Chapter 17 - Section 17.4 - Study Guide - Assess Your Learning Outcomes - Page 669: 8

Several hormones require second messengers to activate target cells. These hormones include hydrophilic hormones, such as peptide hormones and amines, that bind to cell surface receptors. Second messengers are intracellular molecules that transmit signals from the cell membrane, where the hormone-receptor interaction occurs, to the cell's interior. Three important second messengers are cyclic AMP (cAMP), diacylglycerol (DAG), and inositol trisphosphate (IP3). **1. Cyclic AMP (cAMP):** Cyclic AMP is a second messenger that is commonly used by many hormones to transmit signals to target cells. The process involves the following steps: - Hormone Binding: A hormone, such as epinephrine or glucagon, binds to its cell surface receptor. - G Protein Activation: The hormone-receptor interaction activates a G protein, which is a molecular switch located on the inner side of the plasma membrane. - Activation of Adenylyl Cyclase: The activated G protein activates an enzyme called adenylyl cyclase, which converts adenosine triphosphate (ATP) into cyclic AMP (cAMP). - cAMP Production: Cyclic AMP acts as a secondary messenger that diffuses throughout the cell. - Activation of Protein Kinase A (PKA): Inside the cell, cAMP binds to and activates protein kinase A (PKA), a protein that phosphorylates target proteins. - Cellular Response: Phosphorylation of target proteins by PKA leads to various cellular responses, such as changes in enzyme activity, gene expression, and ion channel regulation. **2. Diacylglycerol (DAG) and Inositol Trisphosphate (IP3):** These second messengers are involved in a signaling pathway known as the phospholipase C (PLC) pathway. Here's how it works: - Hormone Binding: Hormones, such as certain neurotransmitters and peptide hormones, activate a receptor that couples to a G protein. - Activation of Phospholipase C: The activated G protein activates an enzyme called phospholipase C (PLC). - Generation of DAG and IP3: PLC cleaves a phospholipid in the plasma membrane into two products: diacylglycerol (DAG) and inositol trisphosphate (IP3). - DAG's Role: DAG remains in the membrane and activates protein kinase C (PKC), an enzyme that phosphorylates target proteins. - IP3's Role: IP3 diffuses into the cytoplasm and binds to receptors on the endoplasmic reticulum, leading to the release of stored calcium ions (Ca²⁺). - Cellular Response: Calcium ions and PKC modulate various cellular processes, including gene expression, enzyme activity, and muscle contraction. In summary, second messengers such as cAMP, DAG, and IP3 are crucial intermediaries in signal transduction pathways. They amplify and transmit the signals from the cell surface to the interior, ultimately leading to various cellular responses. These mechanisms are essential for coordinating the physiological effects of hormones on target cells.

Several hormones require second messengers to activate target cells. These hormones include hydrophilic hormones, such as peptide hormones and amines, that bind to cell surface receptors. Second messengers are intracellular molecules that transmit signals from the cell membrane, where the hormone-receptor interaction occurs, to the cell's interior. Three important second messengers are cyclic AMP (cAMP), diacylglycerol (DAG), and inositol trisphosphate (IP3). **1. Cyclic AMP (cAMP):** Cyclic AMP is a second messenger that is commonly used by many hormones to transmit signals to target cells. The process involves the following steps: - Hormone Binding: A hormone, such as epinephrine or glucagon, binds to its cell surface receptor. - G Protein Activation: The hormone-receptor interaction activates a G protein, which is a molecular switch located on the inner side of the plasma membrane. - Activation of Adenylyl Cyclase: The activated G protein activates an enzyme called adenylyl cyclase, which converts adenosine triphosphate (ATP) into cyclic AMP (cAMP). - cAMP Production: Cyclic AMP acts as a secondary messenger that diffuses throughout the cell. - Activation of Protein Kinase A (PKA): Inside the cell, cAMP binds to and activates protein kinase A (PKA), a protein that phosphorylates target proteins. - Cellular Response: Phosphorylation of target proteins by PKA leads to various cellular responses, such as changes in enzyme activity, gene expression, and ion channel regulation. **2. Diacylglycerol (DAG) and Inositol Trisphosphate (IP3):** These second messengers are involved in a signaling pathway known as the phospholipase C (PLC) pathway. Here's how it works: - Hormone Binding: Hormones, such as certain neurotransmitters and peptide hormones, activate a receptor that couples to a G protein. - Activation of Phospholipase C: The activated G protein activates an enzyme called phospholipase C (PLC). - Generation of DAG and IP3: PLC cleaves a phospholipid in the plasma membrane into two products: diacylglycerol (DAG) and inositol trisphosphate (IP3). - DAG's Role: DAG remains in the membrane and activates protein kinase C (PKC), an enzyme that phosphorylates target proteins. - IP3's Role: IP3 diffuses into the cytoplasm and binds to receptors on the endoplasmic reticulum, leading to the release of stored calcium ions (Ca²⁺). - Cellular Response: Calcium ions and PKC modulate various cellular processes, including gene expression, enzyme activity, and muscle contraction. In summary, second messengers such as cAMP, DAG, and IP3 are crucial intermediaries in signal transduction pathways. They amplify and transmit the signals from the cell surface to the interior, ultimately leading to various cellular responses. These mechanisms are essential for coordinating the physiological effects of hormones on target cells.