Cell Signaling/ Signal Transduction

Front 1

What mechanisms do Cells use to Communicate?

Back 1

-directly
>through gap junctions-cytoplasmic flow between cells
>cell-cell adhesion
-indirectly
> extra-cellular matrix
>neurotransmitter (fast acting; work at high concentrations)
>hormones- slow acting; work at low concentrations)
-sensory Input (order, taste, light perception)

Front 2

How can Hormones be classified?

Back 2

endocrine (travel through blood)-distant target cells in other organs

paracrine (act on a nearby cell)

autocrine (act on the same cell)

Front 3

How do Hormones work?

Back 3

-they are produced by specialized cells
-they act on target cells to alter cell metabolism

Front 4

What is required by cell for the existence of multicellular organisms?

Back 4

Communication between cells

Front 5

Must happen in order for hormones to work?

Back 5

Hormones can't work unless they bind to receptors!

Front 6

What are some examples of Hormones and neurotransmitters that shows that they vary chemically?

Back 6

proteins or peptides-ex. insulin

amino acid derivatives-ex. small molecules, histamines, adrenaline, dopamine, serotonin

cholesterol derivatives-ex. steroids

Front 7

What do receptors consist of and what is their function?

Back 7

Receptors are proteins

Receptors recognize and bind ONLY certain hormones and trigger some effect ONLY when hormone is present

Front 8

What does binding to a hormone lead to? (for a hydrophilic receptor)

Back 8

Binding of the hormone (first messenger) leads to production of a second messenger inside the cell

Front 9

Where are receptors located?

Back 9

Receptors can be located inside and outside of a cell.

-Inside the cell
> hydrophobic
>are mostly steroids

-On the surface of the cell
>hydrophilic--> do NOT cross the cell membrane
>spans the cell membrane and the cytoplasmic portion of the receptor interacts with enzymes inside the cell

Front 10

What does the binding of a steroid to its receptor trigger?

Back 10

Binding of a steroid to its receptor triggers relocation of the hormone receptor complex to the nucleus, where it binds to DNA and transcribes a specific group of genes that encode other proteins

Front 11

What are the diseases associated with Steroids?

Back 11

1. testicular feminization syndrome- testosterone receptor cannot bind the steroid testosterone--> cells lack genes needed for male characteristics: genetically male but appears female

2. Diabetes Mellitus
> Type I- cannot produce insulin hormone--> take daily insulin shots to lower blood sugar levels
>Type II- insensitivity to insulin

Front 12

What is receptor mediated endocytosis and why is it important?

Back 12

It occurs when cell surface receptors are internalized and degraded.

This process allows cells to stop responding to hormones, (desensitization or receptor down regulation)

Front 13

What are the four classes or Hormone Receptors?

Back 13

1. Ligand gated ion channels (Acetylcholine, Nicotine)
2. G-Protein coupled receptors
3. Growth factor receptors
4. Steroid receptors

Front 14

How does the Ligand Gated Ion Channel work and what does it lead to?

Back 14

The binding of a hormone (or neurotransmitter) triggers opening of a channel formed by the receptor protein, allowing small molecules to cross the cell membrane.

This leads to the depolarization of the cell.

Front 15

What is a good example of Ligand Gated Ion Channel ?

Back 15

Aceptylcholine receptor

also nicotinic cholinergic receptor where nicotine is the ligand

Front 16

What is the structure of Aceptylcholine receptors and what is its ligand?

Back 16

ligand: Acetylcholine (a nuerotransmitter)

structure:
-4 subunits (2alpha:beta:gama:delta)
-4 inter-membrane segments
-form pore int he middle of the membrane when ligand is bound.

Front 17

What is the binding subunit of the Aceptylcholine receptor and how many acetylecholone molecules bind to each one?

Back 17

The active binding subunit is the alpha subunit.

Each alpha subunit binds two acetylcholonic molecules

Front 18

What is the structure of G-Protein Coupled Receptors (GPCR)?

Back 18

consists of one subunit: alpha

have 7 membrane spanning segments and each membrane forms an alpha helix

several of the alpha helices form the hormone binding pocket.

Front 19

What kind of mechanism does a cell using G-Protein Coupled Receptors (GPCR) use?

Back 19

Indirect communication- GPCR do not themselves pass molecules across the cell

Front 20

How do G-Protein Coupled Receptors (GPCR) work?

