Mtc.ex2.c.hormones&signaltransduction

Front 1

What is the molecular action of lipophilic hormones?

Back 1

The molecular action of lipophilic hormones (e.g. steroids) is to control gene expression. Lipophilic hormones diffuse through the plasma membrane and bind to intracellular receptors that are transcriptional regulators.

Front 2

What is the molecular action of hydrophilic hormones?

Back 2

The molecular action of hydrophilic hormones is ultimately to control activities of cellular proteins, including enzymes, transcription factors, cytoskeletal proteins. Hydrophilic hormones bind to extracellular domain of cell surface receptors that span the membrane. This extracellular binding event "activates" the cytoplasmic domain of the receptor and results in the stimulation of one or more intracellular signaling pathways. Signaling pathways are comprised of several different classes of molecules that function in a relay.

Front 3

What classes of proteins typically regulate activation states of intracellular signal relay components in response to a hydrophilic hormone binding to its cell-surface receptor?

Back 3

G Proteins and protein kinases typically regulate activation states of intracellular signal relay components in response to a hydrophilic hormone binding to its cell-surface receptor.

Front 4

What mathematical relationship of the key equilibrium and catalytic rate constants governs the amount of G-protein in the active state?

Back 4

(Active G-protein)/(Inactive G-protein) = (Kd-GDP)/(kcat-GTP)

Front 5

What classes of regulatory proteins function to increase the fraction of G-protein that is in the “active state”? What step in the G-protein cycle does each of these regulate? What is the mechanism underlying their regulatory effect?

Back 5

Guanine nucleotide exchange factor (GEF) affects the release of GDP (Kd-GDP). It increases the rate of GDP exchange for GTP and promotes G-GTP formation.

GTPase inhibiting protein (GIP) acts on the hydrolysis of GTP (kcat-GTP). It decreases the rate of GTP hydrolysis and traps G protein in the bound state.

Front 6

What mechanisms are commonly used to inhibit activity of protein kinases in the cell? Which of these is relieved by the action of second messengers?

Back 6

Mechanisms: Negative phosphorylation that serves to stabilize an inactive conformation of the protein kinase. Binding of negative regulatory subunits (or domains) to the catalytic subunit (or domain) of a protein kinase. This binding interaction inhibits activity of the kinase catalytic subunit (or domain).

Reverse inhibition is relieved by the action of second messengers. The second messenger binds to the negative regulatory subunit (or domain) of a protein kinase. Binding weakens the interaction of the negative subunit (or domain) with the catalytic subunit (or domain).

Front 7

What are producer cells?

Back 7

Producer cells sense specific stimuli from the external environment and produce hormones in response to these stimuli.

Front 8

What are target cells?

Back 8

Target cells have receptors for hormones. Hormone binding to receptor causes the target cell to respond by changing cellular functions.

Front 9

What are hormones?

Back 9

Hormones are intercellular information carriers between sensor/producer cells that perceive changes in the environment and target cells that respond to the change. Hormones coordinate activities of different cells in a multicellular organism.

Front 10

What are receptors?

Back 10

Receptors are proteins that bind hormones with high specificity and affinity.

Front 11

What are responses?

Back 11

Responses are changes in cellular processes including gene expression, enzyme activities, proliferative capacity, secretory properties, and morphology.

Front 12

What are the three classifications of hormones, when classified by radius of action?

Back 12

1. Endocrine hormones are produced by endocrine glands and transported by the circulatory system to the target cells.

2. Paracrine Hormones are produced by cells that are in close proximity to the target cells.

3. Autocrine Hormones are produced by the cells that are also the target of action.

Front 13

Which second messenger is catalyzed by the effector Adenylate cyclase?

Back 13

cyclic AMP (cAMP) is the second messenger for Adenylate cyclase.

Front 14

Which second messenger is catalyzed by the effector Guanylate cyclase?

Back 14

cyclic GMP (cGMP) is the second messenger for Guanylate cyclase.

