Carbohydrate and Lipid Metabolism Flashcards

Glycogen Synthesis

• Glycogen serves as the cell's readily available energy store.

• It consists of glucose polymers, reaching up to 100 million daltons.

• The majority of the macromolecule comprises straight chain links with α1→4 linkages.

• Branch points are created through α-1,6-linkages.

• Metabolism primarily involves adding or removing α1→4 units from the nonreducing end.

• At the core of glycogen is a protein called glycogenin.

• Glycogen breakdown initiates with splitting, involving glycogen synthase and glycogen phosphorylase (which also breaks down α-1,6-links).

• This process rapidly supplies glucose for energy.

• Glycogen functions as a glucose buffer.

  • Liver: Provides energy to the rest of the body during fasting and stress.

  • Muscle: Used exclusively by the muscle.

  • Found in other tissues in small amounts.

Glycogen Metabolism

• Glucose-6-P is incorporated into glycogen.

  • Step 1: Formation of glucose-1-P by phosphoglucomutase (near equilibrium).

    • Similar mechanism to phosphoglyceromutase in glycolysis.

  • Step 2: Formation of uridine diphosphate glucose.

Glycogen Synthesis

• Formation of uridine diphosphate glucose (UDP-glucose).

  • Direct nucleophilic substitution.

  • Formation is driven by splitting two high energy phosphate groups.

  • This leaves low levels of P_i in the cell.

Glycogen Synthesis

• Glycogen Synthase.

  • Rate-limiting step of glycogen synthesis.

  • Displacement of the UDP portion of UDP glucose produces a glucose carbocation.

  • The carbocation attacks the 4-OH group of the nonreducing end of glycogen.

  • Glucolactone can inhibit the enzyme.

  • Similar structurally to the native substrate (UDP-glucose).

Glycogen Synthesis

• Branching step.

  • Occurs once 10 glucosyl units have been added.

  • Glycogen branching enzyme catalyzes the reaction.

  • Creates α-1,6-bond called a branch point.

Glycogen Synthesis

• Glycogenin.

  • Required for complete de novo synthesis.

  • Serves as both an enzyme and a scaffold.

  • Structure:

    • Dimer with two active sites.

  • Mechanism:

    • UDP-glucose donates glucose residue to tyrosine hydrolysis.

  • Further buildup of glycogen involves glycogen synthase and branching enzyme.

  • Most glycogen synthesis involves adding to existing glycogen rather than de novo synthesis.

Glycogenolysis

• Glycogen phosphorylase accomplishes almost all glycogen breakdown.

  • (glucose)n + Pi \rightarrow (glucose)_{n-1} + glucose-1-P

  • Removal of glucose residues from the nonreducing end.

  • Glycogen phosphorylase is the enzyme catalyst.

  • Pyridoxal phosphate is a bound cofactor.

    • Derived from dietary pyridoxine (vitamin B6).

Glycogenolysis

• Mechanism is similar to glycogen synthetase due to carbocation intermediate.

• Glucolactone is also an inhibitor of glycogenolysis.

• The enzyme is glycogen phosphorylase.

• Cannot cleave α-1,6-bonds.

Glycogenolysis

• Removal of branch points is facilitated by debranching enzyme.

  • Removal of polymer containing all but one glucosyl subunit in the α-1,6-bond.

  • The last glucosyl unit is removed by glucosidase (debranching enzyme).

• The final product is free glucose.

Physiological Context of Glucose Metabolism

• Glycogen in the muscle.

  • Used as fuel to support contraction.

  • Muscle contraction and glycogenolysis occur due to an increase in Ca^{2+}.

    • Released in response to nerve-directed depolarization.

  • Contraction of muscle involves actin and myosin.

  • Glycogen breakdown provides energy for muscle contraction.

  • Synthesis of glycogen is under control of insulin.

    • The hormone is secreted into the bloodstream during feeding conditions.

    • Most of the glucose goes to skeletal muscle after feeding.

Physiological Context of Glucose Metabolism

• Glycogen in the liver.

  • Functions as a glucose buffer for the entire body.