Back 20

1. The binding of hormone to the receptor triggers a change in receptor conformation, or overall shape.
> allows the cytoplasmic portion of the receptor to bind G-Proteins inside the cell
2. G-Proteins release GDP, bind GTP, and activate effector proteins

Front 21

What are effectors?

Back 21

Effectors are enzymes that produce a second chemical messengers

Front 22

What is a good example of G-Protein Coupled Receptors (GPCR)?
[name ligand and effector protein]

Back 22

B-adrenergic receptor

ligand: epinephrine (adrenaline)-secreted by adrenal medulla

Effector Proten: adenyl cyclase produces second messenger cAMP
> cAMP (promotes changes in cell metabolism)
* ex. glycolysis, gluconeogenesis, and lipolysis (fight or flight)

Front 23

What are G Proteins?
[location?, subunits?]

Back 23

-called G proteins because they bind the guanine nucelotides, GDP and GTP
- heterotrimers (made of 3 different subunits)]
-3 subunits: G[α], G[β], and G[gamma]

Front 24

What subunit of the G-proteins carries the binding site?

Back 24

G[α]

Front 25

What are the different types of G[alpha] and what do they stimulate?

Back 25

-Gs[α]:stimulates adenylyl cyclase-->second messenger cAMP--> effector enzyme Ad. cyc-->activates PKA
-Gi[α]: inhibits adenylyl cyclase--> second messenger cAMP-->effector enzyme Ad. cyc
-Gq[α]:stimulates phopholipase C-->second messenger DAG and IP3--> effector enzyme PLCβ-->activate PKC
-G12[α] and G13[α]: stimulate cytocketelton rearrangements

Front 26

How does G-Proteins relate to Pharmacology?

Back 26

-Allergy-treated with antihistamine receptor antagonists (bind to receptor but don't activate it)

-Asthma-treated with β-adrenergic receptor agonists like adrenaline--> relax lungs but speed up heart

Front 27

What diseases do G-proteins relate to?

Back 27

Cholera- caused by a toxic peptide made by the bacterium that modifies Gs[alpha] subunit in the intestinal epithelium so they no longer hydrolyze GTP
> permanent activation of adenylyl cyclase-->elevated cAMP--> sodium and water secretion into the gut--> diarrhea

-whopping cough- caused by toxic peptide made by the bacterium
> modifies Gi[alpha] subunit in lungs--> no long binds to receptors

Front 28

What are second messengers and what are some examples of them?

Back 28

Second messengers are small mobile messengers produced inside the cell by effector enzymes that are responsible for transmitting the signal to protein inside the cell

ex. Adenylyl Cyclase: converts ATP to cAMP
>cAMP- promotes glycogen breakdown and inhibits glycogen synthesis

ex. Phospolipase C (PLC):conversts a PIP2 to DAG and IP3.
> IP3 increases cytoplasmic calcium by binding to and opening calcium channels int he endoplasmic reticulum

ex. ion channels

Front 29

What are the steps to G-Protein Activation and Inactivation

Back 29

1. Hormone binding to receptor. Receptor is ON.
2. Receptor binding to G protein, G protein releases GDP
3. G protein binds GTP, G protein undergoes conformational change.
4. G protein leaves the receptor, separates into alpha and beta:gamm subunits. G protein in ON.
5. Either the G protein alpha or beta:gamma subunits (or both) activate effector enzymes.
6. Galpha subunit hydrolyzes GTP to GDP, subunits reassemble and the signal stops. G protein is OFF. The G protein is the GTPase!
7. G protein GTP hydrolysis is accelerated by a fourth protein called RGS (a GTPase accelerating protein or GAP).

Front 30

What are Protein Kinases?

Back 30

enzymes that transfer a phosphate from ATP to an amino acid side chain within a protein, a process called phosphorylation

Front 31

How can phosphorylation alter the activity of an enzyme?

Back 31

altering its shape (conformation), its ability to bind other proteins, or its location within the cell

Front 32

What is a kinase cascade and what does it do?

Back 32

Protein kinases often phosphorylate and activate other kinases - are part of a kinase cascade that allows signal amplification

Front 33

What do Kinases usually do?

Back 33

Add phosphates and turn proteins ON

Front 34

What do protein phosphatases usually do?

Back 34

dephosphorylates the protein; turning it OFF

Front 35

What are the two classes of Protein Kinase?

Back 35

1. Tyrosine Kinases-phosphorylate tyrosine amino acids (RARE)
2. Serine/Threonin Kinases-phosphorylate serines and threonine
ex. cAMP dependent protein kinase (PKA)
ex. CaM Kinase
ex. Protein Kinase- DAG and Calcium both activate this kinase

Front 36

What is the function of phosphodiesterases?

