First Semester/Mini 1 part 2

How does glucocorticoids diffuse into the membrane?

– a steriod hormone

1. diffuses passively across the plasma membrane

2. binds to a receptor protein in cytosol or nucleus

3. the hormone-receptor diffuse across the NPC

4. binds to response elements on promotors & enhancers (stimulates transcription)

What are some other hormones undergo passive diffusion in the membrane?

– steriod hormones

– thryoid hormones

– retinoic hormones (vitamine A)

– calcitriol (Vitamine D)

How does water soluble hormones activate 2nd messengers?

1. hormone binds to receptor which cause a conformation change

2. 2nd messenger bind to other cellular parts

Cyclic AMP

-synthesized from ATP & degraded to AMP

– synthesized from ATP by adenylyl cyclase 

– synthesized from cAMP by cAMP           phosphodiesterase

– rxn is reversibel

How does caffein affect the signaling pathway?

– it inactivates the cAMP phosphodiesterase 

– you will have increased amounts of cAMP

– gives you diarrhea

What is the stages when a hormone activates a g- protein?

1. hormone binds and changes the conformation

2. protein loose affinity of GDP thus gaining GTP

3. The α subunit with bound GTP separates from β & γ subunit thus activating it.

4. α subunit binds to adenyl cyclase making cAMP

5. cAMP binds to the regulatory subunit of protein kinase A (activates it)

6. the 2 catalytic subunits are cleaved off

7. PKA phosphorylates CREB – response element that activates transcription

Components of a G-protein?

– contains a α, β, & λ units

– α has binding sites for GDP and GTP

– active G-protein: has GTP bound

– inactive G-Protein: has GDP bound

What does Protein Kinase A do?

– phosphorylates CREB

– a response element

– this activates transcription

How can Adenylate cyclase be inhibited?

– cAMP can be increased or decreased by hormone

– Gsα-protein is activated thus increasing cAMP

– Giα-protein is activated thus decreasing cAMP

The effects of pseudohypoparathyroidism

– TPH (parathyroid hormone) raises blood Ca level by raising cAMP

– Abnormalities

     * sign of PTH deficiency (hypocalcemia, tetany)

     * causes: mutation in the PTH receptor

  -abnormal Gsα making insufficient coupling adenylate cyclase

Effects of thyroid nodules

– TSH (thyroid stimulating hormone) from the pituitary gland stimulates hormone by raising cAMP

– Problem

* benign tumors in thyroid gland can over produce the hormone

– Cause:

1. activated somatic mutation in the gene

2. a Gsα mutation (accumulation of cAMP)

How does Cholera toxin affect the pathway?

– cause of diarrhea 

– binds to Gsα and inactivate hydrolysis of GTP

– leaves protein in permanent active state

– you get an accumulation of cAMP


– uses NAD to attach nicotinamide to g-protein


– opium to inhibit diarrhea 

How does Pertussis Toxin affect the pathway?

aka “whooping cough”

– it binds to Giα protein and inhibits it

– Giα cant stop the Gsα from making cAMP

– you get an accumulation of cAMP

The mechanism involving the phosphylation of 2nd messenger inositol lipids

1. phosphatidylinositol (PI) is phosphorylated by PI kinase using ATP

2. it undergoes another phosphorylation by PIP kinase using ATP

-structure is split by PLCβ giving a DAG (left in PM) + an inositol portion (IP3) thats hydrophilic

3. DAG binds to Protein kinase C (PKC)

4. IP3 releases Ca from ER which activates PKC


Effects of increased Ca in the enthothelia cells

– increased Ca causes relaxation

– produces NO (nitrous oxide) that bind to guanylate cyclase

Effects of Viagra (PDE5 inhibitor) on the cycle

– inhibits the cGMP phosphodiesterase

– accumulating more cGMP 

What are three intracellular receptors of Ca?

– Protein Kinase C: activated by DAG + Ca

– Calmodulin: activates smooth muscles

– Troponin C: activates skeletal and cardiac

Mechanism for GMP activation

1. GTP to cGMP by enzyme guanylate cyclase

2. cGMP to GMP by cGMP phosphodiesterase

***note guanylate cyclase activated in 2 forms

1. membrane-bound guanylate cyclase by hormones

2. cytoplasmic guanylate cyclase by NO

What is an atrial natriuretic factor (ANF)

– ANF receptor is a ligand-activated guanylate cyclase

– ANF binds to ANF receptor activating PKG

– PKG relaxes smooth muscles

Mechanism of Growth factors receptors 

(tyrosine K)

1. dimer binds on tyrosine kinase causing receptors to cluster & phosphorylate each other

2. undergoes a conformational change and autophosphorylates (SH2 domain)


Growth factors signaling activation of IP3

1. growth factor binds to receptor

2. it undergoes autophosphorylation

3. next it binds onto phospholipase C-γ

-PLCγ phosphorylates PIP2 making IP3

**Note: PLCβ is activated by G-proteins


Growth Factor Signaling the activation of Protein Kinase Akt

1. Growth factor binds to Tyrosind K

2. it undergoes autophosphorylation

3. it activates IP3 kinase which makes PIP3

4. PIP3 attracts proteins PDK1 & Akt which activates protein kinase B (PKB)

5. PDK1 & mTOR phosphorylates & activates Akt

6. Akt dissociates (active form)


RAS and the MAP (mitogen kinase) kinase cascade

– RAS has only one subunit w/ GDP bound

– MAP kinase activates small G-protein Ras

– upon activation it gets a GTP that will lead to protein kinase cascade

– this is an important target for cancer mutations

Insulin receptors

– similar to growth factors except:

       * receptor is a disulfide-bonded tetramer

– mediated by IRS-1 & IRS-2

  *if one receptor is missing = Leprechaun or Dorphism

1. insulin binds to TK which undergo autophosphorylation

2. insulin substrate (IRS-1) binds & become tryosine-phosphorylated

3. SH2- containing proteins bind signaling other proteins to bind

Desensitization of G-Protein Coupled



1. stimulated (unstimulated) receptor becomes phosphorylated

2. uncouples receptor from G-protein + BARK attracts arrestin

3. Arrestin binding cause: 

a. receptor sequestration (receptor is recycled)

b. receptor down-regulation (receptor is not recycled)

– sent to lysosome