<p>Inosine-uridine preferring nucleoside hydrolase (<db_xref db="EC" dbkey="3.2.2.1"/>) (IU-nucleoside hydrolase or IUNH) is an enzyme first identified in protozoan [<cite idref="PUB00000434"/>] that catalyses the hydrolysis of all of the commonly occuring purine and pyrimidine nucleosides into ribose and the associated base, but has a preference for inosine and uridine as substrates. This enzyme is important for these parasitic organisms, which are deficient in <i>de novo</i> synthesis of purines, to salvage the host purine nucleosides. IUNH from <taxon tax_id="5656">Crithidia fasciculata</taxon> has been sequenced and characterised, it is an homotetrameric enzyme of subunits of 34 Kd. An histidine has been shown to be important for the catalytic mechanism, it acts as a proton donor to activate the hypoxanthine leaving group.</p><p>A highly conserved region located in the N-terminal extremity contains four conserved aspartates that have been shown [<cite idref="PUB00000435"/>] to be located in the active site cavity.</p><p>IUNH is evolutionary related to a number of uncharacterised proteins from various biological sources.</p><p>This entry represents the structural domain of IUNH.</p>