BMS100 BCH 2.02 - Carbohydrates Metabolism PDF

BCH 2.02 - Carbohydrates metabolism In-class Dr. Rhea Hurnik BMS100 Plan Pre-learning Review Glycolysis and Gluconeogenesis In-class Glycolysis continued Regulation of glycolytic Metabolism of other monosaccharides Gluconeogenesis continued Regulation of gluconeogenic enzyme Pentose phosphate shunt...

BCH 2.02 - Carbohydrates metabolism In-class Dr. Rhea Hurnik BMS100 Plan Pre-learning Review Glycolysis and Gluconeogenesis In-class Glycolysis continued Regulation of glycolytic Metabolism of other monosaccharides Gluconeogenesis continued Regulation of gluconeogenic enzyme Pentose phosphate shunt General pathway Regulation Glycogenesis & Glycogenolysis General pathways Regulation Learning outcomes Diagram the connections between glycolysis, gluconeogenesis, pentose phosphate shunt, glycogenesis, and glycogenolysis Glycolysis § Briefly review glycolysis, including reactants & products, enzyme names, and any allosteric and hormonal regulators of the enzymes. Irreversible steps of glycolysis are testable on the next quiz, test, and final exam, include substrate, product, enzyme, and enzyme regulation § Describe the metabolism of fructose and how it feeds into glycolysis Gluconeogenesis § Briefly review gluconeogenesis, including reactants & products, enzyme names, and any allosteric and hormonal regulators of the enzymes Bypass reactions of gluconeogenesis are testable on the next quiz, test, including substrate, product, enzyme, and enzyme regulation § Note: Gluconeogenic substrates are testable on the final exam. Describe the pentose phosphate shunt include starting substrates and glycolytic products. § Describe the key product of the pentose phosphate shunt, including NADPH and pentose phosphates § Describe the allosteric and hormonal regulation of glucose-6-Phosphate dehydrogenase Outline glycogenesis and glycogenolysis pathways, including key enzymes involved. § Describe allosteric and hormonal regulation of glycogen synthase and glycogen phosphorylase Cytosol Glycogen ATP Glucose 6-P Glucose NADPH ATP *NADH Pyruvate Triglycerides Glycerol PentoseP-Sugars ATP Fatty AcylCoA *NADH Fatty Acids Oxaloacetate *NADH Pyruvate Mitochondria CO2 *NADH, FADH2 O2 *NADH, FADH2 ATP AcetylCoA Oxaloacetate Citrate Citric Acid Cycle Ketones *NADH, FADH2 Fatty AcylCoA ATP Electron Transport Chain ATP ATP H2O Acetyl CoA Metabolism Review § Once Glucose is converted to Glucose-6-P it can enter several pathways? § Glycolysis § Which enzyme is needed to commit glucose-6-P to glycolysis? § Pentose Phosphate Shunt § Glycogenesis Glycolysis: Regulation The enzymes of the three irreversible reactions are also the main regulated enzymes of glycolysis § What are they? Glycolysis: Allosteric Regulation Hexokinase What reaction does Hexokinase catalyze? § Hexokinase in muscle Reversibly regulated by glucose-6-phosphate This is an example of product inhibition § When cellular concentrations of glucose-6-P rise above normal, Hexokinase is temporarily inhibited to bring rate of glucose-6-P into balance with its rate of utilization Glycolysis: Allosteric Regulation – PFK1 § What reaction does PFK-1 catalyze § PFK-1 is under complex allosteric regulation § 1. ATP inhibits PFK-1 by binding to an allosteric site § Lowers the affinity of the enzyme for fructose-6-P § AMP and ADP relieve the inhibition of PFK-1 § Explain why this makes sense logically § 2. Citrate § Key intermediate in CAC § High citrate levels inhibit PFK-1 § High citrate serves a signal that the cell is meeting its current needs for energy Glycolysis: Allosteric Regulation – PFK1 § PFK-1 is under complex allosteric regulation § 3. Fructose 2,6-bisphosphate § Most significant allosteric regulator § Activates PFK-1 § Fructose-2,6-bisphosphate enhances the affinity of PFK-1 for fructose-6-P § Stimulates glycolysis § Inhibits FBP-1, slowing gluconeogenesis Glycolysis: Allosteric Regulation – PFK1 § PFK-1 is under complex allosteric regulation § 3. Fructose 2,6-bisphosphate continued § Fructose 2,6-bisphosphate is formed by phosphorylating Fructose-6-P, catalyzed by phosphofructokinase-2 (PFK-2) CHO-rich meal Blood glucose rises Glycolysis: Allosteric Regulation – Pyruvate Kinase § What reaction does pyruvate kinase catalyze? § Pyruvate kinase is allosterically regulated by: § Inhibition: try to explain each logically § 1. ATP § 2. Acetyl-CoA § 3. Fatty acids § Activation: § Fructose 1,6-bisphosphate Glycolysis: Hormonal Regulation § Glycolysis is regulated hormonally by insulin and glucagon. § Insulin promotes the transcription of Hexokinase, phosphofructokinase-1, and pyruvate kinase § Reminder: Insulin also promotes the activation of PFK1 ( & inhibition FBP-2) § Covered on slide 21 § Glucagon reduces the expression of hexokinase, PFK-1, and pyruvate kinase § Glucagon promotes the activation of FBP-2 (& inhibition PFK-2) Glycolysis Review Glycolytic enzyme Hexokinase PFK-1 Pyruvate Kinase Activated by Inhibited by Metabolism of other important sugars Fructose, Mannose, and galactose are converted into glycolytic intermediates § We are only going to look at Fructose metabolism in more detail Predominately occurs in the liver Fructose Fructose -1-Phosphate DHAP + glyceraldehyde Glycolysis Lipogenesis Fructose metabolism Fructose Fructose metabolism – thinking questions Which monosaccharide is metabolized faster, glucose or fructose, why? Gluconeogenesis - Regulation The first two bypass reactions are the main regulated reactions of gluconeogenesis § What were these bypass reactions? Gluconeogenesis: Allosteric Regulation – Pyruvate carboxylase § Pyruvate can be converted either to: § A) acetyl CoA à enter into CAC § B) Oxaloacetate à enter into GNG § What enzyme catalyzes conversion of pyruvate to oxaloacetate § C) Lactate (Cori cycle) § Acetyl CoA is a allosteric activator of pyruvate carboxylase Gluconeogenesis: Allosteric Regulation – FBP-1 § What reaction does FBP-1 catalyze § Allosterically inhibited by AMP § Allosterically inhibited by Fructose 2,6-bisphosphate Blood glucose is low Gluconeogenesis: Hormonal Regulation § Gluconeogenesis is regulated hormonally by insulin and glucagon. § Glucagon promotes activation of FBP-2, lowering levels of Fructose-2,6bisP § Previous slide § Glucagon can also induce the transcription of PEP carboxykinase Gluconeogenesis Review Gluconeogenesis enzyme Pyruvate carboxylase PEP carboxykinase FBP-1 Activated by Inhibited by Metabolism Review § Once Glucose is converted to Glucose-6-P it can enter several pathways? § Glycolysis § Which enzyme is needed to commit glucose-6-P to glycolysis? § Pentose Phosphate Shunt § Glycogenesis Pentose Phosphate Shunt One fate of Glucose-6-phospate is the pentose phosphate Shunt Alternative metabolic pathway for glucose that “shunts” molecules into and out of glycolysis Purpose: Generates two main products: § NADPH § Ribose-5-Phosphate (“pentose phosphates) Also generates Fructose-6-Phosphate and glyceraldehyde3-Phosphate, which can feed back into glycolysis § Why do you think it is called a “shunt?” § Location? Pentose Phosphate Shunt Glucose-6-P 6-Phosphoglucono-𝛿 -lactone 6-Phosphogluconate Two phases: § Oxidative Ribose 5-P Glyceraldehyde-3-P D-Ribulose 5-phosphate Irreversible reactions Sedoheptuloase-7-P § Non-oxidative Xylulose-5-P Reversible reactions Glyceraldehyde-3-P Erythrose-4-P Fructose-6-P Fructose-6-P Oxidative phase Glucose-6-phosphate à Ribulose-5-Phosphate § Generates 2 NADPH