Crystal structure of Urocanase from B. subtilis.
Symbol Urocanase
Pfam PF01175
InterPro IPR000193

Urocanase[1] (also known as imidazolonepropionate hydrolase or urocanate hydratase) is the enzyme that catalyzes the second step in the degradation of histidine, the hydration of urocanate into imidazolonepropionate.

urocanate + H2O \rightleftharpoons 4,5-dihydro-4-oxo-5-imidazolepropanoate

Inherited deficiency of urocanase leads to elevated levels of urocanic acid in the urine, a condition known as urocanic aciduria.

Urocanase is found in some bacteria (gene hutU), in the liver of many vertebrates and has also been found in the plant Trifolium repens (white clover). Urocanase is a protein of about 60 Kd, it binds tightly to NAD+ and uses it as an electrophil cofactor. A conserved cysteine has been found to be important for the catalytic mechanism and could be involved in the binding of the NAD+.


  1. ^ Retey J (1994). "The urocanase story: a novel role of NAD+ as electrophile". Arch. Biochem. Biophys. 314 (1): 1–16.  

External links