BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 15. INTEGRATION OF METABOLIC PATHWAYS. HORMONES

15.4. Mechanisms of Hormone Action

15.4.1. Mechanisms of Action of Peptide Hormones and Catecholamines

Signal Transduction via Membrane Receptors involves the following steps: interaction of the hormone (primary messenger) with its receptor, Formation of the hormone-receptor complex, Activation of a membrane enzyme that catalyzes the synthesis of a second messenger, and subsequent activation of specific Proteins by these messengers to influence intracellular processes. Second messengers can include molecules such as cAMP, cGMP, IP₃, DAG, Ca²⁺, and NO.

Structurally, receptors for Peptide Hormones are proteins—predominantly Glycoproteins—and may consist of a variable number of subunits. The binding of a hormone to its receptor triggers the activation of Intracellular regulatory systems. The hormone-recognition domain of the receptor is located on the outer surface of the membrane, while the domain responsible for coupling the receptor to the effector system resides within the membrane. Over the past decades, the genes for nearly all known hormones and growth factors, their receptors, and individual Structural components of post-receptor signaling mechanisms have been successfully localized and cloned.

The most extensively studied hormonal signaling pathway is the adenylate cyclase pathway (Fig. 15.6, A), in which hormones act through a three-component system comprising a receptor protein, a G protein, and the enzyme adenylate cyclase. ATP serves as the substrate for adenylate cyclase, generating cAMP, which is a crucial intracellular messenger in hormone signal transduction. The adenylate cyclase complex is activated by many peptide hormones as well as catecholamines.

There are approximately 200 known G proteins, which are heteropolymers consisting of three subunits: α, β, and γ. The α-subunit possesses specific binding sites for GTP and GDP and mediates the interaction with both the receptor and the adenylate cyclase enzyme. The inactive form of the G protein is

- the αβγ-GDP complex, which is activated upon interaction with the hormone-receptor complex. The formation of the hormone-receptor complex induces Conformational Changes in the α-subunit, the exchange of GDP for GTP, and the dissociation of the βγ dimer. There are two types of α-subunits: α_s and α_i. If the receptor is coupled with a G_s protein, adenylate cyclase is activated by the α_s-GTP subunit, whereas if it is coupled with a G_i protein, it is inhibited by the α_i-GTP subunit. A single hormone-bound receptor can activate numerous G protein molecules, thereby amplifying the extracellular signal.

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Fig. 15.6. Signal transduction through membrane receptors: PKA - protein kinase A,

PKC - protein kinase C, PIP - phosphatidylinositol diphosphate, IP3 - Inositol trisphosphate, DAG — diacylglycerol

The next step involves the dephosphorylation of GTP bound to the α-subunit, returning the G protein to its inactive αβγ-GDP form.

Adenylate cyclase, a key enzyme found in all Cell types, is an integral membrane protein containing 12 transmembrane domains. Its extracellular domains are glycosylated, while the cytoplasmic domains feature two catalytic centers responsible for cAMP formation. Cyclic AMP regulates The activity of protein kinase A.

Protein kinase A consists of four subunits: two regulatory (R) and two catalytic (C). The R2C2 complex lacks catalytic activity. The binding of cAMP to the Regulatory Subunits, which contain specific cAMP-binding sites, leads to the dissociation of the complex:

The C subunit represents the active form of protein kinase A, which catalyzes the phosphorylation of specific proteins at Serine and Threonine residues. This protein modification alters their conformation and activity, thereby regulating the rate and direction of specific cellular processes.

G_sα mediates the signal transduction of parathyroid hormone (PTH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), antidiuretic hormone (ADH), corticotropin (ACTH), and Glucagon.

Signal transduction from many G protein-coupled receptors is mediated by inositol-1,4,5-trisphosphate (IP₃) and 1,2-diacylglycerol (DAG), which are generated through the Hydrolysis of membrane Phospholipids (Fig. 15.6, B). The inositol phosphate system comprises three primary Membrane Proteins: the receptor, phospholipase C, and a G protein. The activation of phospholipase C, triggered by hormone binding to the receptor, results in the hydrolysis of cell membrane phosphatidylinositols, producing inositol trisphosphate (IP₃), which is released into the Cytosol, and diacylglycerol (DAG), which remains in the membrane and participates in the activation of protein kinase C. IP₃ binds to specific sites on The Endoplasmic reticulum membrane, thereby opening Ca²⁺ channels. The concentration of Calcium Ions in the cytosol increases, which enhances The rate of their interaction with cytosolic protein kinase C and the protein calmodulin. The translocation of protein kinase to the membrane allows the enzyme to bind with DAG.

Protein kinase C consists of two domains: regulatory and catalytic. The regulatory domain exhibits a high affinity for Ca²⁺. The active form of protein kinase C phosphorylates proteins at serine and threonine residues.

Another pathway involves receptor autophosphorylation at Tyrosine residues (receptor tyrosine kinase activity) and the phosphorylation of tyrosine residues on intracellular protein substrates (Fig. 15.6, B). This mechanism mediates the cellular actions of Insulin, insulin-like growth factor I (IGF-I), and related factors.

A classic example of a receptor tyrosine kinase is the insulin receptor. The receptor consists of two α- and two β-subunits, which are glycoproteins linked by Disulfide Bonds. The α-subunits are located on the outer surface of the membrane and contain the insulin-binding site, whereas the β-subunits span the membrane. The α-subunit is responsible for insulin binding, while the catalytic center is located on the β-subunit. Hormone binding to the recognition site triggers the activation of the protein kinase, with the enzyme itself serving as the substrate. Autophosphorylation of the β-subunit occurs at tyrosine residues, leading to Enzyme Activation and a shift in substrate Specificity. The protein kinase then begins to phosphorylate intracellular proteins, modulating their activity. A key protein phosphorylated by tyrosine kinase is insulin receptor substrate 1 (IRS-1), which, in its phosphorylated state, can activate proteins essential for The regulation of various cellular processes, for instance.

There is also a system that generates cGMP as a second messenger. The synthesis of cGMP from GTP is catalyzed by the enzyme guanylate cyclase, which exists in both cytosolic and membrane-bound forms within The Cell. The cytosolic form of the enzyme consists of two subunits and contains a heme prosthetic group. Membrane-bound guanylate cyclase is a glycoprotein whose extracellular domain acts as a receptor, while the intracellular domain exhibits enzymatic activity. cGMP molecules can activate Ion Channels or protein kinase G. Nitric oxide (NO), synthesized from Arginine, can also function as a signaling molecule. This reaction is catalyzed by NO synthase, which is found in Nervous Tissue, vascular endothelium, and platelets. In target Cells, NO interacts with the Active Site of guanylate cyclase, accelerating The conversion of GTP to cGMP.