Daohai Zhang, Nan Li, S. Lok
Sep 12, 2003
Citations
11
Influential Citations
49
Citations
Journal
Journal of Biological Chemistry
Abstract
Isomaltulose synthase from Klebsiella sp. LX3 (PalI, EC 5.4.99.11) catalyzes the isomerization of sucrose to produce isomaltulose (α-d-glucosylpyranosyl-1,6-d-fructofuranose) and trehalulose (α-d-glucosylpyranosyl-1,1-d-fructofuranose). The PalI structure, solved at 2.2-Å resolution with an R-factor of 19.4% and Rfree of 24.2%, consists of three domains: an N-terminal catalytic (β/α)8 domain, a subdomain between Nβ3 and Nα3, and a C-terminal domain having seven β-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.