Metabolism Notes

Introduction to Metabolism

Learning Objectives

  • Describe catabolism and anabolism.
  • Explain the regulation of metabolism.
  • Explain second messenger systems.
  • Explain action mechanisms of G proteins.
  • Explain action mechanisms of protein kinases, protein phosphatases, and phosphodiesterase.
  • Explain the effects of Vibrio cholera and Bordetella pertussis toxins on G proteins.
  • Define the effect of caffeine on phosphodiesterase.

Introduction to Metabolism

  • Catabolic Reactions:
    • Break down complex molecules (proteins, polysaccharides, lipids) into simple molecules (CO2, NH3, H2O).
  • Anabolic Pathways:
    • Form complex end products from simple precursors (e.g., glycogen from glucose).
  • Cycles:
    • Pathways that regenerate a component.

Catabolic Pathways

  • Capture chemical energy in the form of ATP from the degradation of energy-rich fuel molecules.
  • Convert molecules from the diet (or nutrient molecules stored in cells) into building blocks for synthesizing complex molecules.
  • Typically oxidative and require oxidized coenzymes such as nicotinamide adenine dinucleotide (NAD+).

Energy Generation - 3 Stages

  • Stage I:
    • Hydrolysis of complex molecules into building blocks.
    • Carbohydrates → Monosaccharides
    • Fats → Glycerol, fatty acids
    • Proteins → Amino acids
  • Stage II:
    • Conversion of building blocks to acetyl CoA (or other simple intermediates).
  • Stage III:
    • Oxidation of acetyl CoA; oxidative phosphorylation.
    • TCA cycle => ATP, CO2

Metabolic Map

The image illustrates various metabolic pathways and their interconnections. Key pathways and molecules include:

  • Glycogen synthesis and degradation, involving glucose 1-P, glucose 6-P, and UDP-glucose.
  • Glycolysis, converting glucose to pyruvate.
  • The citric acid cycle (TCA cycle), oxidizing acetyl CoA to produce energy.
  • Fatty acid synthesis and degradation, involving fatty acyl CoA and malonyl CoA.
  • Amino acid metabolism, showing the conversion of several amino acids into metabolic intermediates.
  • The urea cycle, for the disposal of nitrogenous waste.

Anabolic Pathways

  • Combine small molecules (amino acids) to form complex molecules (proteins).
  • Require energy (endergonic), provided by ATP hydrolysis to ADP and inorganic phosphate (Pi).
  • Involve chemical reductions, often using NADPH as an electron donor.

Catabolism vs. Anabolism

  • Catabolism: Complex molecules are broken down into energy-poor end products like carbon dioxide, water, and ammonia, yielding chemical energy in the form of ATP and NADH.
  • Anabolism: Precursor molecules like amino acids, sugars, fatty acids, and nitrogenous bases are assembled into complex molecules like proteins, polysaccharides, lipids, and nucleic acids, requiring energy input.

Regulation of Metabolism

  • Pathways must be coordinated to meet the cell's needs for energy production and synthesis of end products.
  • Cells are part of interacting tissues, requiring a communication system to coordinate body functions.
  • Regulatory signals include hormones, neurotransmitters, and nutrient availability, influencing signals within the cell.

Intracellular Communication

  • The rate of a metabolic pathway can respond to regulatory signals within the cell.
  • Influenced by:
    • Substrate availability
    • Product inhibition
    • Allosteric activators or inhibitors
  • Typically elicit rapid responses.
  • Important for moment-to-moment regulation of metabolism.

Intercellular Communication

  • Essential for organism development and survival.
  • Signaling between cells provides long-range integration of metabolism.
  • Usually results in a response, such as a change in gene expression (slower than intracellular signals).
  • Communication can be mediated by:
    • Surface-to-surface contact
    • Gap junctions
    • Chemical signaling via bloodborne hormones or neurotransmitters (most important for energy metabolism).

Second Messenger Systems

  • Hormones or neurotransmitters act as signals, and their receptors act as signal detectors, linking extracellular events to intracellular changes.
  • Many receptors initiate reactions that result in a specific intracellular response upon ligand binding.
  • Second messengers intervene between the original messenger and the ultimate effect on the cell.
  • Part of the cascade that translates hormone or neurotransmitter binding into a cellular response.

Common Second Messenger Systems

  • Calcium/phosphatidylinositol system
  • Adenylyl cyclase (adenylate cyclase) system (important in regulating intermediary metabolism pathways).

Adenylyl Cyclase

  • Recognition of a chemical signal by cell membrane receptors (e.g., β- and α2-adrenergic receptors) triggers an increase or decrease in adenylyl cyclase (AC) activity.
  • Adenylyl cyclase is a membrane-bound enzyme that converts ATP to cyclic AMP (cAMP).
  • Chemical signals are often hormones or neurotransmitters, each binding to a unique type of membrane receptor.
  • Tissues responding to multiple chemical signals have different receptors, each linked to AC.

