The bifunctional enzyme, formiminotransferase-cyclodeaminase (FTCD) catalyses two independent but sequential reactions in the histidine degradation pathway in mammalian liver.  The full length FTCD is a single chain of 62 kDa and is arranged as a tetramer of dimers resulting in the formation of two different subunit interfaces.  One interface is essential for the transferase reaction and the second interface is necessary for the deaminase reaction  The full length enzyme can channel g-linked polyglutamylated N5 formimino-tetrahydrofolate between the transferase and deaminase active domains with the efficiency of channeling dependent on the length of the polyglutamyl moiety.  Maximal channeling is observed with a pentaglutamylated substrate.  We have recently determined the structures of both the transferase domain (left figure) and the deaminase domain (right figure).  These structures provide us with an initial view of the enzyme and will enable us to identify structural features that mediate the channeling activity.