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Second messengers Knowledge-Based Study

Biology · Cell Communication & the Cell Cycle · Biology · Cell Communication

Biology topic explanations on Second messengers.

Study notes

  1. What Are Second Messengers?

    In cellular communication, signaling begins when an extracellular signaling molecule, known as the first messenger, binds to a specific receptor on the target cell surface. Because many first messengers are large or hydrophilic, they cannot cross the phospholipid bilayer of the plasma membrane. To relay the message internally, the activated receptor triggers the generation of intracellular signaling molecules known as second messengers.

    Second messengers share several distinct characteristics:

    • Small and non-protein: Unlike enzymes or receptors, they are low-molecular-weight molecules or ions.
    • Rapid diffusion: Due to their small size, water-soluble second messengers can diffuse rapidly throughout the cytosol, while lipid-soluble ones diffuse within the plane of the membrane.
    • Signal amplification: A single activated receptor can stimulate the production or release of thousands of second messenger molecules, greatly magnifying the strength of the incoming signal.

    The Adenylyl Cyclase and cAMP Cascade

    One of the most widespread second messengers is cyclic adenosine monophosphate (cAMP). The production of cAMP typically begins when a ligand binds to a G protein-coupled receptor (GPCR), activating a stimulatory G protein (Gs).

    1. Activation of Adenylyl Cyclase: The activated G protein alpha subunit binds to and activates adenylyl cyclase, an enzyme embedded in the plasma membrane.
    2. Synthesis of cAMP: Adenylyl cyclase converts ATP into cAMP by removing two phosphate groups and forming a cyclic bond.
    3. Activation of Protein Kinase A (PKA): Elevated cytosolic levels of cAMP enable it to bind to the regulatory subunits of Protein Kinase A (PKA). This releases the active catalytic subunits, which subsequently phosphorylate target proteins to alter cellular metabolism, gene expression, or ion channel conductance.

    To prevent continuous stimulation, the signal is terminated by the enzyme phosphodiesterase, which hydrolyzes cAMP into inactive AMP.

    Cleavage of PIP2 into Two Second Messengers

    Another major signaling pathway utilizes membrane phospholipids. When specific GPCRs or receptor tyrosine kinases are activated, they stimulate the membrane-bound enzyme phospholipase C (PLC). Phospholipase C cleaves a specific membrane phospholipid called phosphatidylinositol 4,5-bisphosphate (PIP2) into two distinct second messengers:

    • Diacylglycerol (DAG): DAG remains embedded within the plasma membrane due to its hydrophobic nature. Together with calcium ions, DAG recruits and activates Protein Kinase C (PKC), which phosphorylates specific intracellular proteins.
    • Inositol 1,4,5-trisphosphate (IP3): IP3 is hydrophilic and rapidly diffuses through the cytosol toward the smooth endoplasmic reticulum (ER).

    Calcium Ions as Second Messengers

    The ER maintains a much higher concentration of calcium ions than the resting cytosol. When IP3 binds to IP3-gated calcium channels on the ER membrane, the channels open and allow calcium ions to rush down their concentration gradient into the cytosol. The surge in cytosolic calcium concentration activates various calcium-sensitive proteins, such as calmodulin, leading to widespread physiological responses.

    Controlling and Resetting the Signal

    Second messenger systems provide essential advantages in cellular regulation:

    • Cascading Amplification: Binding of a single hormone molecule can activate multiple G proteins, each activating adenylyl cyclase to generate numerous cAMP molecules. Each active kinase can then phosphorylate hundreds of target enzymes, achieving an exponential biological response.
    • Speed and Integration: Small second messengers distribute quickly to coordinate diverse cellular processes simultaneously.

    For a cell to remain sensitive to subsequent signals, second messenger levels must be strictly regulated and quickly returned to baseline:

    • cAMP and cGMP: Degraded into inactive monophosphates by specific phosphodiesterases.
    • Calcium Ions: Actively pumped out of the cytosol back into the lumen of the endoplasmic reticulum and across the plasma membrane by ATP-dependent calcium pumps.
    • Protein Phosphorylation: Reversed by protein phosphatases, which remove phosphate groups added by kinases.