  • Releases or takes up glucose from the blood to maintain a constant level.

  • Breakdown of liver glycogen releases glucose to the blood.

  • Glucagon triggers this.

    • Fasting hormone.

  • Also triggered by cytosolic calcium release of epinephrine does this.

Regulation of Glycogen Metabolism by Glucagon

• α-cells of the pancreas release glucagon into the blood in response to a drop in glucose concentration.

  • Glucagon

    • 29-amino acid peptide.

    • Glucagon targets the glucagon receptor of the liver (spans the membrane).

      • Binds to the exterior face, but does not enter the cell (does not need to enter to exert an effect).

      • Binding induces a conformational change.

      • The cytosolic portion of the receptor binds G-protein.

        • Catalyzes the exchange of GDP for GTP.

        • The G protein in the GTP-bound state leaves the receptor and slides along the surface of the membrane.

Regulation of Glycogen Metabolism by Glucagon

• Once the G-protein is in the GTP-bound state, it dissociates from the glucagon receptor and slides along the surface of the membrane.

  • It then binds a separate membrane-embedded protein (adenylate cyclase).

    • Only the GTP-bound form can bind and activate it.

    • Binding adenylate cyclase creates cyclic AMP (cAMP).

    • PP_i is also created but is hydrolyzed quickly (metabolically irreversible).

    • The G-protein continues to activate adenylate cyclase until turned off.

    • A defect in GTPase of G-protein (inactive) leads to the ras oncogene.

Regulation of Glycogen Metabolism by Glucagon

• cAMP can then bind protein kinase A (PKA).

  • PKA has regulatory and catalytic subunits.

  • In its resting state, PKA is a tetramer.

  • When cAMP binds to regulatory subunits, active catalytic monomers are released.

  • Phosphodiesterases remove cAMP.

    • cAMP + H_2O \rightarrow AMP

Regulation of Glycogen Metabolism by Glucagon

• Active PKA catalyzes the phosphorylation of two proteins in glycogen metabolism.

  • 1) Glycogen synthase is converted to an inactive form, reducing the rate of glycogen formation.

  • 2) Glycogen phosphorylase kinase is converted to its active form.

    • Acts on glycogen phosphorylase, catalyzing phosphorylation and activation of the enzyme (leads to increased glycogen breakdown).

Regulation of Glycogen Metabolism by Epinephrine

• Epinephrine, unlike glucagon (which increases cAMP), can produce different responses.

  • In muscle, it binds the β-adrenergic receptor coupled with G-proteins.

    • cAMP and PKA are activated.

  • In the liver, it binds the α-adrenergic receptor.

    • Binds a separate G-protein and activates phospholipase C, which catalyzes hydrolysis of phosphatidylinositol-4,5-P2 (PIP_2).

    • This hydrolysis produces inositol triphosphate (IP3) and diacylglycerol (DAG).

    • PIP2 \rightarrow IP3 + DAG

Regulation of Glycogen Metabolism by Insulin

• A rise in blood sugar after eating triggers insulin release.

  • Anabolic signal.

  • In muscle and liver, leads to increased glycogen synthesis.

• Insulin leads to activation of glycogen synthase.

  • A key regulator of glycogen synthase is glycogen synthase kinase (GSK3).

    • Insulin binds its receptor, which leads to phosphorylation and inactivation of GSK3, increasing the activity of glycogen synthesis.

Regulation of Glycogen Metabolism by Insulin

• Mechanism of regulation by insulin.

  • Binds to the exterior portion of the insulin receptor, triggering a conformational change to the intracellular portion of the receptor.

  • Activates intrinsic kinase activity of the insulin receptor.

  • Phosphorylated IRS-1 can bind to Src homology-2 (SH2) binding proteins.

  • PI3K (phosphatidylinositol phosphate 3-kinase) becomes activated.

    • Phosphatidylinositol-P2 (PIP2) + ATP \rightarrow Phosphatidylinositol-P3 (PIP3) + ADP

Regulation of Glycogen Metabolism by Insulin

• Mechanism of regulation by insulin.

  • PIP_3 anchors phospholipid-dependent kinase (PDK).

