5′-Nucleotidases and their new roles in NAD+ and phosphate metabolism

5′-Nucleotidase (EC 3.1.3.5) designates a set of enzymes, which catalyze the hydrolysis of ribonucleoside and deoxyribonucleoside monophosphates into the corresponding nucleosides plus orthophosphate. 5′-Nucleotidases are classified according to subcellular localization, nucleobase specificity and their ability to hydrolyze deoxynucleoside monophosphate substrates. Membrane-bound 5′-nucleotidases are ectoenzymes principally involved in salvage of extracellular nucleosides, and often display a preference toward adenosine monophosphate, thereby modulating signal transduction cascades involving purinergic receptors. Cytosolic 5′-nucleotidases are members of the haloacid dehalogenase superfamily of enzymes, which are two-domain proteins containing a modified Rossman fold as the core and a variable cap structure. Extracellular and intracellular 5′-nucleotidase activities participate in purine and pyrimidine salvage to support balanced synthesis of nucleotides, which is critical for maintaining high fidelity DNA replication. While the production of ribonucleosides from ribonucleotides by 5′-nucleotidases remains the most well studied function, it appears that the physiological functions of these activities are more broad. Indeed, Sdt1, previously termed a pyrimidine-specific 5′-nucleotidase, and Isn1, previously termed an inosine monophosphate (IMP)-specific 5′-nucleotidase, have recently been implicated in catabolic processes in nicotinamide adenine dinucleotide (NAD+) metabolism, and are regulated by the NAD+ precursor vitamin nicotinic acid, glucose and phosphate availability in the medium. In addition, Usha, Pho5, Sdt1 and Phm8 are phosphate starvation-induced 5′-nucleotidases with diverse substrate specificities that liberate phosphate under phosphate starvation conditions. Here we review 5′-nucleotidase enzyme structure, catalytic mechanism and substrate specificity and focus on new biological roles for these enzymes in nucleotide, NAD+ and phosphate metabolism.