The roles of the residues in the catalytic active site of pyrimidine nucleoside phosphorylase from Bacillis subtilis have been elucidated using hybrid quantum-mechanical/molecular-mechanical methods.

Pyrimidine nucleoside phosphorylase functions in the nucleotide synthesis salvage pathway by catalyzing the reversible phosphorolysis of pyrimidines. Both uridine and thymidine are substrates.

One unit will convert 1 μmole each of 2′-deoxyuridine and phosphate to uracil and 2-deoxyribose 1-phosphate per minute at pH 7.4 and 37 °C.