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37.2C: Plasma Membrane Hormone Receptors

  • Page ID
    13973
  • Hormones that cannot diffuse through the plasma membrane instead bind to receptors on the cell surface, triggering intracellular events.

    LEARNING OBJECTIVES

    Describe the events that occur when a hormone binds to a plasma hormone receptor

    KEY TAKEAWAYS

    Key Points

    • When a lipid (fat) insoluble hormone binds to a plasma membrane hormone receptor, this triggers specific actions inside the cell that alter the cell’s activities, such as gene expression.
    • Because the first event in this sequence is the binding of the hormone to the plasma membrane receptor, the hormone is called the “first messenger”, while the molecule that is activated within the cell and carries out intracellular change is called the ” second messenger “.
    • In many cases, a hormone binding to a plasma membrane receptor activates a special kind of protein called a G protein, which in turn activates an enzyme that generates cAMP, a second messenger.
    • cAMP activates another group of proteins called protein kinases, which can change the structure of other molecules by adding a phosphate group to them; these activated molecules can then affect changes within the cell.

    Key Terms

    • second messenger: any substance used to transmit a signal within a cell, especially one which triggers a cascade of events by activating cellular components
    • cyclic adenosine monophosphate: cAMP, a second messenger derived from ATP that is involved in the activation of protein kinases and regulates the effects of adrenaline
    • G protein: any of a class of proteins, found in cell membranes, that pass signals between hormone receptors and effector enzymes

    Plasma Membrane Hormone Receptors

    Amino acid-derived hormones and polypeptide hormones are not lipid-derived (lipid-soluble or fat-soluble); therefore, they cannot diffuse through the plasma membrane of cells. Lipid-insoluble hormones bind to receptors on the outer surface of the plasma membrane, via plasma membrane hormone receptors. Unlike steroid hormones, lipid-insoluble hormones do not directly affect the target cell because they cannot enter the cell and act directly on DNA. Binding of these hormones to a cell surface receptor results in activation of a signaling pathway; this triggers intracellular activity to carry out the specific effects associated with the hormone. In this way, nothing passes through the cell membrane; the hormone that binds at the surface remains at the surface of the cell while the intracellular product remains inside the cell. The hormone that initiates the signaling pathway is called a first messenger, which activates a second messenger in the cytoplasm.

    One very important second messenger is cyclic adenosine monophosphate (cAMP). When a hormone binds to its membrane receptor, a G protein that is associated with the receptor is activated. G proteins are proteins separate from receptors that are found in the cell membrane. When a hormone is not bound to the receptor, the G protein is inactive and is bound to guanosine diphosphate, or GDP. When a hormone binds to the receptor, the G protein is activated by binding guanosine triphosphate, or GTP, in place of GDP. After binding, GTP is hydrolyzed by the G protein into GDP and becomes inactive.

    image

    Second messenger systems: The amino acid-derived hormones epinephrine and norepinephrine bind to beta-adrenergic receptors on the plasma membrane of cells. Hormone binding to receptor activates a G protein, which in turn activates adenylyl cyclase, converting ATP to cAMP. cAMP is a second messenger that mediates a cell-specific response. An enzyme called phosphodiesterase breaks down cAMP, terminating the signal.

    The activated G protein in turn activates a membrane-bound enzyme called adenylyl cyclase. Adenylyl cyclase catalyzes the conversion of ATP to cAMP. cAMP, in turn, activates a group of proteins called protein kinases, which transfer a phosphate group from ATP to a substrate molecule in a process called phosphorylation. The phosphorylation of a substrate molecule changes its structural orientation, thereby activating it. These activated molecules can then mediate changes in cellular processes.

    The effect of a hormone is amplified as the signaling pathway progresses. The binding of a hormone at a single receptor causes the activation of many G-proteins, which activates adenylyl cyclase. Each molecule of adenylyl cyclase then triggers the formation of many molecules of cAMP. Further amplification occurs as protein kinases, once activated by cAMP, can catalyze many reactions. In this way, a small amount of hormone can trigger the formation of a large amount of cellular product. To stop hormone activity, cAMP is deactivated by the cytoplasmic enzyme phosphodiesterase, or PDE. PDE is always present in the cell, breaking down cAMP to control hormone activity; thus, preventing overproduction of cellular products.

    The specific response of a cell to a lipid-insoluble hormone depends on the type of receptors that are present on the cell membrane and the substrate molecules present in the cell cytoplasm. Cellular responses to hormone binding of a receptor include altering membrane permeability and metabolic pathways, stimulating synthesis of proteins and enzymes, and activating hormone release.

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