Hormones and Signaling Quiz

Signal Transduction I: Cell Surface Receptor

• This lecture covers the definition of hormones and their differentiation from enzymes.
• It discusses hormones' classification based on chemical structure.
• Different hormones are discussed based on biosynthesis, storage, secretion, and transportation.
• It examines the feedback mechanism regulating hormone biosynthesis.
• The role of receptors in hormone action is explained.
• Mechanisms of action producing metabolic and physiological responses are detailed.

What are Hormones?

• Substances in an organism that carry a signal to cause metabolic changes at the cellular level.
• Hormones are also known as first messengers.
• Hormones interact with specific receptors to amplify signals through second messengers.
• Various types of second messengers exist within the cytosol and membrane-associated receptors

Three basic types of second messengers

• Hydrophilic molecules: water-soluble (e.g., cAMP, cGMP, IP3, and Ca2+), located within the cytosol.
• Hydrophobic molecules: water-insoluble (e.g., diacylglycerol, phosphatidylinositols), membrane-associated, and diffusing from the plasma membrane.
• Gas molecules: gases (e.g., nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide (H2S)) that can diffuse through both cytoplasm and cellular membranes.

Classification of Hormones

• Peptide hormones: insulin, glucagon, growth hormone, hCG, FSH, ACTH, LH, etc.
• Amino acid-derived hormones: epinephrine, norepinephrine, thyroid hormones, TSH.
• Steroid hormones: aldosterone, cortisol, testosterone, estrogen.

Hormone Interaction With Receptor

• Receptors on plasma membranes for peptide hormones and amino acid-derived hormones.
• Intracellular receptors for some steroid hormones and thyroid hormones.

Mechanism of Second Messenger Synthesis

• Agonist activates membrane-bound receptor.
• G-protein is activated and produces effector.
• Effector stimulates second messenger synthesis.
• Second messenger activates intercellular processes through a cascade reaction.

General Mechanism of Signal Transduction Across Cell Membrane

• Hormone binds to cell surface receptor.
• Conformational changes enable association with a G-protein.
• Activated G-protein causes enzyme activation.
• Enzyme synthesizes intracellular second messenger.

Specificity & Regulation of Receptor

• Receptors have a high degree of specificity (e.g., b1 and b2-adrenergic receptors).
• High affinity (e.g., 10⁹-10¹¹ M⁻¹).
• Receptors can be regulated (e.g., phosphorylated receptors binding to arrestin).
• Hormone-receptor complexes can be internalized (endocytosis).

G Protein

• G-proteins (also called transducer proteins) transmit external stimuli to effector enzymes.
• Composed of three subunits (α, β, γ).
• GTPase activity: hydrolyzes GTP to GDP.
• Inactive state: GDP-bound, active state: GTP-bound.
• Subunits (αs, αi) affect adenylate cyclase activity.

Hormone-Receptor Interaction: Adenylyl Cyclase Signaling Pathway

• Resting state: receptor, G-protein, and enzyme do not interact.
• Hormone binding to receptor.
• Conformational change exposing site for G-protein attachment.
• G-protein activation via GTP exchange.
• α-GTP disassociates from βγ-complex and activates effector enzyme.

cAMP - Protein Kinase A Pathway

• cAMP is synthesized from ATP by adenylate cyclase.
• cAMP activates protein kinase A (PKA)
• PKA phosphorylates proteins, altering cellular function.

Role of cAMP in the breakdown of Glycogen

• Glucagon and epinephrine trigger glycogen breakdown via PKA.
• PKA phosphorylates glycogen phosphorylase kinase, activating glycogen phosphorylase, leading to glucose release.

Role of cAMP in the action of hormones

• Glucagon and epinephrine stimulate glycogen breakdown (glycogenolysis) and increase blood glucose via PKA activation.
• Insulin regulates glycogen synthesis.

Regulation of glycogen degradation

• Hormones like epinephrine and glucagon increase glycogen breakdown by activating glycogen phosphorylase.

Second Messengers

• Any compound acting intracellularly in response to an extracellular signal.
• Hormones need a second messenger to enter a cell as they cannot directly cross the cell membrane themselves.
• Various types of second messengers exist: cyclic AMP (cAMP), inositol trisphosphate (IP₃), diacylglycerol (DAG), Ca²⁺, cyclic GMP (cGMP), and tyrosine kinase pathways.