G Protein-Coupled Receptors (GPCRs)

  • Characterized by:
    • An extracellular ligand-binding domain
    • 7 transmembrane α helices
    • An intracellular domain that interacts with G proteins

GTP-Dependent Regulatory Proteins (G Proteins)

  • α subunit binds guanine nucleotides (GTP and GDP).
  • Form a communication link between the receptor and adenylyl cyclase.
  • In the inactive form, the α subunit is bound to GDP.

G Protein Activation

  1. Unoccupied receptor does not interact with Gs protein.
  2. Hormone binding causes a conformational change in the receptor, triggering the replacement of GDP with GTP on the α subunit of the G protein.
  3. The GTP-bound α subunit dissociates from the βγ subunits and activates adenylyl cyclase.
  4. Adenylyl cyclase converts ATP to cAMP + PPi.

Gα–GTP Complex

  • The GTP-bound form of the α subunit dissociates from the βγ subunits and then moves to adenylyl cyclase, activating it.
  • Many active Gα protein molecules are formed by one activated receptor.

Gα–GTP Complex Deactivation

  • Actions of the Gα–GTP complex are short-lived.
  • Gα has inherent GTPase activity, rapidly hydrolyzing GTP to GDP.
  • This inactivates Gα, causing it to dissociate from adenylyl cyclase and reassociate with the βγ dimer.

Hormone/Neurotransmitter Effect on Adenylyl Cyclase

  • The ability of a hormone or neurotransmitter to stimulate or inhibit adenylyl cyclase depends on the type of Gα protein linked to the receptor.
  • Gs stimulates adenylyl cyclase.
  • Gi inhibits adenylyl cyclase.

Abnormal G Protein Signaling - Cholera Toxin

  • Cholera toxin is encoded by a bacteriophage inside Vibrio cholera.
  • The enterotoxin contains two A subunits and 5 B subunits.
  • The B subunit binds to ganglioside GM1 on the surface of intestinal mucosal cells.
  • The A subunit enters the membrane, leading to ADP ribosylation of the alpha subunit of Gs protein.
  • This inhibits inherent GTPase activity and causes irreversible G protein activation.
  • Adenylyl cyclase remains continuously active, keeping cyclic AMP levels high.
  • This prevents absorption of salts from the intestine, leading to watery diarrhea and loss of water.

Cholera Toxin Effects

  • Chronic elevation of cAMP in the large intestine results in a sustained PKA-mediated phosphorylation of chloride channels (CFTR) that normally regulate salt and water transport.
  • Hyperactivity of these channels results in loss of NaCl with watery diarrhea (liquid stools), which can be fatal.
  • Patients may lose as much as 1 L of water per hour.

Abnormal G Protein Signaling - Pertussis Toxin

  • Pertussis toxin ADP ribosylates the alpha subunit of Gi protein.
  • Prevents the Gi-GDP complex from interacting with the activated receptor.
  • The action of hormones acting through Gi is inhibited.

Toxins and Adenylyl Cyclase

  • Toxins from Vibrio cholerae (cholera) and Bordetella pertussis (whooping cough) cause inappropriate activation of adenylyl cyclase through covalent modification (ADP-ribosylation) of different G proteins.
  • With cholera, the GTPase activity of Gαs is inhibited in intestinal cells.
  • With whooping cough, Gαi is inactivated in respiratory-tract cells.
  • The result in both situations is increased adenylyl cyclase activity and excess production of cAMP.

Protein Kinases

  • The next key link in the cAMP second messenger system is activation by cAMP of a family of enzymes called cAMP-dependent protein kinases, such as protein kinase A.
  • cAMP activates protein kinase A by binding to its two regulatory subunits, causing the release of two active, catalytic subunits.
  • Active subunits catalyze the transfer of phosphate from ATP to specific serine or threonine residues of protein substrates.

Protein Kinase A

  • Phosphorylated proteins may act directly on the cell’s ion channels or, if enzymes, may become activated or inhibited.
  • Protein kinase A can also phosphorylate proteins that bind to DNA, causing changes in gene expression.
  • Several types of protein kinases are not cAMP-dependent, for example, protein kinase C.

Dephosphorylation of Proteins

  • Phosphate groups added to proteins by protein kinases are removed by protein phosphatases, enzymes that hydrolytically cleave phosphate esters.
  • This ensures that changes in protein activity induced by phosphorylation are not permanent.

Hydrolysis of Cyclic AMP

  • cAMP is rapidly hydrolyzed to 5'-AMP by phosphodiesterase, one of a family of enzymes that cleave the cyclic 3',5'-phosphodiester bond.
  • 5'-AMP is NOT an intracellular signaling molecule.
  • Therefore, the effects of neurotransmitter- or hormone-mediated increases of cAMP are rapidly terminated if the extracellular signal is removed.

Phosphodiesterase Inhibition

  • Phosphodiesterase is inhibited by the methylxanthine derivative, caffeine.