmembrane helix; the extracellular catalytic PDE domain belongs to the alkaline phosphatase superfamily and uses a binuclear Zn2+ center for catalysis, with an ENPP5-specific third Zn2+ (Zn3) coordinated by Glu159 and Asp192 near the active site contacting ribose 2'/3' oxygens. Unlike other ENPPs, wild-type human ENPP5 has minimal/undetectable activity toward canonical nucleotides such as ATP, ADP, and UTP; substrate access is restricted by a Tyr73 gatekeeper residue in the nucleotide-binding slot (where most ENPPs have Phe), and a Y73F mutant restores nucleotide hydrolysis. The strongest in-vitro activity is on nicotinamide adenine dinucleotide (NAD+); ENPP5 also cleaves ADP-ribose (ADPR) and UDP-glucose (UDPG), but catalytic efficiency for NAD+ is very low (KM > 1 mM; kcat/KM ~0.00067 s-1 uM-1), implying any physiological role likely occurs in niches with locally elevated substrate (e.g., restricted extracellular microenvironments such as synaptic clefts). A role in NAD-based neurotransmission has been hypothesized but awaits genetic validation in knockout models. Crystal structures of human and murine NPP5 ectodomains are available (PDB 5VEM, 5VEN, 5VEO). ENPP5 has been reported as a plasma biomarker associated with frailty and as a candidate factor in dermal fibroblast senescence, though these associations remain correlative.