Inositol trisphosphate (IP3), Diacylglycerol (DAG) & Ca2+ - protein kinase C pathway

• Phospholipase C (PLC) cleaves PIP2 to form DAG and IP3.
• IP3 triggers Ca2+ release from intracellular stores.
• DAG and Ca2+ activate protein kinase C (PKC).
• PKC phosphorylates target proteins
• Cellular response

IP3 and Calcium as Second Messengers

• Hormone binds to receptor (Gq protein mediated).
• Gq-protein releases GDP and binds GTP.
• Phospholipase C (PLC) activated.
• IP3 and DAG created.
• IP₃ binds to receptor releasing Ca²⁺.
• This Ca²⁺ activates proteins & mediates cellular response

Protein Kinase C (PKC) Pathway

• Hormone binding to receptor.
• Activation of phospholipase C (PLC).
• Breakdown of PIP2 into DAG and IP3.
• IP3 causes release of Ca²⁺ from endoplasmic reticulum.
• PKC activation by Ca²⁺ and DAG.
• Phosphorylation of proteins.
• Intracellular effects like enzyme activation and modulation of other proteins

Mode of action of GnRH in the release of FSH and LH

• GnRH activation of gonadotropic cells.
• GnRH binds to cell surface receptors, causing activation of phospholipase C (PLC).
• PLC hydrolysis of PIP2 to DAG and IP3.
• Increase in cytosolic Ca2+ and PKC activation.
• Exocytosis of LH and FSH hormones from cells.

NO as a 2nd Messenger

• NO is synthesized from arginine by nitric oxide synthase (NOS).
• Endothelial forms of NOS (e.g., eNOS) and neural forms (nNOS) exist.
• NO stimulates the production of Cyclic GMP (cGMP).

(iii) Nitric Oxide can stimulate production of cGMP

• Nitric oxide interacts with the heme group of soluble guanylyl cyclase (sGC).
• The interaction converts GTP to cGMP.
• cGMP activates protein kinase G (PKG).

Role Of cAMP, DAG, cGMP

• Adenyl cyclase converts ATP to Cyclic AMP (cAMP) which activates Protein Kinase A.
• Phospholipase C converts PIP2 to IP3 and DAG which activates Protein Kinase C.
• Nitric Oxide activates guanylate cyclase to convert GTP to cyclic GMP (cGMP) which activates Protein Kinase G

(iv) Tyrosine Kinase Pathway

• Protein kinase system involves tyrosine phosphorylation within the cytoplasm.
• Important for growth factor receptors like insulin, EGF, PDGF, NGF, VEGF, and certain oncogenes.

Hypothetical scheme for signal transduction in insulin action

• Insulin binding to receptor triggers tyrosine autophosphorylation.
• Tyrosine phosphorylation regulates essential cellular processes.
• Metabolism reactions involve GLUT4 translocation and glucose uptake, glycogen synthesis and lipid synthesis, cell growth and death or apoptosis

Specific Reading Materials

• Devlin's Textbook of Biochemistry with Clinical Correlations, pp 925-931, 943-953.
• Lippincott Illustrated Review of Biochemistry, pp 79-85.
• Harpers Review of Biochemistry, 25th edition, Chapter 44 (Cell-surface and Intracellular Receptors).

Signal Transduction II: Intracellular Receptors

• This lecture covers intracellular receptors, focusing on the cytoplasmic receptors and nuclear receptors.
• Hormones that bind intracellular receptors are non-polar steroids, that can diffuse across the cell membrane.
• Intracellular receptors are located within the cell (cytoplasm or nucleus).

Summary of Signal Transduction I

• Peptide hormones cannot penetrate the plasma membrane.
• Receptors are located on the plasma membrane.
• Second messengers are generated.
• Existing enzymes are covalently modified.
• Protein phosphorylation affects gene expression.
• Effects happen faster than steroid hormones.

Hormone Interaction With The Cognate Receptor

• Receptors are either on the cell surface (plasma membrane) or intracellular receptors.
• Steroid hormones (and thyroid hormones) bind with intracellular receptors
• The hormone-receptor complex moves into the nucleus.
• The activated complex binds to specific DNA sequences called Hormone Response Elements (HREs).
• Genes are activated or repressed.