  • PDK catalyzes phosphorylation and activation of protein kinase B (PKB).

  • PKB phosphorylates GSK3 (inactivated).

    • Leaves glycogen synthase less phosphorylated and more active.

Regulation of Glycogen Metabolism by AMP Kinase

• AMP-dependent protein kinase (AMPK) is found in almost all cells.

• AMP is elevated under energy deprivation conditions.

• AMPK is involved in cell-cell signaling.

• AMP generation requires the breakdown of cellular ATP to ADP.

  • Adenylate kinase creates AMP.

  • This occurs when ATP utilization outcompetes ADP rephosphorylation.

  • ADP + ADP \rightleftharpoons AMP + ATP

• Phosphorylation of AMPK is essential for its activation.

  • It can then phosphorylate and inactivate GS.

  • This process inhibits the synthesis of glycogen in the muscle and liver.

Specifics Based on Learning Objectives

1. Function and location of glycogen. Function of glycogenin. (Find the answers in the slides.)

2. Critical enzymes and their functions for glycogen synthesis and breakdown: GS (rate limiting), GP, branching enzyme, debranching enzyme.

3. Two types of glycosidic bonds in glycogen (Find the answers in the slides).

4. Regulation of Glycogen Metabolism by Glucagon.

   • Step 1: glucagon binds to its cell surface receptor, and then multiple subsequent steps lead to an increase in cAMP in cytoplasm.

5. Regulation of Glycogen Metabolism by Epinephrine. Different from glucagon, and liver and muscle use different pathways.

6. Regulation of Glycogen Metabolism by Insulin.

7. Regulation of Glycogen Metabolism by AMPK.

   • Its reaction: ADP + ADP \rightleftharpoons AMP + ATP

   • Phosphorylation of AMPK is essential for its activation. It can then phosphorylate and inactivate GS. This process inhibits the synthesis of glycogen in the muscle and liver

8. Correlate the knowledge to resting and excising muscle and liver

Gluconeogenesis

• Pathway for forming glucose from non-carbohydrate precursors.

  • Lactate, glycerol, and numerous amino acids can be used.

• Occurs almost exclusively in the liver.

• Importance lies in maintaining blood glucose levels.

Learning Objective

1. Explain the process of gluconeogenesis and its role in maintaining blood glucose levels. Identify the major steps and enzymes involved in converting pyruvate to glucose.

2. Outline the role of pyruvate carboxylase and phosphoenolpyruvate carboxykinase (PEPCK) in gluconeogenesis; Compare the regulation of enzymes such as fructose 1,6 bisphosphatase and their opposing actions in metabolic cycles.

3. Analyze how the transition between feeding and fasting states impacts carbohydrate metabolism in the liver and muscles; Explain the metabolic role of the pentose phosphate pathway, including its oxidative and nonoxidative phases.

4. Discuss how glucose metabolism adapts during rest and exercise, focusing on energy demands in muscles and the liver.

5. Trace the metabolism of galactose obtained from lactose and its integration with glycogen metabolism.

Gluconeogenesis

• Pyruvate to phosphoenolpyruvate (PEP).

  • Rate-limiting step in gluconeogenesis.

  • Requires two enzymes: pyruvate carboxylase and phosphoenolpyruvate carboxykinase.

    1. Pyruvate carboxylase (metabolically irreversible). Exists in the matrix.

       • Cytosolic pyruvate is transported in through the pyruvate transport system.

       • Pyruvate + CO2 + ATP \rightarrow Oxaloacetate + ADP + Pi

       • Incorporation of CO_2 requires biotin (bound cofactor, vitamin).

    2. Phosphoenolpyruvate carboxykinase (PEPCK).

       • Oxaloacetate + GTP \rightarrow PEP + GDP + CO_2

       • Rate-limiting enzyme.

Gluconeogenesis

• PEPCK can catalyze the conversion of oxaloacetate to PEP in the cytosol.

  • Indirect transport is then necessary; two separate conversions occur.

    1. Reduction to malate (Malate dehydrogenase).

       • Oxaloacetate + NADH \rightleftharpoons Malate + NAD^+ (near equilibrium).

