LECTURE 9 NEW

Question 1

What is signal transduction

Answer 1

Signal transduction is the process through which cells convert external signals into functional responses.

Components of signalling pathways include signals, receptors, and intracellular messengers.

Question 2

What are the three types of cell signalling

Answer 2

Endocrine
Paracrine
Autocrine

Question 3

Types of cell signalling: Endocrine

Answer 3

Definition: Endocrine signaling involves long-range communication between cells through blood-borne hormones.

Mechanism:
Hormones are secreted by endocrine glands into the bloodstream.
They travel long distances to target organs or tissues, where they bind to specific receptors.

Characteristics:
Generally mediates long-term effects such as growth, metabolism, and reproductive functions.
Examples include insulin, which regulates glucose metabolism

Question 4

Types of cell signalling: Paracrine

Answer 4

Definition: Paracrine signaling facilitates short-range communication between nearby cells.

Mechanism:
Cells secrete signaling molecules, such as growth factors and cytokines, that diffuse through the interstitial fluid.
These molecules act on adjacent cells, stimulating responses without entering the bloodstream.

Characteristics:
Typically involved in local tissue responses, such as inflammation and tissue repair.

Question 5

Types of cell signalling: Autocrine

Answer 5

Definition: Autocrine signaling occurs when a cell secretes a factor that acts on itself.

Mechanism:
The signaling molecules released by the cell bind to receptors on the same cell, influencing its behavior.

Characteristics:
Often involves growth factors and cytokines that regulate cell proliferation and differentiation.

Question 6

What are extracellular mediators and the three types

Answer 6

Hormones: Act in an endocrine manner, generally producing short-term metabolic effects.

Growth factors: Operate in paracrine or autocrine modes, typically leading to long-term proliferation and developmental changes.

Steroid hormones: Interact with intracellular receptors, producing long-term effects.

Question 7

Receptors in signal transduction: G protein-coupled receptors (GPCRs)

Answer 7

Function: GPCRs are a large family of receptors that induce rapid, short-term changes in cell function.

Mechanism:
They transmit signals through the activation of G proteins, which then influence various intracellular pathways.

Question 8

Receptors in signal transduction: Enzyme-coupled receptors

Answer 8

Function: Enzyme-coupled receptors include those with intrinsic enzymatic activity and those associated with enzymes, leading to significant long-term changes in cellular function.

Types:
Receptor Serine-Threonine Kinases: Such as Transforming Growth Factor Beta (TGF-β) receptors, which phosphorylate serine and threonine residues.

Receptor Tyrosine Kinases (RTKs): Examples include the Platelet-Derived Growth Factor receptor (PDGF-R) and the Insulin receptor, which phosphorylate tyrosine residues and are critical for cell growth and metabolism.

Question 9

Receptors in signal transduction: Ion channel receptors

Answer 9

Function: Ion channel receptors, specifically ligand-gated ion channels, are activated by external signals, leading to changes in ion flow across the cell membrane.

Mechanism:
When the ligand (such as a neurotransmitter) binds to the receptor, it causes conformational changes that open the channel, allowing ions (e.g., Na⁺, Ca²⁺) to flow in or out of the cell.

Examples: Transient Receptor Potential (TRP) channels are a type of cation channel involved in sensory transduction, including pain and temperature sensations.

Question 10

Criteria for classification as a second messenger

Answer 10

Must be a small molecule.
Must allow for rapid concentration changes.
Controlled by extracellular stimuli.
Regulates specific enzyme activity or protein function.
Amplification must occur within the system

Question 11

Second messenger examples

Answer 11

cAMP: Activates protein kinase A, affecting lipid breakdown and glycogen synthesis.
cGMP: Activates protein kinase G, opens cation channels in rod cells.
DAG: Activates protein kinase C, influencing transcription and glycogen synthesis.
IP3: Opens calcium channels in the endoplasmic reticulum and activates calcium-dependent protein kinases

Question 12

How does protein phosphorylation work in signalling cascades

Answer 12

Protein phosphorylation is a critical biochemical process that involves the addition of phosphate groups (usually from ATP) to specific amino acids in a protein

Phosphorylation can activate or inhibit protein’s function and thereby affecting cellular signaling pathways.

The process of phosphorylation is reversible; protein kinases are responsible for adding phosphate groups, while protein phosphatases remove them.

Question 13

Step by step of a signalling cascade

Answer 13

1. Extracellular Signal Molecule: A signaling molecule (such as a hormone or neurotransmitter) binds to a specific receptor on the cell membrane.
   Receptor Activation: The binding of the signal molecule induces a conformational change in the receptor, activating it.

2.Second Messenger Production: The activated receptor may stimulate or inhibit enzymes that produce second messengers from precursor molecules
Rapid Concentration Change: The levels of second messengers increase rapidly, facilitating signal amplification.

4. Kinase Cascade Initiation: The second messengers activate protein kinases
   Phosphorylation: The active kinases transfer phosphate groups from ATP to specific target proteins, creating a phosphorylation cascade. Each kinase can activate multiple downstream kinases - amplification.
5. Target Protein Activation: The final active protein in the cascade (which could be an enzyme or transcription factor) brings about the desired cellular response
6. Deactivation of Kinases: Enzymes called protein phosphatases remove phosphate groups from the phosphorylated proteins, returning them to their inactive state.

Question 14

Signalling cascade example: cAMP

Answer 14

1. One epinephrine molecule binds to one GPCR (β-adrenergic receptor).
2. The GPCR activates many G-proteins
3. Each active G-protein then activates one adenylyl cyclase.
4. Each adenylyl cyclase enzyme produces hundreds to thousands of cAMP molecules from ATP.
5. Each cAMP molecule then activates one protein kinase A
6. Each activated PKA phosphorylates many target proteins/enzymes (amplifying the signal even further).

Question 15

Signal transduction leads to actual changes in the cell, such as:

Answer 15

Modification of metabolism
(e.g., breaking down glycogen in muscle during fight-or-flight).

Changes in movement
(e.g., white blood cells moving toward infection).

Altered gene expression
(e.g., growth factors leading to new protein synthesis).

Control of cell proliferation or differentiation
(e.g., stem cells committing to specific fates during development).