Hormone Receptor Interaction

• Free steroids enter cytoplasm and interact with their receptor.
• Receptor-ligand complex dissociates heat shock protein and translocates to the nucleus.
• The complex acts as a transcription factor to either augment or suppress gene transcription on DNA.

Two classes of Nuclear Receptors

• Type I receptors have HSP, present in the cytoplasm and translocate to the nucleus after binding to ligand.
• Type II receptors lack HSP are located directly in the nucleus.

Steroid hormone mechanism of action

• Steroid hormones pass through the cell membrane and bind to intracellular receptors.
• The hormone receptor complex moves to the nucleus.
• Complex binds to specific DNA sequences.
• mRNA production is regulated, leading to protein synthesis.

Steroid Hormone Response

• Steroid hormones bind to receptors in the cytoplasm or nucleus.
• The hormone-receptor complex translocates to the nucleus.
• Complex binds to specific DNA regulatory sites (HREs).
• Transcription of target genes is regulated, leading to protein production.

Steroid Receptors

• Steroid receptors are transcription factors.
• They may be located in the cytoplasm or the nucleus depending on the receptor-type.
• They bind to the response element (HRE) on the DNA.
• Heat shock proteins (HSPs) can bind to the receptor and regulate its activity until the specific hormone is present

Steroid Receptor has 3 Functional Domains

• Ligand/hormone-binding domain: Binding of ligand, heat shock proteins inactivation and translocating to the nucleus
• DNA-binding domain: Interacting with DNA, until ligand is bound.
• Variable (immunogenic domain): Modulates transcriptional activation

Steroid Receptor has 4 Functional Domains

• Variable domain: Most variable domain between various receptors.
• DNA binding domain (DBD): Highly conserved and involved in DNA binding to the response element (HRE, part of the promoter).
• Hinge region: Involved in receptor movement into the nucleus, and interacts with chaperones (e.g., HSP90 and HSP56).
• Hormone binding domain (LBD): Modulating the magnitude of the response, with cytoplasmic conformation and homo/heterodimer formation

General Model of Steroid Hormone Action

• Hormone dissociates from binding protein.
• Diffuses into cytosol or nucleus.
• Binds to an intracellular receptor.
• Hormone-receptor complex is activated.
• Complex moves into nucleus and binds to HRE on DNA.
• Gene expression is affected.
• Synthesis of new proteins.
• Alteration of cell phenotype or metabolic activity

Features of the hormone response element (HRE)

• Specific segments of chromosomal enhancers,
• Binding elements for different steroid hormone receptors (glucocorticoids, mineralocorticoids, progesterone, and androgens).
• Extent of expression determines hormone sensitivity in a cell type

Steroid Hormone Receptors and their Response Elements

• Steroid hormone receptors are proteins with binding sites for specific steroid molecules.
• Receptor complexes bind to specific DNA sequences (response elements) within promoters of target genes.
• Gene activation or repression is mediated by the binding of receptor complexes to HREs.
• The presence of the hormone's receptor in a cell determines if the cell responds to the hormone.

How a steroid hormone regulates gene transcription

• Hormone binds to specific receptors.
• Hormone-receptor complex forms a dimer.
• The complex travels to the nucleus.
• The complex binds to a DNA region called response element.
• Gene transcription is affected.

Positive and Negative Response Elements

• Some glucocorticoid response elements activate gene transcription when bound by the hormone receptor complex.
• Others inhibit gene transcription.
• Examples: cortisol binding to positive response elements activating gluconeogenesis and to negative response elements diminishing insulin synthesis.

Evidence to say that Not all cells have receptors for a specific hormone

• Endometrial cells concentrated radioactivity after progesterone/estrogen administration; it's specific for hormones.
• Other cells do not concentrate the hormone.
• Hormone localization to a certain target cells. Cells without hormone receptors do not have the same response.

Usually hormone exert → positive effect i.e. induction of gene expression.

• Hormone exerts positive effect → induction of gene expression
• Hormone exerts negative effect → repression.
• Receptor acts as homodimer or heterodimer.
• Ligand-activated transcription factors - modulate gene expression.

