Biochem Lectures 7&8

Question 1

What is signal transduction?

Answer 1

Process by which an outside signal produces a reaction inside the cell.

Question 2

What are the four basic categories of signaling?

Answer 2

Autocrine: short distance, to same cells
Paracrine: short distance, to different cells
Endocrine: Long distances
Neuronal: Long and short distances

Question 3

What is contact dependent signaling?

Answer 3

Direct contact is made between the cells. This can be classified as autocrine or paracrine also.
It is used as cell recognition and aids in restricting cell growth.

Question 4

What are some common endocrine hormones?

Answer 4

Estrogen
Progesterone
Testosterone
Insulin
Glucagon
Cortisol
Epinephrine

Question 5

What two phases of signal transferring is required in neuronal signaling?

Answer 5

1. Through the neuron via electrical potential.

2. Across the synapse through action of a neurotransmitter.

Question 6

What are the two mechanism for ligand-induced signaling?

Answer 6

1. Intracellular receptors: the ligand is hydrophobic and diffuses across the membrane to a receptor inside the cell.
2. Cell surface receptors: hydrophilic signal molecules bind to receptors on the cell surface and the transmembrane proteins transfer the signal into the cell.

Question 7

How are steroids transferred through the body and into cells?

Answer 7

Steroids are derived from cholesterol and are therefore non-polar. They need carrier proteins to be transported through the blood stream, but they diffuse on their own across the plasma membrane.

Question 8

What are the five main steroids that we should know?

Answer 8

Testosterone
Estradiol
Progesterone
Cortisol
Aldosterone

Question 9

What is the general mechanism of steroid action inside the cell?

Answer 9

The steroid crosses the membrane and binds to its receptor protein in the cytosol. The receptor complex relocates to the nucleus and binds to the DNA at a regulatory site for a gene. It induces transcription of the gene and the corresponding protein is produced.

Question 10

Explain nitric oxide signaling.

Answer 10

NO is produced by nitric oxide synthase (NOS) which converts arginine to NO and citrulline.
NO is soluble in aqueous and lipid media. Once made, it only affects local cells because it has a short half life.
Acetylcholine is released from a nerve terminal and is received by an endothelial cell of the blood vessel. This causes eNOS to produce NO which diffuses out of the cell and into a nearby smooth muscle cell. NO reacts with iron in the active site of guanylyl cyclase which produces cyclic GMP. cGMP initiates a pathway that signals rapid relaxation of the smooth muscle cell. This cause vessel dilation.

Question 11

How is nitroglycerin related to nitric oxide?

Answer 11

Nitroglycerin is converted to NO in the body and causes dilation of blood vessels. This is used to treat angina where too little oxygen is getting to the heart. This process does not require NO synthase.

Question 12

What proteins phosphorylate other proteins?

Answer 12

Kinase proteins. They phosphorylate serine, threonin, and tyrosine because they have hydroxyl groups.

Question 13

What proteins dephosphorylate?

Answer 13

phosphoratases

Question 14

Extracellular ligands can bind to two types of transmembrane receptors that cause phosphorylation cascades, what are they?

Answer 14

G-Protein coupled receptor (GPCR): a single protein coiled through the membrane 7 times
Receptor tyrosine kinase (RTK): these are two separate proteins that work together

Question 15

What do phosphorylation cascades do to the signal? How is the signal changed?

Answer 15

The signal is transmitted very quickly, and it is amplified along the way.

Question 16

What are the targets of kinase proteins?

Answer 16

1. Other kinases: to amplify the signal
2. Phosphates: to increase dephosphorylation of certain substrates
3. Metabolic enzymes: to simultaneously activate or inactivate several enzymes in a given pathway
4. Transcription factors: to have an effect on gene expression

Question 17

What is the speed of the response from phosphorylation and gene expression signals?

Answer 17

Phosphorylation takes seconds to have an effect. Gene expression and protein synthesis can take hours or days.

Question 18

What kind of binding site do most growth factors bind to?

Answer 18

Most bind to receptor tyrosine kinases.

Question 19

What is the largest class o cell surface receptor cells?

Answer 19

G-protein coupled receptors.

Question 20

What kind of receptor do epinephrine and glucagon bind to?

Answer 20

Both bind to GPCR’s.

Question 21

What is the protein inside the cell that transmits the signal from GPCR’s into the cell?

Answer 21

It is a protein that has three parts: alpha, beta, gamma and is a GTP-binding protein.

Question 22

What do the three types of alpha proteins do that are part of the GTP binding protein complex?

Answer 22

Galpha-s: activates the enzyme adenylate cyclase
Galpha-i: inhibits the enzyme adenylate cyclase
Galpha–q: activates the enzyme phospholipase C

Question 23

How do GTPases work?

