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Regulation of Carbohydrate Metabolism

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Regulation of Carbohydrate Metabolism BI/CH 422/622 ANABOLISM OUTLINE: Overview of Photosynthesis Carbohydrate Biosynthesis in Animals Key experiments: precursors Light causes oxygen, which is from water splitting (Hill) Cori cycle NADPH made (Ochoa) Gluconeogenesis Separate from carbohydrate biosynthesis (Rubin & reversible steps Kamen) irreversible steps – four Light Reactions energetics 2-steps to PEP energy in a photon mitochondria pigments Pyr carboxylase-biotin HOW PEPCK Light absorbing complexes FBPase G6Pase Reaction center Photosystems (PS) Glycogen Synthesis PSI – oxygen from water splitting UDP-Glc PSII – NADPH Glycogen synthase Proton Motive Force – ATP Branching Overview of light reactions Pentose-Phosphate Pathway Oxidative phase Carbon Assimilation – Calvin Cycle Non-oxidative/recycling phase Stage One – Rubisco ROS and NADH/NADPH shuttles Carboxylase Oxygenase Regulation of Carbohydrate Metabolism Glycolate cycle Stage Two – making sugar Acetyl-CoA/Pyruvate Stage Three - remaking Ru 1,5P2 Pyruvate/PEP Overview and regulation F6P/FBP: Fru 2,6P Calvin cycle connections to biosyn. 2 C4 versus C3 plants Glc/Glc6P Kornberg cycle - glyoxylate Glycogen Anaplerotic reactions Regulation of Carbohydrate Metabolism Catabolism vs. Anabolism 1 Catabolism vs. Anabolism Regulation of Carbohydrate Metabolism Gene controlled Catabolism Anabolism Glycogenolysis Glycogen versus Glycogen Glycogen Synthase Phosphorylase Phosphorylase Glycogen Glycogen Synthesis Synthase UDP-Glc UDP-glucose Glycolysis pyrophosphorylase versus Glc 1-P Gluconeogenesis phosphoglucomutase Hexokinase Glucose 6- • Regulated enzymes phosphatase often correspond to points in the pathways Phosphofructo- Fructose 1,6- kinase-1 bisphosphatase that have the same -1 substrate and product, but a different enzyme. • Also where there are junctions. • Can you name those Pyruvate Kinase – PEPCK – Pyruvate enzymes? Pyruvate carboxylase Dehydrogenase ✓ Acetyl-CoA Complex Regulation of Carbohydrate Metabolism Regulation of Pyruvate All tissues Kinase • Allosterically activated by fructose-1,6-bisphosphate – increase flow through glycolysis – Feed-forward activation • Allosterically inhibited by signs of abundant energy supply. –ATP –acetyl-CoA and long-chain fatty acids –alanine (enough amino acids) Liver only (under hormonal control) Inactivated by phosphorylation in response to signs of glucose depletion (low blood- glucoseàglucagon) (liver only) This is not the only time we’ll see Glucose from liver is exported to the brain and other hormonal control of these vital organs. pathways. 2 Regulation of Carbohydrate Metabolism Regulation of Pyruvate All tissues Kinase • Allosterically activated by fructose-1,6-bisphosphate – increase flow through glycolysis – Feed-forward activation • Allosterically inhibited by signs of abundant energy supply. –ATP –acetyl-CoA and long-chain fatty acids –alanine (enough amino acids) Liver only (under hormonal control) Inactivated by phosphorylation in response to signs of glucose depletion (low blood- glucoseàglucagon) (liver only) This is not the only time we’ll see Glucose from liver is exported to the brain and other hormonal control of these vital organs. pathways. Regulation of Carbohydrate Metabolism Regulation of Pyruvate Carboxylase • Allosteric activation of pyruvate carboxylase by Acetyl-CoA – stimulates glucose synthesis via gluconeogenesis because plenty of acetyl-CoA signals plenty of CAC intermediates • Notice the reciprocal control of PDH Complex by acetyl-CoA 3 Regulation of Carbohydrate Metabolism Catabolism Anabolism Glycogen Glycogenolysis Glycogen Glycogen Synthase versus Phosphorylase Phosphorylase Glycogen Synthase Glycogen Synthesis UDP-Glc UDP-glucose pyrophosphorylase Glc 1-P Glycolysis phosphoglucomutase versus Hexokinase Glucose 6- Gluconeogenesis phosphatase • Regulated enzymes Phosphofructo- Fructose 1,6- often correspond to kinase-1 bisphosphatase points in the pathways that have the same substrate and product, but a different enzyme. • Can you name those Pyruvate Kinase – PEPCK – Pyruvate enzymes? Pyruvate carboxylase Dehydrogenase Acetyl-CoA Complex Regulation of Carbohydrate Metabolism Regulation of High [AMP] Phosphofructokinase-1 Low [AMP] versus Fructose 1,6-bisphosphatase-1 Homeostatic level of Fru 6-P •Fructose-6-phosphate à fructose 1,6-bisphosphate is the commitment step in glycolysis. •While ATP is a substrate, ATP is also a negative effector. – Do not spend glucose in glycolysis if there is plenty of ATP. – Same for citrate, if there is plenty of citrate, do not waste glucose • Low energy charge inhibits biosynthesis of Glc. Bumble bees are missing an FBPase • Go glycolysis if AMP is high and ATP is low. Is this a typo? that responds to AMP • Go gluconeogenesis