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催化学报 2010
PossibleRoleofCarboxylandImidazoleGroupsintheCatalysisofPummeloLimonoidGlucosyltransferase, PP. 1445-1451 Keywords: chemicalmodification,catalyticresidue,carboxylgroup,imidazolegroup,uridinediphosphateglucosyltransferase,pummeloalbedo Abstract: ？Limonoidbitternessisaseriousprobleminthecitrusindustryworldwide.Limonoidglucosyltransferaseisanenzymethatcatalyzestheconversionofbitterlimonoidintonon-bitterlimonoidglucosidewhileretainingthehealthbenefitoflimonoidsinthejuice.Theimmobilizationofthisenzymeinacolumncansolvethejuicebitternessproblem.Moreinformationaboutthecatalyticresiduesoftheen-zymeisneededinthisimmobilizationprocess.Glutamate/aspartate,histidine,lysine,tryptophan,serine,andcysteineresidueswerechemi-callymodifiedtoinvestigatetheirrolesinthecatalyticfunctionoflimonoidglucosyltransferase.Inactivationoftheenzymefollowingmodi-ficationofcarboxylandimidazolemoietieswasaconsequenceofalossinsubstratebindingandcatalysisintheglucosyltransferreaction.ThemodificationofasinglehistidineresiduecompletelydestroyedtheabilityoflimonoidglucosyltransferasetotransfertheD-glucopyranosylunit.Tryptophanseemedtohavesomeroleinmaintainingtheactiveconformationofthecatalyticsite.Lysinealsoseemedtohavesomedirectorindirectroleinthiscatalysisbutthemodificationofserineandcysteinedidnothaveanyeffectoncatalysis.There-fore,weconcludethatthecarboxylandimidazolegroupscontainaminoacidsareresponsibleforthecatalyticactionoftheenzyme.
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