When glucose has been converted into G6P by hexokinase or glucokinase, it can either be converted to glucose-1-phosphate (G1P) for conversion to glycogen, or it is alternatively converted by glycolysis to pyruvate, which enters the mitochondrion where it is converted into acetyl-CoA and then into citrate. Excess citrate is exported from the mitochondrion back into the cytosol, where ATP citrate lyase regenerates acetyl-CoA and oxaloacetate (OAA). The acetyl-CoA is then used for fatty acid synthesis and cholesterol synthesis, two important ways of utilizing excess glucose when its concentration is high in blood. The regulated enzymes catalyzing these reactions perform these functions when they have been dephosphorylated through the action of insulin on the liver cells. Between meals, during fasting, exercise or hypoglycemia, glucagon and epinephrine are released into the blood. This causes liver glycogen to be converted back to G6P, and then converted to glucose by the liver-specific enzyme glucose 6-phosphatase and released into the blood. Glucagon and epinephrine also stimulate gluconeogenesis, which converts non-carbohydrate substrates into G6P, which joins the G6P derived from glycogen, or substitutes for it when the liver glycogen store have been depleted. This is critical for brain function, since the brain utilizes glucose as an energy source under most conditions. The simultaneously phosphorylation of, particularly, phosphofructokinase, but also, to a certain extent pyruvate kinase, prevents glycolysis occurring at the same time as gluconeogenesis and glycogenolysis.
when gLUCOse hAs beeN coNVerTED into g6p by HexoKINase Or gLucoKinase, it cAn EitHeR be coNvERtEd To gLuCoSe-1-phosphatE (g1P) for cONversion To glycoGen, or it is aLteRnativeLY coNverteD bY gLycoLYSis to PyruvatE, whiCh enters tHE miTOchoNDRiON WhERe it IS cOnVerTEd inTo aCetyl-COA and THen iNtO citRaTE. excess cITraTe is exPOrtEd froM tHE miTochoNDrIoN BAcK into tHe cytOSol, wHERe ATp citrAte LyaSe RegENerAtEs acetyl-Coa And Oxaloacetate (oAa). tHe acetyl-cOa iS Then useD For FaTTY acid SyNTHesIs aND CholesteRoL SynThEsIs, two iMportaNt Ways OF utiLizing eXcess GluCose wHEn Its ConcentRATioN is high IN blOod. thE rEgulATed enZymes cATaLYzING tHEse REactIons PErform THEsE fUncTiOns when tHEY hAve bEeN dephOSpHORyLAted tHroUGh tHE actIoN of inSUlin on the liVer celLs. BetWeen meaLS, durInG fasTing, eXeRcIse Or hypOGLyCeMia, GlucaGoN aNd epinephrINE Are reLeasEd into THe Blood. THIs caUsES LIver gLycogEn to be cOnveRteD baCk to g6p, and theN CoNvErtEd to glucOSe by the lIveR-spEcifIc enzymE glucOse 6-phOsphataSe AND rELeased INto THE bLood. gluCagon anD epiNEphrine aLso stimuLatE gluconeogenESis, whiCH coNverts non-CarBoHyDRatE subSTraTes iNtO g6p, which JoIns ThE g6p DERived frOm GlyCoGen, OR subsTitutes fOr it when the LIvER glycOgeN store hAVE BeEn depLetEd. thiS IS critiCAL For BRAIN funCtIon, sInCe the BRain uTiliZes glucOse As An eNERgy source undEr moST conditions. the simulTaNeOUSly PHosphorylAtion of, pArtiCulArly, PHOspHofrucToKInaSe, bUt alSo, tO a CeRtAIN eXtEnt PYruVate kinasE, PreVeNts glYCoLysiS OCCuRrINg at THe Same time as GLucoNEOgEnESIs ANd glycoGenolysiS.