Chp 11: Chemical Messengers

Question 1

1. Beginning with the release of a chemical messenger in response to a stimulus, list the common characteristics of all chemical messenger systems.

Answer 1

• Secretion of chemical messenger → secretory cell releases chemical messengers (hormone, first messenger) in response to a stimulus
• Messenger diffuses or is transported through blood or extracellular fluid to target cells (cells with receptors)
• The messenger will then bind specifically to a receptor either on or in the target cell (a plasma membrane receptor or intracellular receptor.
• Amino acid/protein messengers (hydrophilic) bind to plasma membrane receptors while lipid-soluble messengers (hydrophobic) bind to intracellular receptors
• Binding of the messenger to the receptor elicits a response
• The signal is terminated and ceases

Question 2

2. What is a signal transduction pathway?

Answer 2

A sequence of chemical reactions after the chemical messenger binds to a receptor.

Question 3

2. Name two types of targets of signal transduction pathways.

Answer 3

• Activation or inhibition of control enzymes
• Induction or repression of genes

Question 4

3. Beginning with the response to a stimulus, list the common characteristics of all chemical messenger systems as they apply to the chemical messenger acetylcholine at the neuromuscular junction

Answer 4

• The nerve cell action potential (stimulus) reaches the presynaptic membrane where it opens Ca²⁺ channels, resulting in an influx of Ca²⁺ inside the nerve cell.
• Increased intracellular Ca²⁺ triggers fusion of acetylcholine vesicles with the presynaptic membrane and then the release of acetylcholine into the synaptic cleft.
• Acetylcholine moves by diffusion across the synaptic cleft and binds to acetylcholine receptors.
• A conformational change takes place as acetylcholine binds to these receptors, opening the gated ion channels and allowing Na⁺ to diffuse in and K⁺ to diffuse out.
• This initiates the muscle cell potential that eventually results in contraction of the fiber
• Acetylcholinesterase (AChe), an enzyme located on the postsynaptic membrane that cleaves acetylecholine by hydrolysis, terminates the message

Question 5

4. When is a chemical messenger acting as an endocrine substance?

Answer 5

• Endocrine cells secrete the hormone into the blood
• Exerts its action on specific target cells that can be very far away - Usually at high enough concentrations to react with cells all over the body

Question 6

4. When is a chemical messenger acting as a paracrine substance?

Answer 6

• The paracrine substance is secreted from cells that are not normally thought of as endocrine cells. They have other names like liver cells or muscle cells.
• Actions are performed on nearby cells.
• Very low amounts are too dilute to affect distance cells.
• Location of cell plays a role in the specificity of the response.

Question 7

4. When is a chemical messenger acting as an autocrine substance?

Answer 7

• Act on the cell from which it is secreted or on nearby cells that are the same type of cell as the secreting cell
• Most autocrine cells are also paracrine cells

Question 8

5. What is a major difference between chemical messengers that are specific for intracellular receptors and those that are specific for plasma membrane receptors?

Answer 8

• The major difference is that they are hydrophobic or hydrophilic
• Hydrophobic chemical messengers are specific for intracellular receptors
• Hydrophilic chemical messengers are specific for plasma membrane (or cell-surface) receptors

Question 9

5. What are examples of intracellular receptors?

Answer 9

Intracellular receptors are proteins found in both the cytosol and nucleus of a cell, that bind to chemical messengers that are hydrophobic.

Ex: thyroxine and steroid hormone cortisol

Question 10

5. What are examples of plasma membrane receptors?

Answer 10

Plasma membrane receptors are proteins, exist on the cell surface and have extracellular binding domains, bind with chemical messengers that are hydrophilic/unable to cross through the cell’s membrane.

Ex: proteins like insulin and glucagon, or amino acid derivatives like epinephrine

Question 11

6. Describe the path taken by cortisol from the time it is released from the adrenal cortex until the time it affects gene transcription.

