Peptides, steroids and amine hormones

Overview of Hormones

• All hormones can be classified into three broad classes: peptidic, steroidal, and amine hormones. These classifications are based on their chemical structure, which fundamentally dictates their synthesis, transport, receptor interactions, and ultimate physiological effects.

• The structure of hormones profoundly influences their action, a crucial physiological principle.

• Learning objectives include understanding:

  • Nature of hormones: Chemical signaling molecules secreted by endocrine glands into the bloodstream to regulate distant target cells.

  • Receptor location: Where hormones bind on or within target cells (e.g., cell surface vs. intracellular).

  • Speed and mechanism of action: How rapidly the hormone elicits a response and the biochemical pathways involved (e.g., second messenger systems vs. gene expression).

  • Half-life of hormones: The time it takes for the concentration of a hormone to decrease by half in the blood, influencing duration of action.

Classes of Hormones

Peptide Hormones

• Composed of linked amino acids, ranging from small peptides to large proteins (e.g., insulin is a well-known example, consisting of two chains linked by disulfide bonds). These are synthesized like other proteins: transcription of DNA to mRNA, translation on ribosomes into a preprohormone, processing in the endoplasmic reticulum and Golgi apparatus (cleavage, glycosylation, folding) to form a prohormone and then an active hormone. They are stored in secretory vesicles and released via exocytosis upon stimulation.

• Since they are made of amino acids, peptide hormones are classified as proteins.

Steroid Hormones

• Derived from cholesterol, a complex lipid molecule. The synthesis involves a series of enzymatic modifications of cholesterol, primarily in the adrenal cortex, gonads, and placenta. They are not stored in vesicles; instead, they are synthesized on demand and immediately diffuse out of the cell after production due to their lipophilic nature.

• Examples include testosterone, progesterone, and estrogen.

• Therefore, steroid hormones are classified as lipids.

Amine Hormones

• Modified from single amino acids (e.g., epinephrine and norepinephrine are derived from tyrosine; melatonin from tryptophan). These hormones can act as both neurotransmitters and hormones.

• Catecholamines are stored in vesicles and released by exocytosis, similar to peptide hormones, while thyroid hormones are stored extracellularly in the thyroid colloid and released by proteolysis.

• Amine hormones, particularly catecholamines, are chemically related to proteins.

Structure Influencing Function

• A fundamental physiological theme is: structure dictates function. The chemical structure of a hormone (e.g., whether it's lipid-soluble or water-soluble) directly determines its physiological properties.

• Understanding the characteristics and mechanisms of hormone action requires a thorough examination of their inherent chemical structure and how it interacts with cellular components.

Detailed Comparison of Peptide and Steroid Hormones

Lipophilicity

• Lipophilic: Refers to substances that 'love lipids' and are thus lipid-soluble. They can readily dissolve in and pass through lipid-based cell membranes.

  • Steroid hormones are highly lipophilic, allowing them to diffuse freely across the plasma membrane of target cells.

• Lipophobic: Refers to substances that 'fear lipids' and are water-soluble, meaning they are not lipid-soluble. They cannot easily pass through lipid-based membranes.

  • Peptide hormones are lipophobic (hydrophilic) and thus cannot cross the lipid bilayer of the plasma membrane without specific transport mechanisms, which are generally not present for their entry.

Transport in Blood

• Steroid Hormones:

  • Due to their lipophilic nature, they cannot dissolve directly in water-based blood plasma. They would aggregate and be rapidly metabolized or excreted.

  • To overcome this, they are transported in the bloodstream primarily by binding to specific plasma proteins (e.g., albumin, sex hormone-binding globulin (SHBG), corticosteroid-binding globulin (CBG)). This binding increases their water solubility, prevents rapid degradation, and extends their half-life.

• Peptide Hormones:

  • Being water-soluble, they can easily dissolve directly in blood plasma.

  • They generally do not require protein binding for transport, though some larger peptide hormones might associate non-specifically with plasma proteins. Their half-lives are typically much shorter due to rapid enzymatic degradation.

Receptor Locations

• Steroid Hormones:

  • Because of their lipophilic nature, they can readily diffuse through the plasma membrane and enter the cytoplasm or nucleus of target cells.

  • Therefore, they bind to intracellular receptors located either in the cytosol or within the nucleus.

• Peptide Hormones:

  • Since they are lipophobic and cannot cross the lipid-based plasma membrane, they must bind to specific cell surface receptors (transmembrane proteins) located on the outer surface of the plasma membrane. This binding initiates an intracellular signaling cascade.

Mechanisms of Action

Peptide Hormones

• Upon binding to cell surface receptors, peptide hormones typically activate second messenger systems. The hormone itself (1^{st} messenger) does not enter the cell, but rather a signal is transduced across the membrane.

• A common example involves the activation of adenylyl cyclase, an enzyme that converts ATP into cyclic AMP (cAMP). cAMP then acts as a second messenger.

• This often leads to the activation of protein kinases (e.g., Protein Kinase A for cAMP pathway), which catalyze the phosphorylation of various intracellular proteins. The addition of a phosphate group can alter the protein's conformation and thus activate or inhibit its activity.