Functions: § Fatty acid synthesis – more to come next week § Reduces glutathione (antioxidant) + CO2 3 Glucose-6-P 6-Phosphoglucono-𝛿 -lactone 6-Phosphogluconate D-Ribulose 5-phosphate NADPH: antioxidant Glutathione is an antioxidant § 3 amino acid peptide Glycine-cystineglutamate § Neutralizes (reduces) hydrogen peroxide to water by donating H’s § NADPH regenerates glutathione by replacing the donated H’s Oxidative phase regulation Glucose-6-phosphate à Ribulose-5-Phosphate § Rate-limiting step: Glucose-6-P à 6-phosphoglucono-𝛿-lactone § Enzyme: glucose-6-Phosphate dehydrogenase (G6PD) Regulated by ratio of NADPH:NADP+ High NADPH:NADP+ ratio inhibits G6PD Also inhibited by high levels of Acyl CoAs – why? Upregulated by insulin 3 Glucose-6-P 6-Phosphoglucono-𝛿 -lactone 6-Phosphogluconate D-Ribulose 5-phosphate G6PD deficiency G6PD deficiency is an X-linked trait Therefore, is it more common in men or women? § FYI - Highest prevalence in those of Middle Eastern, Mediterranean, and tropical African or Asian descent Results in hemolytic anemia when an individual is exposed to oxidant stress § Ie. Some drugs, infections, fava beans Non-oxidative phase Riboluse-5-P is converted to Ribose-5-P Ribose 5-P Glyceraldehyde-3-P D-Ribulose 5-phosphate § What do you suppose Ribose-5-P can be used for? Sedoheptuloase-7-P Xylulose-5-P OR into glycolytic intermediates § Which two molecules are the glycolytic intermediates? Reactions all FYI Glyceraldehyde-3-P Erythrose-4-P Fructose-6-P Fructose-6-P Metabolism Review § Once Glucose is converted to Glucose-6-P it can enter several pathways? § Glycolysis § Which enzyme is needed to commit glucose-6-P to glycolysis? § Pentose Phosphate Shunt § Glycogenesis Glycogen metabolism Glucose is stored in polymeric form as glycogen mostly in the liver and skeletal muscle. High glucose/energy levels will triggers glycogen synthesis § Glycogenesis Glucose can be rapidly delivered to the blood stream when needed by degradation of glycogen in the liver § Glycogenolysis Glycogenesis Glucose is transferred onto a growing chain of glycogen as UDP-glucose § Steps: UTP Glucose-6-P Glucose-1-P Phosphoglucomutase Enzyme names FYI UDP-glucose + PPi UDP-glucose pyrophosphorylase Glycogenesis UDP-glucose is added to an existing strand of glycogen by the enzyme glycogen synthase Glycogenn + UDP-glucose → Glycogenn+1 + UDP Glucose is added to the non-reducing end in what type of link? Glycogenesis - branching Branching enzyme catalyzes the transfer of 4-8 glucose residues to a branch point § Branching enzyme is also called amylo-α(1,4à1,6) glucosyl transferase Glycogenesis Primer Glycogen synthase cannot initiate a new glycogen chain de novo, it requires a primer § Primer contains pre-formed (alpha1à4) polyglucose chain with at least 4-8 glucose residues § Found within glycogenin § Contains the primer and the enzyme needed to build the primer Glycogenesis Primer creation - visual Notes Glycogenolysis Glycogenolysis is a catabolic progress § Breakdown of glycogen into glucose units § Occurs in the liver and muscle Glycogenolysis Glucose is removed from nonreducing ends of glycogen § Enzyme: glycogen phosphorylase Sequentially cleaves α (1 à4) linkages from the non-reducing ends until 4 units away from a branch point § Glucose is released as glucose-1-P Once all chains degraded to within 4 units of a branch point, the molecule is called a limit dextrin Glycogenolysis A de-branching enzyme has two functions: § 1) transfers the outer 3 glucose residues from the branch to another non-reducing end (leaving only 1 reside behind at the branch point) Aka Oligo-α(1,4)-α(1,4)glucotransferase § 2) Removes the final glucose residue in the alpha(1à6) linkage Aka Amylo-α(1,6)-glucosidase