Answer 11

• Cortisol is released from the adrenal cortex and diffuses into the bloodstream. It is hydrophobic so it must travel attached to serum albumin and steroid hormone binding globulin (SHBG).
• When it reaches those target cells, it passes through the plasma membrane into those cells.
• The cortisol binds to intracellular cortisol receptors in the cytosol and cause a conformation change in the receptors.
• The conformation change causes dimerization of the receptors and exposes a nuclear translocation signal that allows the hormone-receptor complex to cross the nuclear membrane into the nucleus. (Dimerization – compound formed by two identical molecules)
• In the nucleus, the hormone-receptor complex acts as a transcription factor. It binds to a portion of the DNA called the hormone response element, or glucocortoid response element (GRE).
• This results in either induction or repression in gene transcription depending upon the location of the GRE
• The signal is terminated by the lowering of the cortisol concentration. Cortisol is regularly destroyed by the liver.

Question 12

7. How is the signal transduced through plasma membrane receptors?

Answer 12

• The signal is tranduced by turning it from the form it was outside the cell into the form it is inside the cell
• The signal outside the cell (a protein or amino acid) is tranduced into an increase in concentration of a different organic molecule/protein inside the cell.

Question 13

7. Chemical messenger binding produces what two major effects on the cell?

Answer 13

• Regulation of pathways by rapid and immediate activation or inhibition of control enzymes.
• Induction or repression of RNA synthesis, which is slower and less obvious.
• In most cases, the chemical messenger performs both these actions.

Question 14

8. What are signal transduction proteins?

Answer 14

• Any proteins within the cell bound to a membrane-bound hormone receptor.
• A signal transducer protein changes conformation in response to a chemical messenger binding to a receptor outside the cell.
• Each of these signal transduction proteins changes the original signal into a different form, is a member of the cascade/chain of events, and is amplified.

Question 15

9. When a protein contains a src homology 2 domain (SH2 domain), what does it bind to? Is the binding specific?

Answer 15

Yes, SH2 domains are very specific because they recognize

• The phosphotyrosine residue
• The conformation of amino acids around the phosphotyrisine residue
• Each phosphotyrosine residue on the other protein will have a different group of amino acids and therefore a different conformation, making it very specific.
• For example, PI-3-kinase, PLC, and Grb2 all have a SH2 domain but each binds to different sites on the IRS protein.
• Saying that a protein has a SH2 domain does not say the protein will bind to any other protein with a phospotyrosyl residue. It is specific for the conformation around the phosphotyrosyl group.

Question 16

10. In the Ras and MAP kinase pathway, how does the occupied receptor activate Grb2?

Answer 16

Upon binding of the hormone to the receptor, autophosphorylation occurs on the inner side of the membrane, thus forming a phosphorylated tyrosyl residue that the SH2 domain of the Grb2 can bind to.

Binding activates the Grb2

Question 17

10. What is the last step in the Ras and MAP pathway that is catalyzed by MAP-kinase and what is the effect?

Answer 17

The induction or repression of gene transcription

Question 18

11. A mutated form of the G-protein Ras is found in many cancers. How are these mutations thought to affect the cell?

Answer 18

• In a normal cell, the binding of growth factors to receptors sends a signal to the cell nucleus that the cell should divide.
• The strength of the message reaching the nucleus is dependent upon how much Ras is bound to GTP, among other things, and this normally depends on the rate of hydrolysis of GTP by Ras.
• In many cancer cells, the Ras protein is mutated, so that its GTPase function is inhibited (i.e., it won’t hydrolyze GTP to GDP as fast as normal).
• The internal clock of the GTPase is said to be broken. While there may be very little growth factor on the outside of the cell, the nucleus is receiving a strong message to divide from the MAP kinase pathway.
• (The “internal clock for the length of the signal” of any G proteins = its rate of enzymatic hydrolysis of GTP to GDP)

Question 19

12. What are the functions served by phosphatidylinositol phosphates in signal transduction?

Answer 19

• Phosphatidylinositol 4’,5’-bisphosphate (PI-4,5-bisP) can be cleaved by phospholipase C (PLC), producing two intracellular second messengers:
  
  • Diacylglycerol (DAG) stays in membrane, activates protein kinase C which phosphorylates target proteins
  • Inositol triphosphate (IP₃) binds to ER or SR receptor, which opens channels allowing Ca²⁺ to flood into the cell. Ca²⁺ binds to a small protein called calmodulin forming the Ca2+ calmodulin complex. The complex activates enzymes like glycogen phosphorylase kinase
• Phosphatidylinositol 4’,5’-bisphosphate (PI-4,5-bisP) can also be phosphorylated by phosphatidylinositol-3’kinase (PI 3-kinase) to form phosphatidylinositol 3’,4’,5’-trisphosphate (PI-3,4,5-trisP)

Question 20

13. What are the substrates and products of the reaction catalyzed by phospholipase C?

Answer 20

• Substrates: Phosphatidylinositol 4,5-bisphosphate (PI-4,5-bisP) + H2O
• Products: Diacylglycerol (DAG) + Inositol trisphosphate (IP3)

Question 21

14/ What are the substrates and products of the reaction catalyzed by phosphatidylinositol 3’ kinase?