• These cascades allow for signal amplification: a single hormone molecule can trigger the production of many second messenger molecules, leading to a large cellular response.

• Speed of action: Generally fast, often within minutes to seconds, as it involves modifying existing proteins.

• Duration of action: Relatively short half-life, requiring continuous secretion for sustained effects.

Steroid Hormones

• After diffusing into the cell and binding to their intracellular receptors, the hormone-receptor complex translocates to the nucleus (if initially in the cytosol).

• The hormone-receptor complex then acts as a transcription factor, binding directly to specific DNA sequences called Hormone Response Elements (HREs) within the promoter regions of target genes.

• This binding leads to the activation or repression of gene transcription, resulting in the synthesis of new messenger RNA (mRNA). The mRNA is then translated into new proteins (enzymes, structural proteins, regulatory proteins), which mediate the hormone's long-term effects.

• Speed of action: Slower, typically taking hours to days to elicit a full response due to the time required for gene expression and protein synthesis.

• Duration of action: Longer half-life and sustained effects due to the production of new proteins.

Summary of Differences Between Peptide and Steroid Hormones

Specifics of Signaling Pathways for Peptide Hormones

• Types of Receptors:

G Protein Coupled Receptors (GPCRs)

• These are transmembrane proteins that, upon hormone binding, activate an associated G protein (guanine nucleotide-binding protein). G proteins consist of \alpha , \beta , and \gamma subunits.

• Activation leads to the dissociation of the \alpha subunit, which can then:

  1. Directly act on ion channels: For example, opening calcium or potassium channels to alter membrane potential or ion concentrations.

  2. Activate or inhibit enzymes: Such as adenylyl cyclase (leading to cAMP production) or phospholipase C (leading to IP_3 and DAG production, which elevate intracellular Ca^{2+} and activate Protein Kinase C, respectively). These enzymatic activations lead to a cascade of downstream effects, often involving phosphorylation of target proteins.

Receptor Enzymes

• These receptors have an intrinsic enzymatic activity (or are closely associated with an enzyme) that is activated upon hormone binding. They are generally simpler than GPCRs in their direct signaling.

• A common type is the Receptor Tyrosine Kinase (RTK). When a hormone (e.g., insulin, growth factors) binds, the receptor typically dimerizes and undergoes autophosphorylation on its tyrosine residues.

• These phosphorylated tyrosine residues then serve as docking sites for various intracellular signaling proteins, which become activated and propagate the signal through the cell, often by phosphorylating other proteins.

Mechanisms of Action for Steroid Hormones (Detailed Steps)

1. Transport in blood: Steroid hormones are largely inactive when bound to their protein carriers in the blood. Only the small fraction of free hormone is biologically active and can diffuse into target cells, maintaining an equilibrium between bound and free forms.

2. Binding and diffusion: The free hormone dissociates from its carrier in the vicinity of target cells and diffuses passively across the lipophilic plasma membrane directly into the cytoplasm. This process is energy-independent.

3. Intracellular interaction: Once inside the cell, the hormone binds to specific cytoplasmic or nuclear receptors. These receptors are often associated with heat shock proteins (HSPs) in their inactive state. Hormone binding causes a conformational change in the receptor, leading to the dissociation of HSPs and activation of the hormone-receptor complex.

4. Gene binding: The activated hormone-receptor complex then translocates into the nucleus (if it was initially cytoplasmic) and binds to specific Hormone Response Elements (HREs) on the DNA. This binding acts as a transcriptional regulator, either enhancing or inhibiting the transcription of specific target genes, ultimately altering cellular function through the synthesis of new proteins.

Amine Hormones

• Amine hormones are subdivided into two main classes based on their synthesis, transport, and mechanisms of action:

  • Catecholamines: (e.g., epinephrine, norepinephrine, dopamine) - These are derived from tyrosine, synthesized in the adrenal medulla and neurons, stored in vesicles, and released via exocytosis. They are lipophobic, dissolve readily in blood, and bind to cell membrane receptors (e.g., adrenergic receptors). Their actions are rapid, mirroring those of peptide hormones via second messenger systems.

  • Thyroid Hormones: (e.g., T3 (triiodothyronine), T4 (thyroxine)) - These are also derived from tyrosine but are uniquely synthesized by iodination of tyrosine residues within a large glycoprotein called thyroglobulin, which is stored extracellularly in the thyroid gland's colloid. They require protein carriers (e.g., thyroxine-binding globulin, albumin) in the blood because they are lipophilic. Thyroid hormones bind to intracellular receptors (primarily nuclear receptors) and act slowly by regulating gene transcription, similar to steroid hormones, despite their amino acid origin.

Conclusion

• The ability to differentiate between peptide, steroid, and amine hormones based on their chemical properties and physiological characteristics is crucial for understanding endocrine regulation.

• A key overarching concept is that lipid solubility (lipophilic or lipophobic) dictates nearly every aspect of a hormone's journey and action: its synthesis and storage, how it is transported in the blood, the location of its receptors on or within target cells, its specific mechanism of action, and ultimately, the speed and duration of its physiological effects.