title: At the periphery of the amidohydrolase superfamily: Bh0493 from Bacillus halodurans catalyzes the isomerization of D-galacturonate to D-tagaturonate.
abstract: The amidohydrolase superfamily is a functionally diverse set of enzymes that catalyzes predominantly hydrolysis reactions involving sugars, nucleic acids, amino acids, and organophosphate esters. One of the most divergent members of this superfamily, uronate isomerase from Escherichia coli, catalyzes the isomerization of d-glucuronate to d-fructuronate and d-galacturonate to d-tagaturonate and is the only uronate isomerase in this organism. A gene encoding a putative uronate isomerase in Bacillus halodurans (Bh0705) was identified based on sequence similarity to uronate isomerases from other organisms. Kinetic evidence indicates that Bh0705 is relatively specific for the isomerization of d-glucuronate to d-fructuronate, confirming this functional assignment. Despite a low sequence identity to all other characterized uronate isomerases, phylogenetic and network-based analysis suggests that a second gene in this organism, Bh0493, is also a uronate isomerase, although it is an outlier in the group, with <20% sequence identity to any other characterized uronate isomerase from another species. The elucidation of the X-ray structure at a resolution of 2.0 A confirms that Bh0493 is a member of the amidohydrolase superfamily with conserved residues common to other members of the uronate isomerase family. Functional characterization of this protein shows that unlike Bh0705, Bh0493 can utilize both d-glucuronate and d-galacturonate as substrates. In B. halodurans, Bh0705 is found in an operon for the metabolism of d-glucuronate, whereas Bh0493 is in an operon for the metabolism of d-galacturonate. These results provide the first identification of a uronate isomerase that operates in a pathway distinct from that for d-glucuronate. While most organisms that contain this pathway have only one gene for a uronate isomerase, sequence analysis and operon context show that five other organisms also appear to have two genes and one organism appears to have three genes for this activity.
authors.affiliation: Department of Chemistry, P.O. Box 30012, Texas A&M University, College Station, Texas 77842-3012, USA.
authors.forename: Tinh T
authors.initials: TT
authors.lastname: Nguyen
authors.forename: Shoshana
authors.initials: S
authors.lastname: Brown
authors.forename: Alexander A
authors.initials: AA
authors.lastname: Fedorov
authors.forename: Elena V
authors.initials: EV
authors.lastname: Fedorov
authors.forename: Patricia C
authors.initials: PC
authors.lastname: Babbitt
authors.forename: Steven C
authors.initials: SC
authors.lastname: Almo
authors.forename: Frank M
authors.initials: FM
authors.lastname: Raushel
mesh_terms: D000581:Amidohydrolases / Q000737:chemistry / Q000145:classification / Q000302:isolation & purification / Q000378:metabolism*; D001407:Bacillus / Q000201:enzymology* / Q000235:genetics; D001665:Binding Sites; D002384:Catalysis; D002876:Chromosomes, Bacterial / Q000235:genetics; D019295:Computational Biology; D018360:Crystallography, X-Ray; D006603:Hexuronic Acids / Q000737:chemistry / Q000378:metabolism*; D007536:Isomerism; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015394:Molecular Structure; D010802:Phylogeny; D021122:Protein Subunits / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment
chemical_list: D006603:Hexuronic Acids; D021122:Protein Subunits; C007819:galacturonic acid; D000581:Amidohydrolases