       • Malate + NAD^+ \rightleftharpoons Oxaloacetate + NADH (near equilibrium).

    2. Transamination to aspartate (aspartate aminotransferase).

       • Oxaloacetate + Glutamate \rightleftharpoons Aspartate + 2-Ketoglutarate (near equilibrium).

       • Aspartate + 2-ketoglutarate \rightleftharpoons Oxaloacetate + Glutamate (near equilibrium).

Fructose-1,6-P to Fructose-6-P

• Bypass of PFK step in glycolysis.

• Catalyzed by fructose-1,6-P2 phosphatase.

  • Fructose-1,6-P2 \rightarrow Fructose-6-P + Pi (metabolically irreversible).

  • Mechanism: hydrolysis of the phosphate bond.

  • Allosterically activated by citrate.

  • Allosterically inhibited by fructose-2,6-P2.

    • Opposite to PFK.

  • Constitutes a substrate cycle (opposing) with PFK and FBPase (result splits ATP).

    • PFK: ATP + Fructose-6-P \rightarrow ADP + Fructose-1,6-P_2

    • FBPase: Fructose-1,6-P2 \rightarrow Fructose-6-P + Pi

Glucose-6-P to Glucose

• Catalyzed by Glucose-6-phosphatase.

• Regulation is mainly through genetic means.

  • Increased during starvation, directed by glucagon.

  • Suppression in a fed state, directed by insulin.

Pathway Integration: Glycolysis, Glycogen Metabolism, and Gluconeogenesis

• Feeding-Fasting Transition

  • During a meal, glucose moves from the digestive tract to the bloodstream.

    • Triggers insulin release, activating glycogen synthesis in the liver and muscle.

    • Suppression of liver gluconeogenesis.

    • Glycogenolysis is depressed in liver and muscle.

    • Stimulation of glucose transport in muscle.

  • Prominent carbohydrate pathways in the fed state.

    • Glycogen synthesis.

    • Glycolysis.

The Feeding-Fasting Transition

• After a meal (postprandial state).

  • Glucose concentration drops.

    • Decreases insulin release.

    • Stimulates glucagon release.

    • Epinephrine is released in response to low blood glucose.

      • Stimulates glycogen breakdown in the liver.

      • Rise in intracellular Ca^{2+} by epinephrine.

The Resting-Exercise Transition

• Skeletal muscle is a major consumer of glucose in the resting state.

• During exercise, glucose utilization is increased 100-fold.

• Regulation is largely intrinsic.

  • Increased AMP that results from the extensive adenine nucleotide turnover activates AMPK.

    • Increased glucose uptake, inhibits glycogen formation, stimulates glycolysis.

• During exercise, blood flow is shunted away from the liver.

  • When exercise is finished, blood flow returns.

    • Lactate is brought to the liver, where it is converted to glucose by gluconeogenesis.

    • Exercise also causes a rise in intracellular [Ca^{2+}].

      • Epinephrine increases cytosolic [Ca^{2+}], stimulating liver glycogen breakdown.

      • In muscle, [Ca^{2+}] is elevated during the contraction cycle, activating glycogen breakdown.

The Pentose Phosphate Shunt

• Occurs in nearly all cells.

• Two distinct contributions to the cell:

  1. Partially oxidize glucose-6-P and generate NADPH.

  2. Produce sugar phosphates, including ribose phosphates.

• In vitro, NAD^+/NADH and NADP^+/NADPH have identical redox states and \Delta E°.

  • However, in the cell, NADP^+/NADPH is more reduced (~5-fold more).

Oxidative Stage

• Metabolically irreversible.

• Glucose-6-P + 2 \ NADP^+ \rightarrow Ribulose-5-P + CO_2 + 2 \ NADPH

  • The first step is catalyzed by glucose-6-P DH (oxidative decarboxylation).

    • Results in 6-phosphogluconolactone (GPG lactone or PGL).

    • Allosterically inhibited by NADPH.

  • The second step is catalyzed by lactonase.