Answer 21

• Substrates: PI 4,5-bisphosphate (PI-4,5-bisP) + ATP
• Products: PI-3,4,,5-trisphosphate (PI-3,4,5-trisP) + ADP
• A phosphate added to carbon #3 of the inositol ring makes it the docking site for pleckstring homology domains

Question 22

15. What is the function of a pleckstrin homology domain?

Answer 22

• Pleckstrin homology domain - a membrane docking site for proteins that contain a certain sequence of amino acids
• Function is to restrict the diffusion of a protein to the inner surface of the plasma membrane
• The pleckstrin homology domain of a protein binds to the hydrophilic end of PI-3,4,5-trisP while the hydrophobic end of PI-3,4,5-trisP remains embedded in the membrane.
• The attached protein and the PI-3,4,5-trisP can then move freely along the plane of the membrane.
• This reduces the time it takes to find other proteins that are also attached to the inner surface of the membrane.

Question 23

16. What is the effect of insulin on protein synthesis, glucose uptake, and glycogen synthesis in muscle cells? Does this help to explain muscle wasting and hyperglycemia in a diabetic?

Answer 23

• Insulin increases protein synthesis.
  
  • Proteins are constantly being broken down into amino acids and synthesized from amino acids.
  • This breakdown and synthesis is in equilibrium and insulin plays a large part in maintaining the equilibrium. If insulin is lowered, the breakdown exceeds the synthesis and muscle wasting occurs.
• Muscle wasting occurs when protein breakdown occurs without adequate synthesis, or restocking of protein stores in the muscle cells. Some protein synthesis can occur without insulin but the adequate replacement of muscle protein requires insulin.
• Insulin increases the rate of glucose transport across the cell membrane into muscle cells. Also, insulin increases the conversion of glucose into glycogen. After a meal, increased insulin increases both the rate of cell uptake of glucose and the conversion of glucose into glycogen. If insulin is low, the glucose from the meal is not removed from the blood at an adequate rate so blood sugar rises (hyperglycemia)

Question 24

17. Be able to draw a cartoon of the insulin receptor that shows the cell membrane, two alpha-beta subunits, the membrane spanning region of the dimers, the insulin-binding site, the sites of tyrosine kinase domains, and the sites of auto-phosphorylation. Draw two IRS proteins bound to the receptor and indicate some of the sites phosphorylated on the IRS by the insulin receptor tyrosine kinases.

Answer 24

Fig 11.13

Question 25

17. Why do proteins bind to the phosphorylated IRS sites?

Answer 25

Because they have the correct SH2 domains

Question 26

18. In the insulin signal transduction pathway that begins with the activation of phosphatidylinositol 3’ kinase, name the down stream active kinase that dissociates from the membrane. Is this kinase a tyrosine or a serine/threonine kinase?

Answer 26

• The downstream active kinase that dissociates from the membrane is Protein Kinase B (PKB).
• This kinase is a serine/threonine kinase.

Question 27

19. In the insulin signal transduction pathway that leads to the activation of MAP kinase, what is the signal transduction protein that binds to the IRS? Why does it bind to the IRS?

Answer 27

• Grb2 binds to the phosphorylated IRS and initiates the MAP kinase cascade.
• Because it has a src 2 homology (SH2) domain.

Question 28

20. In the insulin signal transduction pathway that leads to increases in the diacylglycerol and inositol trisphosphate second messengers, what is the first signal transduction protein that binds to the IRS? Why does it bind to the IRS?

Answer 28

• The first signal transducer protein that binds to the IRS is Phospholipase C (PLC)
• It binds to IRS because of its SH2 domain.

Question 29

21. Explain the sequence of reactions that occur following the binding of glucagon or epinephrine to a heptahelical receptor. How long does the Gαs protein stay active?