Back 36

degrades the second messenger

ex. cAMP is degraded by cAMP phosphodiesterase which converts cAMP to AMP

Front 37

What are the step of G Protein Activation and Deactivation?

Back 37

1. Hormone binding to receptor. Receptor is ON
2. receptor binding to G protein--> releases GDP
3. G protein binds GTP, G-protein undergos conformational change
4. G protein leaves the receptor, separates into alpha and beta:gamma subunits. G-Protein is ON
5. Either the G protein alpha or beta:gamma subunits reassemble and the signal stops. G-protein is OFF. The G-Protein is the GTPase
7. G-protein GTP hydrolysis is accelerated by a fourth protein calle RGS (AKA GAP)

Front 38

What are the mechanisms that are key off switches?

(signal inactivation)

Back 38

1. Removal of ligand from the receptor.
2. Receptor internalization.
3. Uncoupling from the G protein
4. GTP hydrolysis by the G protein α subunit.
5. Protein de-phosphorylation.

Front 39

What are growth factor receptors?

Back 39

-the receptor and the effector are the same protein
-Are typically tyrosine kinase receptors

Front 40

What is the structure of growth factor receptors?

Back 40

-2 subunits encoded by 1 or 2 genes that form a dimer
- Each receptor spans the membrane 1 time
Intracellular domain is an enzyme

Front 41

what are examples of growth factor receptors?

Back 41

1. Tyrosine kinase (e.g. EGF, PDGF).
2. Serine/Threonine kinase (e.g. insulin)
3. Tyrosine phosphatase.

Front 42

What are the steps of receptor tyrosine kinase activation and inactivation?

Back 42

1. Hormone binding to receptor.
2. Two receptors assemble to form a dimer. Receptor is ON.
3. Receptors phosphorylate each other on several tyrosine amino acids this is commonly known as “receptor autophosphorylation” or trans-phosphorylation.
4. Each phosphorylated tyrosine is recognized by adapter proteins and other enzymes. Sometimes these proteins are also phosphorylated by the receptor

Front 43

What are the steps of Ras activation and inactivation?

Back 43

1. Growth factor hormone binds to receptor.
2. Receptor forms a dimer and autophosphorylates.
3. Phosphorylated receptor binds to an adapter and SOS.
4. SOS binds to Ras. Ras binds GTP, undergoes a conformational change. Ras G protein in ON.
5. Ras recruits a kinase to the membrane and activates it.
6. A series of kinases are activated leading to new gene transcription.
7. Ras hydrolyzes GTP to GDP, and the signal stops. Ras is OFF. GTP hydrolysis is accelerated by another protein called GAP (GTPase accelerating protein). Ras is the GTPase!

Front 44

What is Steroid hormone action?

Back 44

-steroid are hydrophobic
- steroid hormones Diffuse through the lipid bilayer of the cell membrane and bind to a receptor in the cytoplasm.
-Steroid binding triggers release of the receptor from a cytoplasmic protein complex, allowing entry into the nucleus where it can bind DNA.
-Steroid hormone receptors bind directly to DNA to control gene expression.
-The hormone-receptor complex binds DNA as a dimer at specific sites called hormone response elements (HREs) and starts transcription of nearby genes.

Front 45

True or False: Steroid receptors are not transcription factors

Back 45

FALSE

Front 46

What are some examples of Steroid hormones?

Back 46

-Androgens (e.g. testosterone)-development and maintenance of male sex characteristics

-Progestins (e.g. progesterone)-developement and maintenance of pregnancy; oral contraceptive

-Estrogens (e.g. estradiol)-development and maintenance of female sex characteristics
- replacement therapy used to prevent osteoporosis and symptoms of menopause

Front 47

what are somefacts about Steroid hormones?

Back 47

Steroid Hormones =Direct Gene Activation
* Steroid hormones are lipid soluble (Hydrophobic ).
* Steroids can diffuse through the membrane
*Have internal receptors

Front 48

How is steroid synthesis typically regulated?

Back 48

g-protein coupled receptors

Front 49

what is a Step by step of how steroid hormones work?

Back 49

1). Diffuse through the membrane
2). Binds & activates intracellular receptor.
3). Steroid-Receptor complex binds to DNA receptor protein
4). Activates a gene.
5). Gene transcribed into messenger RNA.
6). mRNA goes to the ribosomes
7). Translate mRNA into protein