Chemical Signaling
Chemical Signaling in Living Organisms
Chemical signaling is crucial for communication between cells, enabling organisms to respond to their environment and maintain homeostasis.
1. Key Components of Chemical Signaling
Receptors
Definition: Proteins on or in cells that receive chemical signals (ligands).
Function: Facilitate cell-to-cell communication by binding specific ligands, leading to metabolic changes.
Ligands
Definition: Chemical signals that bind to receptors.
Function: Initiate changes in cell metabolism or activity when bound to receptors.
2. Types of Chemical Signaling
Quorum Signaling
Definition: A form of cell-to-cell communication in bacteria that adjusts behavior based on population density.
Example: Bioluminescence in Vibrio fischeri.
Autoinducers
Definition: Signaling molecules produced by bacteria that increase in concentration as the bacterial population grows.
Function: Trigger a cascade of reactions when they bind to receptors, affecting gene expression.
3. Signaling in Bacteria: Quorum Sensing
Mechanism:
Bacteria release autoinducers into the environment.
Increased bacterial numbers lead to higher concentrations of autoinducers.
Autoinducers bind to receptors (e.g., LuxR) on bacterial cells.
Binding triggers gene activation, resulting in behaviors like bioluminescence.
Example:
Vibrio fischeri emits light to aid in camouflage for the bobtail squid. This occurs when the bacterial population is sufficiently large to activate the gene for luciferase, the enzyme responsible for light production.
4. Chemical Signaling in Animals
Types of Signals:
Hormones: Chemical messengers released into the bloodstream affecting distant cells.
Neurotransmitters: Chemicals released by neurons to transmit signals across synapses (e.g., Acetylcholine).
Cytokines: Proteins involved in cell signaling during immune responses.
Calcium Ions: Act as secondary messengers in various signaling pathways.
5. Diversity of Neurotransmitters
Types:
Amino Acids: Basic building blocks that can act as neurotransmitters.
Peptides: Chains of amino acids functioning as signaling molecules.
Amines: Modified amino acids serving as neurotransmitters (e.g., serotonin).
Nitric Oxide (NO): A gas that acts as a neurotransmitter.
6. Mechanisms of Receptor Activation
Type 1: Transmembrane Receptor Proteins
Location: Embedded in the cell membrane.
Function: Ligands bind to the extracellular domain, initiating a signal transduction pathway.
Type 2: Intracellular Receptor Proteins
Location: Found in the cytoplasm or nucleus.
Function: Ligands that are hydrophobic can diffuse through the membrane and bind to these internal receptors.
7. Cell Signaling Pathway
Basic Stages:
Signal Reception: A receptor detects a stimulus.
Transduction: The signal is converted into a form that can trigger a response.
Transmission: The signal is sent to the target cell.
Response: The target cell executes an appropriate action based on the received signal.
8. Importance of Cell Signaling
Coordination: Allows multicellular organisms to manage complex functions and respond to internal and external changes.
Adaptation: Supports evolution through natural selection favoring effective signaling mechanisms.
Hormonal Signaling :
Insulin and glucagon are hormone: Helps maintain homeostasis together in the blood sugar levels throughout the body
Production: pancreas
Role: regulating blood glucose levels
Insulins
Produced : beta-cells when glucose levels increase as a result of eating
Stimulates:
Cells to uptake and metabolize glucose
The liver to convert glucose into glycogen
This stores it for future messages
How does insulin work:
Insulin the hormone binds to the transmembrane receptors with kinase activities, which leads to the following events:
1. To proteins combined and form a dimer which activates the tyrosine kinase
Notes: It all starts with the Tyrosine Kinase Receptors where insulin binds to the tyrosine kinase receptor with this initial sequence of reaction leading to vesicles containing glucose transporters moving to the plasma membrane.
2. The activation of the protein causes the phosphorylation of the tyrosine section of the proteins.
Tyrosine Kinase Receptors (TKRs): group of transmembrane receptors proteins that activates kinase enzymes
The TKRs have an extracellular ligand binding site, with TK sections within the cytoplasm of a cell
The intracellular tyrosine kinase of the Proteins receptors are enzymes that catalyze the phosphorylation of itself
Phosphorylation is the addition of a phosphate to a molecules
Action of Insulin- FINALLY!
Phosphorylated tyrosines attack and binds to other proteins
This inanities a cascades of reaction, which leads to the movement of glucose transporter vesicles to the plasma membrane
Glucose is removed from the bloodstreams
Glucagon:
Produced : alpha cells when glucose levels drop
Stimulates:
The breakdown of the glycogen to glucose molecules in the liver
The resulting glucose are released into the blood for use by body tissues cells
Steroids Hormones
Examples : Oestradiol, progesterone and testosterone
Action of Steroid Hormones:
Testosterone is a hydrophobic steroid hormones that can diffuse through the plasma membrane of cells
Steroid hormones bind to specific receptor proteins in the cytoplasm (or nucleus) to form an active hormone-receptor complex
The active hormones-receptor complex enters the nucleus and binds to specific DNA sequence to promote gene transcription
The steroids hormone controls when an enzymes (or other proteins )is synthesized ultimately controlling activity in the targe cells
Oestradiol
Oestrogen hormone
Secreted by the ovaries
Steroid hormone
How it works : binds to the protein receptors in the cytoplasm of a cell in the hypothalamus, which stimulates the secretion of gonadotropin- release hormones (GnRH)
GnRh stimulates the secretion of follicle stimulates hormones (FSH), which stimulate the development of the follicles during the menstrual cycles
Progesterone
Steriod hormoens
Produced by the overaries
The levels of prosterones increases after ovulations
Prosterones bind to proteins receptor within cells of the endometrium
Progesterone stimulates the thickening of teh endometrium, in preparation for implantation of the developing embryo
Hormone Actions #3
Amino acis dervieves homeone and peptide hormones using G protein-couples receptors to change metabolism within a cell
CASE STUDY
Epinephrine
Hyrdoplhich peptide horme
Cannot pass through the plasma membrane
G Proteins
Found in the cytoplasm of cells coupled with transmembrane receptors proteins known as GPCR proteins
The G proteins is composed of three polypeptide subunits:
alpha (α)
beta (β)
gamma (γ).
In the G proteins’ inactive state, GDP (guanosine diphosphate) is attached to the alpha subunit of the G protein
G Proteins activated
1. When an extracellular ligand such as a hormone binds to a GPCR protein, th GPCR proteins changes shapes
2. The change in shape of a the GPCR protein causes the G protein to release GDP, which is replaced by the GTP (Guanosine triphosphate)
3. This activates the G proteins, causing the alpha subunit to dissociate from the other subunits
4. The alpha subunites interacts with a secondary messenger, which initiates a cascader of reactions within the cells
5. G protein coupled- receptors are the largest class of cell surface receptor in human
Action Mechanism of Epinephrine
Epinephrine is a peptide hormone, and cannot pass through the plasma membrane.
Epinephrine binds to a binding site on a GPCR protein, which changes shape to activate a G protein.
The alpha subunit dissociates from the other two G protein subunits.
The alpha subunit activates an enzyme which converts ATP to cyclic AMP (cAMP).
Notes : AMP is adenosine monophosphate
cAMP is a secondary messenger which triggers a cascade of reactions within the cell, resulting in a change of metabolism.