Answer 29

• Hormone binds
• Conformation change of both the receptor and heterotrimeric G-protein bound to intracellular domain of the receptor
• Exchange of GDP for GTP on the alpha subunit, which dissociates from both the receptor and the beta/gamma subunits
• The Ga_s subunit binds to target enzyme (adenylyl cyclase, which converts ATP to cAMP and PPi)
• Ga_s eventually hydrolyzes GTP to GDP, becoming inactive
• Ga_s separates from adenylyl cyclase and returns to receptor and beta/gamma subunits, waiting for another hormone to bind to the receptor

Question 30

22. What is the effect of activating Gas?

Answer 30

Ga_s activates adenylyl cyclase and the cAMP cascade

Question 31

22. What is the effect of activating Gai?

Answer 31

Ga_i inhibits adenylyl cyclase and the cAMP cascade

Question 32

22. What is the effect of activating Gq?

Answer 32

Ga_q activates PLC which cleaves PI-4,5,-bisP to generate DAG and IP₃

Question 33

23. What is the response when epinephrine binds to an α1-adrenergic receptor? A β-receptor? That is, what kind of G-protein is activated and what are the initial second messengers produced?

Answer 33

When epinephrine binds to an a1-adrenergic receptor, it:

• Activates Ga_q, which activates phospholipase C (PLC)
• This hydrolyzes phosphatidylinositol bisphosphate into two second messengers: diacylglycerol and inositol trisphosphate

When epinephrine binds to a beta-receptor, it:

• Activates Ga_s, which activates adenylyl cyclase
• This converts ATP into cAMP and PPi

Question 34

24. Which enzyme synthesizes cAMP?

Answer 34

Adenylyl cyclase

Question 35

24. Which enzyme hydrolyzes cAMP?

Answer 35

Phosphodiesterase (or cAMP phosphodiesterase)

Question 36

24. Which enzyme is allosterically activated by cAMP?

Answer 36

Protein kinase A

Question 37

24. Which cAMP enzyme is affected by insulin?

Answer 37

The cAMP phosphodiesterase reaction. This reaction lowers the concentration of cAMP and helps to stop further phosphorylation of proteins by the cAMP cascade.

Question 38

25. When phosphatidylinositol bisphosphate is hydrolyzed by phospholipase C, what is the next step in the signal transduction pathway for diacylglycerol? What are the next several steps in the signal transduction pathway for inositol trisphosphate?

Answer 38

• The next step in the signal transduction pathway for DAG is the activation of protein kinase C.
• The target proteins are those that are then phosphorylated by protein kinase C.
• The next several steps in the signal transduction pathway are:
  
  • The release of calcium from the sarcoplasmic reticulum or the endoplasmic reticulum, depending on the cell type.
  • Calcium binds to calmodulin and changes the conformation of the calmodulin molecule.
  • The Ca2+ calmodulin complex binds to target proteins (calmodulin binding proteins) and changes their conformation and activity.
  • Some of these calmodulin binding proteins are phosphokinases like glycogen phosphorylase kinase

Question 39

26. Glucagon is released when blood sugar is low. How is its signal terminated (or lowered) following a high carbohydrate meal that increases the blood sugar?

Answer 39

The glucagon signal is terminated in several ways:

• Increased insulin inhibits glucagon release from alpha cells of the pancreas
• Glucagon in blood is regularly destroyed by liver as it goes through (half-life is about 5 minutes)
• GTP bound to Gas is hydrolyzed to GDP, so adenylyl cyclase becomes inactive
• Phosphodiesterase is activated by insulin and destroys cAMP
• Insulin activates phosphatases that dephosphorylate the proteins that were phosphorylated by protein kinase A

Question 40

27. Concerning Mya Sthenia who has myasthenia gravis, explain how her chemical messenger system differs from a normal person. How did this happen? How do anticholinesterase drugs temporarily alleviate the problem?

