Pyrimidine-nucleoside phosphorylase

Pyrimidine-nucleoside phosphorylase (EC 2.4.2.2) is an enzyme that catalyzes several phosphorolysis reactions which convert a pyrimidine nucleoside into the corresponding pyrimidine, by cleavage of the α-D-ribose 1-phosphate sugar unit:^[1]

a pyrimidine nucleoside + phosphate (P_i) ${\displaystyle \rightleftharpoons }$ α-D-ribose 1-phosphate + a pyrimidine base

For example, uridine gives uracil:

The reaction is reversible, so this is also how the pyrimidine nucleosides uridine, cytidine, 2'-deoxyuridine, and thymidine can be made in the bacterium Bacillus stearothermophilus in which the enzyme was characterised.^[2][3][4] It has also been described from Enterobacter aerogenes.^[5]

This enzyme belongs to the family of glycosyltransferases, specifically the pentosyltransferases. The systematic name of this enzyme class is pyrimidine-nucleoside:phosphate alpha-D-ribosyltransferase. This enzyme is also called Py-NPase.^[6]