Page 11 - Demo

259ANALESRANFwwwanalesranfcomMaternalfructoseintakemodulatescholesterolmetabolisminresponsetoaWesterndietintheoffspringPalomaLópezLaizElenaFausteetalAnRealAcadFarmVol88nº3(2022)·pp257-27514StructureoffructoseglucoseandtagatoseMonosaccharidesarecrystallinesolidsthattendtohaveasweettasteTheyaremadeupbycarbonchainsofvaryinglengthwithaketoneoranaldehydegroupThemostrepresentativemonosaccharidesarethehexosesandamongthemglucoseandfructosedue to their freeavailability. Theyare foundin fruitsandhoney.Tagatoseontheotherhandisreferredtoasararesugar,whichmeansthatitisamonosaccharidethatrarelyexistinnatureThesethree compoundshave structuraldifferencesasitis showninFigure1glucosecontainsanaldehydegroupwhilefructoseandtagatosepresentketonegroupsin carbon 2Fructoseand tagatoseare epimersatcarbon4(22526)Duetothedifferencesintheirstructurethesethreemonosaccharidesundergodifferentmetabolicpathways showninFigure2Fructoseis transportedinto the enterocytes throughglucosetransporter 5(GLUT5)aprocess thatisnotdependentoneithersodiumcotransportoradenosinetriphosphate(ATP)ThenitisreleasedtothebloodstreamthroughGLUT2(1228)AfterwardsfructoseuptakeintothelivercellsmainlyoccursthroughGLUT2byanon-insulindependentmechanism(1 29)Fructoseispreferentiallysent to theliver,whereitisphosphorylated through theactionofanenzymecalledfructokinaseyieldingfructose-1-phosphateThisenzymehasahighaffinityforfructosesothereactionrateishigh(30)AsATPisarequiredsubstrateforthephosphorylationlargequantitiesofAMPareproducedwhichinturnincreaseuricacidsynthesis(2)Fructose-6-phosphateisthencleavedbyaldolaseBproducingglyceraldehydeanddihydroxyacetonephosphate(31)Thesetrioses-phosphatecanbeconvertedintopyruvateandenterthetricarboxylicacidcycletofinallyobtainCO2andH2OEvensopartofthetriosephosphateswillresultinlactateandglucoseformation(viagluconeogenesis)whichcanbeconvertedtoglycogen(21432) Afractionofthecarbonsoffructosebecomeconstituentsoflipids throughaprocesscalleddenovolipogenesis(DNL)(33)Ithasbeendemonstratedthatinfructose-fedratslactateisthesubstancewiththehighestlipogeniccapacity(34)Glucoseisabsorbedintheintestinebyasodium-glucosecotransporter(or sodium-glucoselinked transporter, SGLT)(35)Onceitreaches theliver,itcanbedegraded throughglycolysisIn the firstreaction of thispathway,glucoseisphosphorylated toglucose-6-phosphatebytheactionofglucokinaseThisenzymehasalowaffinityforglucose soits conversiondepends onglucose concentrations(36)Thenglucose-6-phosphateis transformedinto fructose-6-phosphatewhichinthenextstepisconvertedintofructose-16-bisphosphateThislastreactioniscatalysedbytheenzymephosphofructokinasewhichisinhibitedby ATPand citrate(37)Fructose-16-bisphosphateisalsocleavedintoglyceraldehyde-3-phosphateanddihydroxyacetonephosphateBoththestrictregulationofphosphofructokinaseand thelowaffinity ofglucokinase forglucoseallowagreater controlofthespeedatwhichglucoseisdegradedthroughglycolysis(2)Figure2GlucosefructoseandtagatosemetabolismThecolourscorrespondtothemetabolismofoneofthehexosesblue-fructosegreen-glucoseandpurple-tagatoseThebluearrowsalsoindicatehowglucoseandglycogenare formedasaresultof fructosemetabolism TheenzymesarerepresentedinitalicsCreated withBioRender.com