The amidohydrolase superfamily comprises a remarkable set of enzymes that catalyze the hydrolysis of a wide range of substrates bearing amide or ester functional groups at carbon and phosphorus ...centers. The most salient structural landmark for this family of hydrolytic enzymes is a mononuclear or binuclear metal center embedded within the confines of a (β/α)8-barrel structural fold. Seven variations in the identity of the specific amino acids that function as the direct metal ligands have been structurally characterized by X-ray crystallography. The metal center in this enzyme superfamily has a dual functionality in the expression of the overall catalytic activity. The scissile bond of the substrate must be activated for bond cleavage, and the hydrolytic water molecule must be deprotonated for nucleophilic attack. In all cases, the nucleophilic water molecule is activated through complexation with a mononuclear or binuclear metal center. In the binuclear metal centers, the carbonyl and phosphoryl groups of the substrates are polarized through Lewis acid catalysis via complexation with the β-metal ion, while the hydrolytic water molecule is activated for nucleophilic attack by interaction with the α-metal ion. In the mononuclear metal centers, the substrate is activated by proton transfer from the active site, and the water is activated by metal ligation and general base catalysis. The substrate diversity is dictated by the conformational restrictions imposed by the eight loops that extend from the ends of the eight β-strands.
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-formimino-l-glutamate. The degradation of N-formimino-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-formimino-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.
