Isomaltulose Synthase ( Pal I) of Klebsiella sp. LX3 CRYSTAL STRUCTURE AND IMPLICATION OF MECHANISM*

Isomaltulose synthase from Klebsiella sp. LX3 ( Pal I, EC 5.4.99.11) catalyzes the isomerization of sucrose to produce isomaltulose ( (cid:1) - D -glucosylpyranosyl-1,6- D fructofuranose) and trehalulose ( (cid:1) - D -glucosylpyranosyl-1,1- D -fructofuranose). The Pal I structure, solved at 2.2-Å resolution with an R -factor of 19.4% and R free of 24.2%, consists of three domains: an N-terminal catalytic ( (cid:2) / (cid:1) ) 8 domain, a subdomain between N (cid:2) 3 and N (cid:1) 3, and a C-terminal domain having seven (cid:2) -strands. The active site architecture of Pal I is identical to that of other glycoside hydrolase family 13 members, suggesting a similar mechanism in substrate binding and hydrolysis. How-ever, 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