Annotating enzymes of unknown function: N-formimino-L-glutamate deiminase is a member of the amidohydrolase superfamily

The functional assignment of enzymes that catalyze unknown chemical transformations is a difficult problem. The protein Pa5106 from Pseudomonas aeruginosa has been identified as a member of the amidohydrolase superfamily by a comprehensive amino acid sequence comparison with structurally authenticated members of this superfamily. The function of Pa5106 has been annotated as a probable chlorohydrolase or cytosine deaminase. A close examination of the genomic content of P. aeruginosa reveals that the gene for this protein is in close proximity to genes included in the histidine degradation pathway. The first three steps for the degradation of histidine include the action of HutH, HutU, and HutI to convert L-histidine to N-forrnimino-L-glutamate. The degradation of N-forrnimino-L-glutamate to L-glutamate can occur by three different pathways. Three proteins in P. aeruginosa have been identified that catalyze two of the three possible pathways for the degradation of N-formimino-L-glutamate. The protein Pa5106 was shown to catalyze the deimination of N-formimino-L-glutamate to ammonia and N-formyl-L-glutamate, while Pa5091 catalyzed the hydrolysis of N-formyl-L-glutamate to formate and L-glutamate. The protein Pa3175 is dislocated from the hut operon and was shown to catalyze the hydrolysis of N-forrnimino-L-glutamate to formamide and L-glutamate. The reason for the coexistence of two alternative pathways for the degradation of N-formimino-L-glutamate in P. aeruginosa is unknown.