Transcript Slide 1
Chapter 12: Gluconeogenesis, Pentose Phosphate Pathway, & Glycogen Metabolism Glucose catabolism for the production of energy requires a source of Glc. glycogen Polysaccharides are degraded and the resulting Glc is stored as glycogen in muscle and liver. Glc also syn from pyruvate (lactate and amino acids) Liver/kidney Glc needed in brain/muscle The pentose phosphate pathway (PPP) is the source of ribose (deoxyribose), and NADPH. NADPH is required for biosynthesis. PPP Pathway Glycolysis Net Reaction: Glucose + 2 ADP + 2 NAD+ + 2 Pi 2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O **Gluconeogenesis Net Reaction:** 2 Pyruvate + 4 ATP + 2 GTP + 2 NADH + 2 H+ + 6 H2O Glucose + 4 ADP + 2 GDP+ 2 NAD+ + 6 Pi #1 #3 Gluconeogenesis - glycolysis going backwards - 3 places differ- control points in glycolysis - 4 new enzymes (eukaryotes) - importance of near equilibrium reactions - ATP energy, NADH reducing equivalents consumed #10 Gluconeogenesis 6 ATP needed total 4 needed to overcome barrier of production of 2 mol of PEP Gluconeogenesis: The Irreversible Steps Pyruvate PEP; reversing the pyruvate kinase step of glycolysis. 4 subunits Biotin Allosteric + acetyl CoA Indicates CAC Backed-up No allosteric reg Hormonal induction Transcriptional regulation + glucagon (fasting) - Insulin (fed state) Gluconeogenesis No ATP needed since Fru-1,6-bisP not high energy intermediate Fru-1,6-biP Fru-6-P; reversing the PFK-1 step of glycolysis. Large – DG and irreversible Allosteric modulation - AMP - 2,6-Fru bisP (opposing effect in glycolysis) Glc-6 Glc; reversing the Glc hexokinase step of glycolysis. Irreversible Allosteric modulation - AMP Enzyme found only in liver, kidneys, small intestine. Bound to ER lumen…leads to release of Glc into bldstream Most cases Glc-6-P is end product---used in other pathways Get to brain And muscle (glycogen syn) Gluconeogenesis: Precursors Major precurser in mammals: Lactate and Amino Acids, Since the body does not transfer pyruvate Lactate Cori cycle Amino Acids Pyruvate in tissues must go to liver First converted to alanine Major source of C for Glc syn during fasting Active muscle-- lactate Amino arise from muscle protein breakdown Lactate to pyruvate in liver Provide temporary and readily available supply of Glc to muscle (exercise) Gluconeogenesis Gluconeogenesis -glucose biosynthesis found in all organisms Some tissues require glucose -brain, muscles After 16-24 hrs, glucose and glycogen reserves depleted Some tissues synthesis glucose from non-carbohydrate precursor -liver, kidney -lactate, alanine Easiest to start with pyruvate -converted from lactate or a.a. Gluconeogenesis: Regulation Low [Glc]: glucagon increases protein kinase A (activates Fru-2,6-bisP phosphatase) lowering [Fru-2,6bisP]. Activate Glc syn and Loss of glycolysis stim neg reg pyruvate kinase Substrate Cycle Dec the net flux of a pathway But allows a point for reg flux Modulate one enzyme effect 2 opposing pathways Inhibit PFK-1 ….. stim Glc syn Regulation of Phosphofructokinase-1 Large oligomeric enzyme bacteria/mammals - tetramer yeast - octamer ATP - product of pathway - allosteric inhibitor AMP - allosteric activator - relieves inhibition by ATP Citrate - feedback inhibitor - regulates supply of pyruvate - links Glycolysis and CAC Fru-2,6-bisphosphate - strong activator - produced by PFK-2 when excess fru-6-phosphate - indirect means of substrate stimulation or feed forward activation Regulation of Pyruvate Kinase + F 1,6 BP Allosteric (feed-forward) activation Fructose-1,6-bisphosphate -allosterically activates -produced in step three -links control steps together High blood [Glc] Inactivation by covalent modification -blood [Glc] drops, glucagon released -liver protein kinase A (PKA) turned on -PKA phosphorylates pyruvate kinase Allosteric inhibition