1.3 Formation of glycosidic linkagesrequires energy and is catalysed byspecific enzymes
1.3 Formation of glycosidic linkages requires energy and is catalysed by specific enzymes
Figure 1.8Formationofaglycosidic linkageHO6_OH4OH2A53OH+HOHO251B3OHHOHO2HOGalactoseGlucoseH20HOβ1-4GlycosidicLOH6ReducinglinkageL4福.OHend6N5anO3HO25HO3OH2HO-NonreducingHOendGalβ1-4Glc=LactoseTaylorandDrickamerIntroductiontoGlycobiology
Uridinediphosphate(UDP）OHHOOCH2?UridineNHOO=O=HOO-P-O-P-O-CH2Galactose--O-O-HOOHRiboseTaylor and DrickamerIntroduction to GlycobiologyGal+UDP+2ATP→UDP-Gal+2ADP+2PCreationofglycosidicbondUDP-Gal+GlcGalB1-4Glc+UDP→TaylorandDrickamerIntroduction toGlycobiology18
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UDP-galactose: glucose β(1,4)-galactosyltransferaseDifferent transferases forsynthesis of lactose and N-acetyl-lactosamine (Galβ1-4GlcNAc)One enzyme-one linkage ruleRecognizemorethanonemannoseacceptorNeuAcd2-3Galβ14GlcNAcβ1-2Manα16Manβ1-4GlcNAcβ1-4GlcNAcβ1-AsnNeuAcd2-3Galβ1/4GlcNAcβ1-2ManaNeuAcGIcNActionGalOManTayiorandDrickamerIntroductiontoGlycobiology
UDP4galactose:(glucose(β(1,4)4galactosyltransferase Different(transferases(for(synthesis(of(lactose(and(N4 acetyl4lactosamine((Galβ144GlcNAc) One(enzyme4one(linkage(rule Recognize(more(than(one(mannose(acceptor(
But,Thereare oftenseveral differentglycosyltransferaseswithsimilaror overlapping specificities.Six sialyltransferases can each add sialic acid in 2-3linkagetogalactoseBreakingof glycosidiclinkages reguiresenzymes,but doesnot require the input of energy.Glycosidasescatalysetheenergeticallyfavourablehydrolysis of glycosidic linkagesSpecific,but Sialidasedoes notdiscriminatebetweena2-3and α2-6 linked NeuAc
But,( There(are(oKen(several(different(glycosyltransferases(with( similar(or(overlapping(specifici%es.( Six(sialyltransferases(can(each(add(sialic(acid(in(243linkage( to(galactose Breaking(of(glycosidic(linkages(requires(enzymes,(but(does( not(require(the(input(of(energy.( Glycosidases catalyse(the(energe%cally(favourable hydrolysis(of(glycosidic(linkages( Specific,(but(Sialidase(does(not(discriminate(between(α243( and(α246(linked(NeuAc