Answer 23

GTPase is inactive when bound to GDP. When it binds a GTP molecule, it becomes active until it hydrolizes the GTP into GDP + P and becomes inactive again. The time this process takes is a kind of molecular timer.

Question 24

What is the step by step process of GPCRs getting activated?

Answer 24

1. The trimeric protein is bound on the inside of the membrane to the inactive GPCR. The alpha subunit is bound to a GDP molecule.
2. Activation of the receptor by a ligand stimulates the alpha subunit to release GDP and bind GTP. This activates it.
3. Activated alpha leaves beta gamma and moves along the membrane to activate other proteins.
4. The beta and gamma units stay together and also move around to activate other proteins.
5. After a given time, the GTP gets hydrolized by alpha, and it becomes inactive.
6. Alpha, beta, gamma reunite to a GPCR and wait for another signal.

Question 25

What are the events following the binding of epinephrine or glucagon to a GPCR?

Answer 25

They activate either an alpha-s or an alpha-i molecule to bing GTP and move along the membrane. Alpha-s will stimulate adenylate cyclase to produce cyclical AMP which binds protein kinase a to stimulate a kinase cascade. The alpha-i protein will inhibit adenylate cyclase and reduce the production of cAMP.

Question 26

What are beta adrenergic receptors vs alpha 2 adrenergic receptors?

Answer 26

Both bind epinephrine, but the beta version is associated with alpha-s which stimulates adenylate cyclase and the alpha 2 adrenergic receptor is associated with alpha-i which inhibits adenylate cyclase activity.

Question 27

What happens with cholera and pertussis toxins?

Answer 27

Both cause increased adenylate cyclase activity.
Cholera: ADP-ribosylated Galpha-s cannot hydrolyze GTP so it continuously signals the production of cAMP. Stuck on.
Pertussis: ADP-ribose is added to Galpha-i which prevents it from releasing GDP. This prevents it from inhibiting adenylate cyclase and cAMP continues to be produced. Stuck off.

Question 28

How does cAMP carry a signal?

Answer 28

cAMP attaches to the regulatory units of Protein Kinase A (PKA) which releases the regulatory units from the active units which go on to phosphorylate proteins and transcription factors.

Question 29

How is the cAMP signal turned down? How are cAMP molecules turned off?

Answer 29

After cAMP binds to PKA to send its signal, cAMP phosphodiesterase converts cAMP into AMP which is no longer a signaling molecule.

Question 30

What is another membrane protein targeted by G proteins besides AC?

Answer 30

Phospholipase C (PLC) is targeted by Gq proteins. This is an alpha1 adrenergic receptor.

Question 31

What does PLC do?

Answer 31

When activated by Gq-alpha protein, PLC cleaves PIP2 into two separate signaling molecules. These are:
Diacylglycerol (DAG) hydrophobic tails
Inositol triphosphate (IP3) hydrophilic head

Question 32

What does IP3 do?

Answer 32

IP3 is negatively charged and goes through the cytoplasm to receptors in the ER which open channels to allow Ca2+ to flow into the cytoplasm. Ca2+ binds to protein kinase C and they find and bind to DAG in the membrane. PKC then becomes active and starts to phosphorylate proteins.

Question 33

What are adrenergic receptors?

Answer 33

GPCRs that bind to specific ligands to regulate metabolic processes. There are three main classes of adrenergic receptors:
Alpha1: G-alpha-q stimulates PLC
Alpha2: G-alpha-i inhibits adenylate cyclase
Beta1,2,3: G-alpha-s stimulates adenylate cyclase

Question 34

What is a second messenger?

Answer 34

A messenger that carries the signal inside the cell after the signal is first received at the cell surface.

Question 35

What is calmodulin and what does it do?

Answer 35

Calmodulin is dumb bell shaped and each end binds two Ca2+ ions which activate calmodulin. It then binds proteins and wraps itself around them to change their shape and function.

Question 36

What is the step by step signaling process for receptor tyrosine kinases?

Answer 36

1. Signaling growth factor binds to an RTK receptor
2. Initial binding causes two RTKs to dimerize
3. Each RTK phosphorylates the other inside the cell
4. Phosphorylation changes the cytoplasmic portion of the receptors creating new binding sites
5. GRB2 binds to RTK and recruits SOS protein
6. SOS binds to a GTP binding protein called RAS. Ras exchanges GDP for GTP and becomes activated
7. RAS finds Raf which starts a phosphorylation cascade known as MAPK

Question 37

What is MAPK?

Answer 37

It is a protein kinase that can phosphorylate other proteins to affect cell behavior such as altered gene expression, cell division, cell survival.