    • Converts 6-phosphogluconolactone to 6-P-gluconate (GPG).

  • The third step is catalyzed by 6-P-gluconate DH.

    • Converts GPG to ribulose-5-P (Ru5P).

Nonoxidative Stage

• All enzymes in this stage are near-equilibrium.

• Ribulose-5-P undergoes two reactions.

  1. Epimerase reaction.

     • Ru5P to X5P (ketone xylulose P).

  2. Isomerase reaction.

     • Ru5P to R5P (ribose 5P).

  • Intermediates (R5P) can be incorporated into ribonucleotides.

Nonoxidative Stage

• Conversion of R5P to two intermediates in glycolysis.

  1. Glyceraldehyde-P (GAP).

  2. Fructose-6-P (F6P).

• Conversion is performed by two enzymes.

  1. Transketolase

     • Cleavage occurs between the carbonyl and alpha carbon.

  2. Transaldolase

     • Cleavage occurs between the carbonyl and beta carbon.

  • For both enzymes, a fragment is added to the aldehyde group.

Distribution Between NADPH Production and R5P

• Cellular demand fluctuates between the requirement for NADPH and ribose-5-P (R5P).

  • Relative needs can be accommodated by the near-equilibrium nature of the nonoxidative segment of the pentose phosphate pathway.

  • If there's a higher need for ribose carbon, more is drawn from the pathway.

  • If there's a higher need for NADPH, then more carbon is returned to glycolytic intermediates, and less R5P is removed.

Galactose Utilization

• Galactose comes from lactose.

  • Hydrolysis of lactose yields glucose and galactose.

• Galactose is metabolized in the liver.

  • The pathway intersects with glycogen metabolism via a UDP-sugar intermediate.

• Catabolism

  • Galactose is converted to galactose-1-P.

    • Liver-specific galactokinase.

  • Galactose switches places with glucose of UDP-glucose.

    • Uridyltransferase (produces glucose-1-P and UDP-galactose).

  • UDP-galactose is converted to UDP-glucose.

    • Epimerase activity.

  • UDP-glucose can become UDP-glucuronate.

Questions Based on Learning Objectives

1. Gluconeogenesis Overview: What is the primary role of gluconeogenesis in the body?

2. Rate Limiting Enzymes: Which of the following enzymes is involved in the rate-limiting step of gluconeogenesis?

3. Regulation of Fructose 1,6 Bisphosphatase: What molecule allosterically inhibits fructose one six bisphosphatase?

4. Feeding and Fasting Transition: During the fasting state, which hormone stimulates gluconeogenesis in the liver?

   • a. Insulin

   • b. Glucagon

   • c. Epinephrine

   • d. Cortisol

5. Pentose Phosphate Pathway: What is the primary product of the oxidative phase of the pentose phosphate pathway?

6. Exercise and Metabolism. During exercise, glucose utilization in muscles increases due to:

   • a. Increased levels of glucagon

   • b. Elevated levels of AMP

   • c. Decreased intracellular calcium

   • d. Suppressed glycolysis

7. Galactose Metabolism. Which organ is primarily responsible for metabolizing galactose?

Lipid Metabolism

Learning Objectives

1. Differentiate between endogenous and exogenous lipid pathways and explain lipid absorption in the small intestine.

2. Trace the role of chylomicrons and the lymphatic system in lipid transport.

3. Summarize the steps of fatty acid oxidation, including activation, transport, and beta-oxidation.

4. Explain the synthesis and role of ketone bodies as energy sources during fasting.

5. Outline the key steps in fatty acid synthesis and the role of regulation.

6. Relate lipid metabolism to health issues such as ketosis and metabolic disorders.

Lipid Metabolism

• Two routes of lipid assimilation in mammals:

  1. Endogenous:

     • Biosynthesis of fatty acids from excess dietary carbohydrates and proteins.

  2. Exogenous:

     • From dietary lipids.

• Chyme is produced when food enters the stomach, and organ muscles produce a liquefied suspension.

• Chyme mixes with bile in the small intestine.

  • Emulsification occurs when bile mixes with lipids to form smaller lipid droplets.