Glycogenolysis Glucose-1-P is converted to glucose-6-P § Enzyme: phosphoglucomutase This is the same enzyme we saw in the first step of glycogenesis The liver can then convert glucose-6-P into glucose with the enzyme _____? § Hint: This enzyme catalyzes the third bypass reaction of gluconeogenesis § Muscles do not have this enzyme, why? What does the muscle do with glucose-6-P instead? Glycogen metabolism regulation Glycogen synthase and glycogen phosphorylase are the two regulated enzymes in glycogen metabolism § Allosteric regulation: Glycogen synthase regulation § Allosterically activated by glucose-6-P Glycogen phosphorylase regulation § Allosterically inhibited by: Glucose-6-P ATP Free glucose (in the liver only) § Allosterically activated by AMP (muscle only) Glycogen metabolism regulation - Hormonal Glycogen synthase and glycogen phosphorylase can also be regulated by covalent modification § Glycogen synthase is de-activated by phosphorylation § Glycogen phosphorylase is activated by phosphorylation Phosphorylation is catalyzed initially by the same protein, protein kinase A (PKA) § How is PKA activated? This prevents both pathways from running at the same time Review activated Inhibited Glycogen metabolism Protein kinase A ATP Protein kinase A ADP ATP Glycogen phosphorylase kinase Glycogen phosphorylase kinase ATP Glycogen phosphorylase Glycogen synthase P ADP Glycogen synthase ADP Glycogen phosphorylase Glycogenolysis P No glycogenesis P activated Review Inhibited Glycogen metabolism continued Protein kinase A ATP Protein kinase A ADP ATP Glycogen phosphorylase kinase Glycogen phosphorylase kinase ATP Glycogen phosphorylase Glycogen synthase P ADP Glycogen synthase ADP Glycogen phosphorylase Glycogenolysis P No glycogenesis P Glycogen metabolism Regulation - Hormonal Covalent modification of glycogen metabolism is under hormonal control § In the presence of glucagon (and epinephrine): Glucagon binds to its GCPR G⍺s activates adenylyl cyclase à cAMP levels rise PKA phosphorylates glycogen synthase, rendering it INACTIVE § Glycogenesis is inhibited Glycogen metabolism Regulation - Hormonal Covalent modification of glycogen metabolism is under hormonal control In the presence of glucagon (and epinephrine): PKA phosphorylates glycogen phosphorylase kinase, rendering it ACTIVE Glycogen phosphorylase kinase phosphorylates glycogen phosphorylase, rendering it ACTIVE Glycogenolysis is promoted Glycogen metabolism Regulation - Hormonal Covalent modification of glycogen metabolism is under hormonal control § In the presence of Insulin Insulin promotes the breakdown of cAMP and thus inactivation of PKA Insulin activates protein phosphatase 1, which removes the phosphate group from glycogen synthase, rendering it active Glycogenesis is promoted Glycogen metabolism Regulation - Hormonal Covalent modification of glycogen metabolism is under hormonal control In the presence of insulin Insulin promotes the breakdown of cAMP and thus inactivation of PKA Insulin activates protein phosphatase 1, which removes the phosphate group from: Glycogen phosphorylase kinase & Glycogen phosphorylase, rendering them both inactive Glycogenolysis is inhibited References Alberts et al. Molecular Biology of the Cell. Garland Science. Lehninger Principles of Biochemistry 4th ed. Figure 21-36. Page 818 Images: § https://upload.wikimedia.org/wikipedia/commons/b/bf /Pentose_phosphate_pathway_Kor.png Study Questions Draw out a basic schematic linking glycolysis, gluconeogenesis pentose phosphate shunt, and glycogenesis, glycogenolysis § In your schematic, include the key regulated reactions and enzymes for each pathway Add the allosteric regulators to your diagram § How does Fructose to feed into glycolysis