General Model of Steroid Hormone Action

• Overview of steroid hormone binding to specific intracellular protein receptors.
• Preformed steroid hormones diffuse into target cells, binding to intracellular receptors.
• Hormone receptor complex translocates to the nucleus and activates specific genes.

Specific Reading Materials

• Devlin's Textbook of Biochemistry with Clinical Correlations, pp 925-931, 943-953.
• Lippincott Illustrated Review of Biochemistry, pp 79–85
• Harpers Review of Biochemistry, 25th edition, Chapter 44

Minerals

• Inorganic substances essential for various bodily functions.
• Classified as macro or micro minerals based on daily requirement.
• Provide structural support, regulate fluid balance and enzyme activity.
• Insufficient intake leads to potential deficiency while excessive intake might prove toxic.

Requirement of Calcium

• Calcium is critical for bone health, muscle contraction, nerve function, and blood clotting.
• Sources: milk, dairy products, and plant-based foods
• Deficiency symptoms: rickets, osteomalacia, osteoporosis

Requirement of Magnesium

• Essential mineral necessary for various enzymatic reactions.
• Stored in bones and soft tissues, including muscles.
• Crucial for neuromuscular function, energy metabolism, and nucleic acid synthesis.
• Deficiency symptoms: muscle weakness and other symptoms

Body Requirement of Macrominerals

• Describes various macrominerals (e.g., sodium, potassium, chlorine, phosphorus, sulfur).
• Includes their major sources, daily intake, functions, and deficiency symptoms.
• Focuses on the importance of maintaining the balance of these elements within the body.

Requirement of Iron

• Iron exists in ferrous (Fe²⁺) and ferric (Fe³⁺) forms; only Fe²⁺ is readily absorbed.
• Essential for hemoglobin and myoglobin (oxygen transport).
• Deficiency in iron leads to anemia.
• Sources of iron include: meat, liver, eggs, vegetables, and potatoes.

Requirement of Cobalt

• Cobalamin (vitamin B₁₂) is an essential compound containing cobalt.
• Essential for red blood cell development.
• Deficiency symptoms include anemia.

Requirement of Manganese

• Manganese is a cofactor for enzymes involved in antioxidant systems (e.g., manganese superoxide dismutase), bone formation and various metabolic pathways.
• Deficiency symptoms could include impaired growth.

Requirement of Zinc

• Zinc is involved in many metabolic pathways, including enzyme function and synthesis of proteins.
• Deficiency symptoms include impaired wound.
• Important for normal growth in children.
• Source: meat, liver and cereals

Requirement of Selenium

• Selenium is involved in antioxidant enzyme function, particularly in glutathione peroxidase.
• Deficiency symptoms include rare cases of cardiac muscle damage.

Requirement of Chromium

• Chromium plays a regulatory role in hormone action and metabolism (e.g., regulating glucose tolerance; insulin sensitivity).
• Deficiency symptoms may include glucose intolerance.
• Sources include: liver, oysters, cheeses, and asparagus.

Body Requirement of Microminerals

• Lists microminerals like copper (Cu), molybdenum (Mo), iodine (I), vanadium (V).
• Details their daily requirements, sources and functions in various processes (e.g., metabolism of carbohydrates and lipids).
• Explains any potential deficiencies or symptoms.

Summary of Mineral Metabolism

• Inorganic mineral elements that have a bodily function must be received from the diet.
• Two main groups: macrominerals and microelements (trace elements).
• Insufficient intake can lead to deficiencies; excessive intake, toxicity.
• Sodium: Extracellular cation, potassium: Major intracellular cation.
• Only ferrous form of iron is absorbed.
• Selenium is crucial for antioxidant enzymes.

Biochemistry Water Soluble Vitamins

• Distinguishes between water-soluble and fat-soluble vitamins.
• Explains the sources, synthesis, and functions of various water-soluble vitamins in humans.
• Discusses avitaminosis as a deficiency syndrome.

Classification of Vitamins

• Categorizes vitamins as water-soluble or fat-soluble.
• Further classifies water-soluble vitamins into Non-B-Complex, B-Complex, and Other categories, based on their functions (energy-releasing, hematopoietic, etc.)