if AMP is low. 4 Regulation of Carbohydrate Metabolism Regulation of High [AMP] Phosphofructokinase-1 Low [AMP] versus Fructose 1,6-bisphosphatase-1 Homeostatic level of Fru 6-P •Fructose-6-phosphate à fructose 1,6-bisphosphate is the commitment step in glycolysis. •While ATP is a substrate, ATP is also a negative effector. – Do not spend glucose in glycolysis if there is plenty of ATP. – Same for citrate, if there is plenty of citrate, do not waste glucose • Low energy charge inhibits biosynthesis of Glc. Bumble bees are missing an FBPase • Go glycolysis if AMP is high and ATP is low. Is this a typo? that responds to AMP • Go gluconeogenesis if AMP is low. Regulation of Carbohydrate Metabolism Fructose 2,6-(bis)phosphate (bD-Fru-2,6P2) •NOT a glycolytic intermediate, only a regulator •Produced specifically to regulate glycolysis and gluconeogenesis bD-Fru-1,6P2 – activates phosphofructokinase-1(PFK-1) (glycolysis) – inhibits fructose 1,6-bisphosphatase (FBPase-1) (gluconeogenesis) Enzyme for synthesis and degradation of Fru 2,6P2 done with a dual-function enzyme: PFK-2/FBPase-2* *6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 5 Regulation of Carbohydrate Metabolism Regulation of Glycolysis and Gluconeogenesis by Fru-2,6P2 • Without Fru 2,6P2, STOP glycolysis • With Fru 2,6P2 (130 nM), Go glycolysis. GO gluconeogenesis. • With Fru 2,6P2 (1300 nM), Stop gluconeogenesis. What controls PFK-2/FBPase-2? Regulation of Carbohydrate Metabolism Structurally, this enzyme, with Regulation of Fru-2,6-P2 Levels its two activities is different than those in glycolysis and gluconeogenesis (i.e., they are conjoined, rather than independent) and are regulated via phosphorylation. Insulin Glucagon Drop relieves the activation of PFK-1, effectively inhibiting glycolysis. Drop increases activity of FBPase-1, stimulating gluconeogenesis 6 Regulation of Carbohydrate Metabolism Structurally, this enzyme, with Regulation of Fru-2,6-P2 Levels its two activities is different than those in glycolysis and gluconeogenesis (i.e., they are conjoined, rather than independent) and are regulated via phosphorylation. Insulin Glucagon Drop relieves the activation of PFK-1, effectively inhibiting glycolysis. Drop increases activity of FBPase-1, stimulating gluconeogenesis Regulation of Carbohydrate Metabolism Catabolism Anabolism Glycogen Glycogenolysis Glycogen Glycogen Synthase versus Phosphorylase Phosphorylase Glycogen Synthase Glycogen Synthesis UDP-Glc UDP-glucose pyrophosphorylase Glc 1-P Glycolysis phosphoglucomutase versus Hexokinase Glucose 6- Gluconeogenesis phosphatase • Regulated enzymes Phosphofructo- Fructose 1,6- often correspond to kinase-1 bisphosphatase points in the pathways that have the same substrate and product, but a different enzyme. • Can you name those Pyruvate Kinase – PEPCK – Pyruvate enzymes? Pyruvate carboxylase Dehydrogenase Acetyl-CoA Complex 7 Regulation of Carbohydrate Metabolism Regulation of There Are Four Isozymes Hexokinase of Hexokinase (I-IV) •Isozymes are different enzymes that catalyze the same reaction. –typically share similar sequences –may have different kinetic properties HK-I versus HK-IV –can be regulated differently •HK I is expressed in all tissues, to different levels. •HK IV (glucokinase) is only expressed in the liver and pancreas. Glucokinase Is Regulated by Sequestration –has higher Km, so responsive to higher [glucose] –not inhibited by glucose-6-phosphate, so can function at higher [glucose] –functions to clear blood glucose at ⊕ higher [glucose] for storage as glycogen Fructose 1-phosphate –Glc activates release/Fru inhibits Regulation of Carbohydrate Metabolism Catabolism Anabolism Glycogen Glycogenolysis Glycogen Glycogen Synthase versus Phosphorylase Phosphorylase Glycogen Synthase Glycogen Synthesis UDP-Glc UDP-glucose pyrophosphorylase Glc 1-P Glycolysis phosphoglucomutase versus Hexokinase Glucose 6- Gluconeogenesis phosphatase • Regulated enzymes Phosphofructo- Fructose 1,6- often correspond to kinase-1 bisphosphatase points in the pathways that have the same substrate and product, but a different enzyme. • Can you name those Pyruvate Kinase – PEPCK – Pyruvate enzymes? Pyruvate carboxylase Dehydrogenase Acetyl-CoA Complex 8 Regulation of Carbohydrate Metabolism Regulation of Glycogen Phosphorylase Glc Glc Nobel Prize 1972 Earl Sutherland 1915-1974 •Glycogen phosphorylase cleaves glucose residues off glycogen, generating glucose-1- phosphate (Glc 1P). •Phosphorylation activates glycogen phosphorylase-b –Phosphorylase-b Kinase –Accentuated by allosteric binding of AMP (muscle only) AMP •Dephosphorylation inhibits glycogen phosphorylase-a –Phosphoprotein phosphatase-1 (PP1) AMP –Accentuated by allosteric binding of Glc (in liver only) Regulation of Carbohydrate Metabolism Regulation of Glycogen Phosphorylase Glc Glc Nobel Prize 1972
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