The structure of Geobacillus stearothermophilus purine nucleoside phosphorylase, an enzyme of biocatalytic interest, is reported and was found to include the presence of an N-terminal tag in the active site of each subunit that belongs to the other subunit in each dimer. This serves as a warning that while tags enable simple and economic purification for proteins of industrial interest, care needs to be taken to ensure that they do not have unwanted effects on protein structure and function.