Answer 40

• People suffering from myasthenia gravis have fewer acetylcholine receptors than a normal person. Due to this lack, these people have trouble depolarizing the postsynaptic membrane. As a result, release of acetylcholine in the neuromuscular junction will result in a less than normal muscle contraction.
• This is an autoimmune disease that is destroying her acetylcholine receptors. Her antibodies bind to her acetylcholine receptors and cause them to cross-link. The resultant clumps are endocytosed and destroyed by lysosomes.
• Also, this process sends a signal to make fewer acetylcholine receptors.
• Anticholinesterase drugs inhibit the normal destruction of acetylecholine in the synaptic cleft. Increased concentrations of acetylcholine make it more likely that an action potential can be started with the remaining acetylcholine receptors

Question 41

28. Concerning Ann O’Rexia who has been fasting and is jogging, what was the stimulus for the release of glucagon, epinephrine, norepinephrine, and cortisol?

Answer 41

• Glucagon – low insulin and stress (low blood sugar/exercise)
• Cortisol – stress (low blood sugar/exercise)
• Epinephrine/norepinephrine – stress (low blood sugar/exercise)

Question 42

29. Concerning Ann O’Rexia who has been fasting, why is her blood glucagon increased?

Answer 42

Because her blood sugar and insulin are both low (as blood sugar drops, insulin release drops, and because insulin is the major controller/inhibitor of glucagon release, more glucagon is released).

Question 43

29. Why does glucagon have an effect upon adipose and liver tissue but not upon skeletal muscle tissue?

Answer 43

There is no effect upon skeletal/striated muscle because it has not receptors for glucagon (cardiac and smooth muscle do have receptors).

Question 44

30. Concerning Ann O’Rexia, do glucagon, epinephrine, norepinephrine, and cortisol react with the same receptors, use the same pathways, and elicit the same cellular response?

Answer 44

• Most hormones are specific for their receptors and will not react with other hormonal receptors.
• One exception would be epinephrine and norepinephrine. They can react with the same receptors, but they bind with a different strength and elicit different levels of cellular response.
• Most second messenger pathways and cellular responses are different but some can be similar. We have seen that both glucagon and epinephrine/ norepinephrine both activate the cAMP case

Question 45

31. Concerning Ann O’Rexia, what are the two general mechanisms (ways) that glucagon, epinephrine and other hormones use to elicit a response in target cells?

Answer 45

• By activating or inhibiting the control enzymes of pathways
• By induction or repression of transcription and protein synthesis
• Ex: epinephrine, norepinephrine, and glucagon inhibit the control enzyme for glycolysis in the liver but activate the control enzymes for making glucose from glycogen or amino acids in the liver. The response is immediate. These hormones also activate and inhibit many other control enzymes in many other pathways.
• Ex: Cortisol induces transcription of the RNA coding for the enzymes of gluconeogenesis in the liver. Induction or repression usually takes one or more hours to occur. Cortisol also causes the induction and repression of many other enzymes in many other pathways.

Question 46

32. Concerning Dennis Veere who has cholera, how does the cholera toxin A change the metabolism of the intestinal cell?

Answer 46

• Cholera toxin enters the mucosal cells from the gut and is processed into an enzyme, Cholera toxin A.
• Cholera toxin A ADP-ribosylates a Ga_s subunit (i.e., it transfers the ADP-ribose portion of NAD to a Ga_s subunit
• The ADP-ribosylated Ga_s remains permanently active because it cannot hydrolyze its bound GTP.
• In turn, the ADP-ribosylated Ga_s causes abnormally high adenyl cyclase activity, abnormally high cAMP concentration, and abnormally high activity of protein kinase A.
• Protein kinase A phosphorylates lots of proteins including the CFTR (cystic fibrosis transmembrane regulator) channel.
• When the CFTR channel is phosphorylated, it allows chloride ion to leak into the intestine.
• Since the abnormally phosphorylated CFTR channel is always active, too much sodium chloride ion enters the lumen and water follows the salt by osmosis. This large volume of water causes diarrhea

Question 47

28. From what cells and what tissue did glucagon originate?

Answer 47

• Alpha cells of the pancreas
• Endocrine tissue

Question 48

28. From what tissues does epinephrine, norepinephrine, and cortisol originate?

Answer 48

• Epinephrine/norepinephrine originates from the adrenal medulla
• Cortisol originates from the adrenal cortex

Question 49

28. What is the effect of the hormones epinephrine, norepinephrine, and cortisol upon the release of glucose from liver and free fatty acids from adipose tissue?

Answer 49

• The increased release of glucose from the liver (via gluconeogenesis and glycogenolysis)
• The increased release of free fatty acids from adipose tissue for energy (ATP)