5'-nucleotidases (EC 3.1.3.5) [1] are enzymes that catalyze the hydrolysis of phosphate esterified at carbon 5' of the ribose and deoxyribose portions of nucleotide molecules. 5'-nucleotidase is a ubiquitous enzyme found in a wide variety of species and which occurs in different cellular locations. The sequences of the following 5'-nucleotidases are currently known:

• Extracellular 5'-nucleotidase from mammals and electric ray. This isozyme is a homodimeric disulfide-bonded glycoprotein attached to the membrane by a GPI-anchor. It requires zinc for its activity
• Vibrio parahaemolyticus 5'-nucleotidase (gene nutA). This isozyme is bound to the membrane by a lipid chain. NutA requires chloride and magnesium ions for its activity. It is involved in degrading extracellular 5'-nucleotides for nutritional needs.
• Periplasmic bacterial 5'-nucleotidase (gene ushA). This enzyme, also known as UDP-sugar hydrolase (EC 3.6.1.45), can degrade UDP-glucose and other nucleotide diphosphate sugars. It produces sugar-1-phosphate which can then be used by the cell. UshA seems to require cobalt for its activity.

5'-nucleotidases are evolutionary related to the following enzyme:

• Periplasmic bacterial 2',3'-cyclic-nucleotide 2'-phosphodiesterase (EC 3.1. 4.16) (gene cpdB). This bifunctional enzyme catalyzes two consecutive reactions: it first converts 2',3'-cyclic-nucleotide to 3'-nucleotide and then acts as a 3'-nucleotidase.
• Mosquito apyrase (EC 3.6.1.5) (ATP-diphosphohydrolase) [2]. This enzyme catalyzes the hydrolysis of ATP into AMP and facilitates hematophagy by preventing ADP-dependent platelet aggregation in the host.

All these proteins share a number of regions of sequence similarity, of which we selected two as signature patterns. Both regions are located in the N-terminal section of these enzymes. The second pattern contains a perfectly conserved pentapeptide.