The level of the allosteric regulatory metabolite fructose-2,6-bisphosphate (F-2,6-bisP) can be changed by altering the activity of the bifunctional enzyme phosphofructokinase-2/fructose-2,6-bisphosphatase (PKFBP). Insulin/Akt-activated protein phosphatase promotes the dephosphorylation of PKFBP, activating its kinase function and reducing its phosphatase function. This leads to the phosphorylation of fructose-6-phosphate (F6P) to fructose-2,6-bisphosphate (F-2,6-bisP). F-2,6-bisP then allosterically activates PFK1, driving glycolysis. It also competitively inhibits fructose-1,6-bisphosphatase, preventing the futile cycle of F6P conversion. Insulin also activates pyruvate kinase (PK) through dephosphorylation by protein phosphatase, increasing enzyme activity. Additionally, insulin enhances PK gene expression through SREBP1C. Fructose-1,6-bisphosphate, a product of reaction 3D, allosterically activates PK.
Fig.39.12) throughchangingtheactivityofthebifunctionalenzymephosphofructokinase2/fructose-2,6-bisphosphatase(PKFBP)andthusthelevelsoftheallostericregulatorymetabolitefructose-2,6-bisphosphate(F-2,6-bisP).Insulin/Akt-activated protein phosphatase F-2,6BPase F-1,6BPase F-1,6-bisPATP, citrate AMP PKFBP-P PFK1 (–) (–) (+) Glucagon & NE/E-activated protein kinase A (PKA) HepatocytecytoplasmPKFBP PFK2 F-2,6-bisP F6P •Fig. 39.14 Metabolisminskeletalmuscleduringdigestive(“D”reactions)vs.fasting(“F”reactions)phases.Reactions/pathways:1D,translocationofGLUT4transportertocellmembrane;1F,lossoftranslocationofGLUT4transportertocellmembrane;2D,glycogensynthesis;2F,glycogenolysis;3D,glycolysisandlactatedehydrogenase,orpyruvatedehydrogenase/TCAcycle/oxidativephosphorylation(OxPhos),dependingonmusclefibertype;3F,β-oxidationofFFAsorketolysisfollowedbytheTCAcycleandOxPhos.GLUT4 GLUT4 Transporters FFAs, KBsAcetyl CoAATP GLUT4 storage vesicle Cell membrane GlucoseGlucoseGlycogen 1D 1F 2F 2D 3D 3F GLUT4 Skeletal myocyte fructose-6-phosphate (F6P) to fructose-1,6-bisphosphate (reaction 3D). This reaction is referred to as the “commitment” reaction for glycolysis. Insulin also inhibits the reverse reaction, as catalyzed by the gluconeogenic enzyme fructose-1,6-bisphosphatase. Insulin regulates these two enzymes through an indirect two-step mechanism that is diagramed in
Fig. 39.14 . This mechanism involves the bifunctional enzyme phosphofructokinase-2/ fructose bisphosphatase (PKFBP; see
Fig. 39.14 ). Insulin/Akt-activated protein phosphatases promote dephosphorylation of PKFBP, thereby activating the kinase function and lessening the phosphatase function. This phosphorylates F6P to fructose-2,6bisphosphate (F-2,6-bisP). F-2,6-bisP in turn allosterically activates PFK1, thereby driving glycolysis. F-2,6-bisP also competitively inhibits fructose-1,6bisphosphatase, thereby blocking the futile cycle of F6P to fructose-1,6-bisphosphate to F6P.
B.	Activatingpyruvatekinase(PK).PKcatalyzestheirreversibleconversionofphosphoenolpyruvate(PEP)topyruvate(seereaction5Din
Fig.
39.12 ).Again,insulin/AktkinaseactivationofaproteinphosphatasedephosphorylatesPK,whichactivatestheenzyme.InsulinalsoincreasesPK geneexpressionthroughSREBP1C.Finally,fructose-1,6-bisphosphate(productofreaction3D)allostericallyactivatesPK.
4.