by ATP -product of pathway and CAC Low blood [Glc] Regulation of Phosphofructokinase-1 Produced in pancreas in response to low [Glc] Dual activities of PFK-2 reg steady-state conc of Fru-2,6-bisP Increased glycolysis Fruc-6P inc….inc F-2,6-bisP Stim PFK-1 Dec F-2,6-bisP PFK-1 less active…..dec glycolysis Activate Protein Kinase A Dec glycolysis Inc glc syn Figure 11-17 PFK-1 and pyruvate kinase Stimulate glycogen breakdown Pentose Phosphate Pathway glycogen The pentose phosphate pathway (PPP) is the source of ribose (deoxyribose), and NADPH. NADPH is required for biosynthesis. Shunt PPP Pentose Phosphate Pathway Shunt Synthesize 3 pentose phosphates Ribulose 5-P Xylulose 5-P Ribose 5-P And NADPH (DNA/RNA) (for the reduction of RNA to DNA) Or NADPH and glycolytic intermediates Rapidly dividing cells need lots of NADPH and DNA High PPP activity The Oxidation Stage of PPP Major reg step Allosteric - NADPH Loss of Carbon The Non-Oxidation Stage of PPP All equilibruim rxns When cells need lot of NADPH and nucleotides - ribulose 5-phosphate ribose 5-phosphate - end of pathway The Non-Oxidation Stage of PPP Convert 5C sugars into glycolytic intermediates Can be used in glycolysis of Gluconeogenesis Pentose Phosphate Pathway Thru PPP 3 Glc-6-P + 6 NADP+ + 3 H2O 2 Fru-6-P + G3P + 6 NADPH + 3 CO2 Recycle 6C sugar Allow sub regeneration via PPP and glyconeogenesis 6 ribulose 5-P Can be metabolized in Glycolysis or Glcneogenesis 6 Glc-6-P + 12 NADP+ 5 Glc-6-P + 12 NADPH + 6 CO2 + Pi 5 Glc 5-P Glycogen Metabolism Glycogen is the storage form of Glc found in muscles and liver. (Plants: stored as Starch) Glycogen complex: single glycogenin molecule (Tyr -OH) and >50,000 glucose residues Stores of Glc in time of plenty and supplies it in times of need Muscle: fuel for contraction Liver: produce Glc…released to Bldstream to other tissues All regulated by hormones: Glucagon, Epinephrin and Insulin Glycogen Metabolism Synthesis: Different enzymes for syn and degradation Driven by PPi hydrolysis Major regulatory step (hormonally regulated) Key regulation by phosphorylation Pre-existing Glycogenin primer UDP-Glc synthases in protists, animals, and fungi. ADP-Glc synthase in plants. Primer of 4 to 8 Glc on a Tyr (-OH) of glycogenin. 1st Glc from UDP-Glc via Glc transferase. Remaining Glc’s tranferred by glycogenin. Amylo-(1,4 1,6)-transglycolase catalyzes the branch point. (Alpha 1-6 link) Degradation: Two subunits, two catalytic sites, allosteric sites. AMP- activator; ATP & Glc-6-P – inhibitor. Phosphorolysis rxn. Generates phosph-sugar not free glc Phosphorylation: active (phosphorylase a). Dephosphorylated: less active (phosphorylase b). Primary regulation Branching inc speed of syn and degradation phosphorolytic Sequential removal of Glc From non-reducing end Reg by ATP and G-6-P Primarily by phosphorylation Stops 4 Glc from branch pt hydrolytic Energy yield from glycogen Higher than from glc Regulation of Glycogen Metabolism Hormonal Regulation: Fed state fasting phosphatase Via cAMP Via PIP3 Decrease glycolysis Insulin: secreted by pancreas when Glc high inc rate of transport into cell and glycogen syn Glucagon: secreted when Glc low GLUT4 Epi: released by adrenal gland in response to neural signal (flight or flight) Sudden energy response Intracellular Regulation of Glycogen Metabolism by Interconvertible Enzymes: Low glc activate kinase and breakdown AMP phosphodiesterase cAMP Low [Glc] Simultaneous effect Regulation of Phosphofructokinase-1 Produced in pancreas in response to low [Glc] Dual activities of PFK-2 reg steady-state conc of Fru-2,6-bisP Increased glycolysis Fruc-6P inc….inc F-2,6-bisP Stim PFK-1 Dec F-2,6-bisP PFK-1 less active…..dec glycolysis Activate Protein Kinase A Dec glycolysis Inc glc syn Figure 11-17 PFK-1 and pyruvate kinase Stimulate glycogen breakdown High [Glc]