Importance of Ascorbic Acid (Vit. C)

• Reducing agent: reducing O2, nitrate, and various cytochromes.
• Involved in iron absorption and enzyme action e.g. collagen synthesis.
• Antioxidant: directly removing peroxyl radicals; indirectly restoring the antioxidant properties of fat-soluble vitamin E.
• Deficiency of this vitamin causes scurvy.

Importance of Thiamine (Vit. B₁)

• Thiamine pyrophosphate (TPP) is a crucial coenzyme in oxidative decarboxylation reactions.
• Plays a key role in carbohydrate metabolism (especially in the Krebs cycle).
• Deficiency leads to beriberi, causing polyneuropathy, muscle wasting, and cardiac failure.

Importance of Riboflavin (Vit. B₂)

• Riboflavin converts into flavin adenine dinucleotide (FAD) and flavin adenine mononucleotide (FMN).
• FAD & FMN are important electron carriers in redox reactions in Kreb's cycle.
• Deficiency symptoms: angular stomatitis, glossitis

Importance of Niacin (Vit. B₃)

• Niacin is essential for the biosynthesis of nicotinamide adenine dinucleotide (NAD+) and nicotinamide adenine dinucleotide phosphate (NADP+).
• NAD+ & NADP+ are crucial coenzymes for many redox reactions, including those in the Krebs cycle.
• Deficiency leads to pellagra, exhibiting symptoms of diarrhea, dermatitis, and dementia.

Importance of Pantothenic Acid (Vit. B₅)

• Pantothenic acid is essential for Coenzyme-A (CoA) synthesis, a crucial carrier for acyl groups.
• CoA plays vital roles in many metabolic processes, including the formation of acetyl-CoA, fatty acid synthesis, and the Krebs cycle.

Importance of Vitamin B₆

• Pyridoxal Phosphate (PLP) is an essential coenzyme involved in amino acid metabolism (especially transamination and decarboxylation).
• PLP is essential for the formation of neurotransmitter and haem synthesis.
• Deficiency could cause anaemia and nerve function problems.

Importance of Cobalamin (Vit. B₁₂)

• Cobalamin is exclusively synthesized by microorganisms.
• Must be consumed via diet.
• Plays a role in the metabolism of amino acids and fatty acids.
• Needed for converting methylmalonyl-CoA to succinyl-CoA and for methyl group transfer reactions. Very important in DNA & RNA development.
• Deficiency can cause pernicious anemia

Importance of Folate (Folic Acid)

• Folate derivatives are carriers of single carbon units, essential for nucleotide synthesis.
• Essential for DNA synthesis in dividing cells.
• Folate deficiency: macrocytic anemias, neural tube defects in developing fetuses

Importance of Biotin

• Biotin is a coenzyme for carboxylation reactions,
• Crucial for gluconeogenesis and fatty acid synthesis.
• Deficiency is rare and usually linked to an overly cooked egg rich diet
• Intestinal flora may have a role in generating this vitamin.

Carbohydrate Metabolism: Glycolysis

• Describes Glycolysis as a set of reactions oxidising glucose to pyruvate or lactate depending on the presence or absence of oxygen.
• Takes place in the cytosol of cells and involves ten reactions.
• Glycolysis has two main stages: priming and energy-yielding.
• Summarizes the overall reaction occurring in aerobic and anaerobic conditions and the energy yield from each of the conditions.

Active Transport

• Transport method where energy is required to move substances against their concentration gradient.
• This is usually done through carrier/transporter proteins.
• The mechanisms include: endocytosis, phagocytosis and pinocytosis.
• Exocytosis as another form of active transport.
• The Na+/K+ pump as an important method.

Cell Surface Receptors

• Integral proteins that span the plasma membrane.
• Recognize and bind with external ligands, such as hormones and neurotransmitters.
• Trigger a series of intracellular events (signal transduction) to create an intracellular response, often involving cascades of second messenger molecules.
• Three main categories of receptors: ion channels, G-protein coupled receptors, and enzyme linked receptors.

Transmembrane Signaling

• Recognition of extracellular signals by integral membrane receptors leads to the generation of intracellular signals that are propagated across the cell.
• Transmembrane proteins have both an extracellular domain which bind to a specific molecule and an intracellular domain which triggers a reaction within the cell.