Bio lecture Cell communication and multicellularity chp. 7

Cell communication = ?

signal transduction

signal transduction pathway

sequence of events leading to a cells response to a signal

3 components of signal transduction

• signal

• receptor

• response

4 chemical signals

• autocrine: signals affect the cells that made them (affects itself)

• Juxtacrine: signals affect only adjacent cells

• paracrine: signals affect nearby cells

• Hormone: long distance cell communication usually via circulatory system (bloodstream)

Responding to a signal

Cell must have a specific receptor to detect signal

where does signal-receptor interaction occur

Cell membrane

signal strength during signal trnasduction

signal gets stronger as it goes through pathway

What may be involved in responses

enzymes and transcription factors that can be activated or inactivated to bring about response

Crosstalk

signal transduction pathways that are interconnected

crosstalk affects

• Pathways can branch: activation of a single protein can activate multiple pathways

• Pathways can converge

• Activation of one pathway can inhibit another

Ligand

chemical signal molecule that binds to specific three dimensional sites on receptor proteins causing the proteins shape to alter. This binding process is reversible without altering the ligand.

other chemicals that can bind to receptors

• agonists: have the same affect as the ligand

• antagonists: binds to site preventing ligand from binding and does not set off signal transduction (inhibitor)

Caffeine an antagonist or agonist

Caffeine binds to same receptor as adenosine which would slow the body and make you tired. Caffeine does not allow this receptor to bind therefore you do not get tired.

Ans. Antagonist

Two types of receptors

• membrane receptors: Large or polar ligands bind to membrane receptor (ex. insulin)

• Intracellular receptor: small or nonpolar ligands that can diffuse across membrane will bind (ex. estrogen)

3 membrane receptors in Eukaryotes

• gated ion channel

• Protein kinase receptor

• G-protein coupled receptors

Gated ion channel

allows ions to enter or leave cell

signal binding results in channel protein shape change which will open the channel

ex. Acetylcholine receptor on muscle cells is a ligand gated ion channel

Protein kinase receptor

catalyzes phosphorylation of themselves or other proteins which changes their shape

G protein-coupled receptors

signal binding to receptor activates a G protein which then moves to activate an effector protein

G protein can have up to 3 subunits, when activated one subunit moves through membrane to activate effector

(activation of G protein replaces inactive GDP with active GTP)

Intracellular receptors

respond to signals like light or chemical that can move through membrane

Usually transcription factors and after binding ligands they move to nucleus to bind to DNA, and alter gene expression

What are G proteins

Abnormal RAS is permanently bound to GTP causing continuous cell division

Protein kinase cascade

one protein kinase activates the next and continuous phosphorylation from one molecule to the other occurs

MAPK

Mitogen activated protein kinase

second messengers

small molecules that diffuse through the cytosol to mediate steps in a cascade. They will amplify and distribute the signal.

Also involved in cross talk. One second messenger can be used for multiple pathways

second messenger example (cyclic AMP cAMP)

Binding of one small molecule to a receptor can lead to the production of many cAMP which activates many enzyme targets

How can second messengers form

from membrane phospholipids

Common membrane phospholipid second messengers

Hydrolysis of PIP3 produces IP3(cytoplasm) and DAG(membrane)

what do IP3 and DAG do

activate protein kinase C (PKC)

Ca2+ ions can be second messengers

Ca2+ is pumped out of cell to keep conc. low

IP3 and other signals open Ca2+ channel leading to increase Ca2+ conc. that will activate PKC

Entry of sperm into egg does what

opens Ca2+ channels

Process of nitric oxide NO being a second messenger between Acetylcholine and muscle relaxation

• Acetylcholine binds to receptor

• Ca2+ channel opens releasing into cytoplasm

• Ca2+ stimulates NO synthesis

• NO diffuses to smooth muscle cell and stimulates production of cGMP

• cGMP promotes muscle relaxation

What is the enzyme that catalyzes the formation of IP3

Phospholipase C

What can IP3 do

It is a secondary messenger that can open Ca2+ channels from endoplasmic reticulum leading to increase Ca2+ conc. in cytoplasm

Signal transduction is highly regulated how

• NO conc depends on how much is synthesized (short lived)

• Ca2+ conc. in cytoplasm depends on gated ion channel activity

• Protein kinases, G proteins, and cAMP is regulated by enzymes

• synthesis and degradation

• activation and inactivation

Phosphodiesterase

enzymes that hydrolyze cAMP and cGMP