title: A catalytic mechanism revealed by the crystal structures of the imidazolonepropionase from Bacillus subtilis.
abstract: Imidazolonepropionase (EC 3.5.2.7) catalyzes the third step in the universal histidine degradation pathway, hydrolyzing the carbon-nitrogen bonds in 4-imidazolone-5-propionic acid to yield N-formimino-l-glutamic acid. Here we report the crystal structures of the Bacillus subtilis imidazolonepropionase and its complex at 2.0-A resolution with substrate analog imidazole-4-acetic acid sodium (I4AA). The structure of the native enzyme contains two domains, a TIM (triose-phosphate isomerase) barrel domain with two insertions and a small beta-sandwich domain. The TIM barrel domain is quite similar to the members of the alpha/beta barrel metallo-dependent hydrolase superfamily, especially to Escherichia coli cytosine deaminase. A metal ion was found in the central cavity of the TIM barrel and was tightly coordinated to residues His-80, His-82, His-249, Asp-324, and a water molecule. X-ray fluorescence scan analysis confirmed that the bound metal ion was a zinc ion. An acetate ion, 6 A away from the zinc ion, was also found in the potential active site. In the complex structure with I4AA, a substrate analog, I4AA replaced the acetate ion and contacted with Arg-89, Try-102, Tyr-152, His-185, and Glu-252, further defining and confirming the active site. The detailed structural studies allowed us to propose a zinc-activated nucleophilic attack mechanism for the hydrolysis reaction catalyzed by the enzyme.
authors.affiliation: The National Laboratory of Protein Engineering and Plant Genetic Engineering, College of Life Sciences, Peking University, Beijing 100871, China.
authors.forename: Yamei
authors.initials: Y
authors.lastname: Yu
authors.forename: Yu-He
authors.initials: YH
authors.lastname: Liang
authors.forename: Erik
authors.initials: E
authors.lastname: Brostromer
authors.forename: Jun-Min
authors.initials: JM
authors.lastname: Quan
authors.forename: Santosh
authors.initials: S
authors.lastname: Panjikar
authors.forename: Yu-Hui
authors.initials: YH
authors.lastname: Dong
authors.forename: Xiao-Dong
authors.initials: XD
authors.lastname: Su
mesh_terms: D000581:Amidohydrolases / Q000737:chemistry*; D000595:Amino Acid Sequence; D001412:Bacillus subtilis / Q000201:enzymology*; D001426:Bacterial Proteins / Q000737:chemistry*; D001665:Binding Sites; D002384:Catalysis; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000201:enzymology; D008956:Models, Chemical; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid
chemical_list: D001426:Bacterial Proteins; D000581:Amidohydrolases; C092896:imidazolonepropionase, Bacillus subtilis