title: Isomaltulose synthase (PalI) of Klebsiella sp. LX3. Crystal structure and implication of mechanism.
abstract: Isomaltulose synthase from Klebsiella sp. LX3 (PalI, EC 5.4.99.11) catalyzes the isomerization of sucrose to produce isomaltulose (alpha-D-glucosylpyranosyl-1,6-D-fructofuranose) and trehalulose (alpha-D-glucosylpyranosyl-1,1-d-fructofuranose). The PalI structure, solved at 2.2-A resolution with an R-factor of 19.4% and Rfree of 24.2%, consists of three domains: an N-terminal catalytic (beta/alpha)8 domain, a subdomain between N beta 3 and N alpha 3, and a C-terminal domain having seven beta-strands. The active site architecture of PalI is identical to that of other glycoside hydrolase family 13 members, suggesting a similar mechanism in substrate binding and hydrolysis. However, a unique RLDRD motif in the proximity of the active site has been identified and shown biochemically to be responsible for sucrose isomerization. A two-step reaction mechanism for hydrolysis and isomerization, which occurs in the same pocket is proposed based on both the structural and biochemical data. Selected C-terminal truncations have been shown to reduce and even abolish the enzyme activity, consistent with the predicted role of the C-terminal residues in the maintenance of enzyme conformation and active site topology.
authors.affiliation: Department of Pathology, National University of Singapore, 5 Lower Kent Ridge Road, Singapore 119074.
authors.forename: Daohai
authors.initials: D
authors.lastname: Zhang
authors.forename: Nan
authors.initials: N
authors.lastname: Li
authors.forename: Shee-Mei
authors.initials: SM
authors.lastname: Lok
authors.forename: Lian-Hui
authors.initials: LH
authors.lastname: Zhang
authors.forename: Kunchithapadam
authors.initials: K
authors.lastname: Swaminathan
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D018360:Crystallography, X-Ray; D005822:Genetic Vectors; D019751:Intramolecular Transferases / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D007709:Klebsiella / Q000201:enzymology*; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism
chemical_list: D011994:Recombinant Proteins; D019751:Intramolecular Transferases; C042002:isomaltulose synthase