Front 15

Which second messenger is catalyzed by the effector Phospholipase C?

Back 15

The Phosphatidylinositol system is comprised of:

Diacylglycerol (DAG)
Inositol 1,4,5-triphosphate (IP3)

These are the second messengers of the effector Phospholipase C.

Front 16

Which second messenger is catalyzed by the effector Phosphatidylinositol-3-kinase?

Back 16

Phosphatidylinositol(3,4,5)P (PIP3) is the second messenger catalyzed by the effector Phosphatidylinositol-3-kinase.

Front 17

What is the second messenger of the ion channel system?

Back 17

Calcium is the second messenger of the ion channel system.

Front 18

What is the role of protein kinases?

Back 18

Protein kinases transfer the γPO4 from ATP to the hydroxyl group of serine, threonine, or tyrosine residues of proteins.

The phosphorylation state of a protein affects its biological function through changing its
activity, protein associations, stability, or cellular localization.

Front 19

What are the three types of protein kinases?

Back 19

Protein kinases are classified as one of three types:

1. Serine/Threonine Kinase
2. Tyrosine Kinase
3. Dual specificity kinase (both 1 and 2)

Front 20

What is the role of protein phosphatases?

Back 20

Protein phosphatases reverse the action of protein kinases by hydrolyzing phospho-serine, phospho-threonine, or phospho-tyrosine residues and restoring the side chains to the free hydroxyl form.

The phosphorylation state of a protein affects its biological function through changing its
activity, protein associations, stability, or cellular localization.

Front 21

What are the four conserved structural features of the protein kinase catalytic domains?

Back 21

1. ATP binding lobe
2. Substrate binding lobe
3. Catalytic center
4. Activation loop

Front 22

What is the function of the activation loop of the protein kinase catalytic domain?

Back 22

Conformation and phosphorylation state of the Activation Loop determines the spatial relationship of the small and large lobes and the orientation of residues in the cleft that are important for catalysis.

Front 23

How do regulatory domains or subunits positively regulate protein kinase activity?

Back 23

Regulatory domains or subunits can positively regulate protein kinase activity by inducing the active conformation of the kinase catalytic cleft.

Front 24

What is the difference between a regulatory domain and a regulatory subunit?

Back 24

A regulatory domain is a part of the same polypeptide as the protein kinase catalytic core. A regulatory subunit is a separate polypetide from that of the protein kinase catalytic core.

Front 25

How do regulatory domains or subunits negatively regulate protein kinase activity?

Back 25

Negative regulation involves protein interactions that disfavor the active conformation of the kinase catalytic core and/or interfere with substrate binding. (This regulation is often reversed by the action of second messengers.)

Front 26

How does one protein kinase positively regulate the activity of another protein kinase?

Back 26

Positive regulation by covalent addition of phosphate(s) promotes active conformation of the kinase (e.g. addition of phosphates to the activation loop). This is a very common mode of regulation.

Front 27

How does one protein kinase negatively regulate another protein kinase?

Back 27

Negative regulation of protein kinase phosphorylation is done by covalent addition of phosphate(s). This phosphorylation promotes an inactive conformation of the kinase (e.g. addition of phosphates to the phosphate anchor.)

Front 28

Which enzymes reverse the positive and negative effects of reversible protein phosphorylation for regulation of protein kinase activity?

Back 28

Positive and negative effects of phosphorylation are reversed by the action of phosphatases.

Front 29

What are the two predominant switch mechanisms for intracellular signal relays?

Back 29

The two predominant switch mechanisms for intracellular signal relays are:
1. Covalent phosphorylation of the target protein by protein kinase
2. Exchange of GDP for GTP by G-proteins

Front 30

What are two key control points in the transition between active and inactive states of G-proteins?

Back 30

kcat-GTP is the rate of GTP hydrolysis to GDP.

Kd-GDP is the dissociation constant for GDP.

Front 31

Which two regulators stimulate G-protein signaling?

Back 31

GEFs and GIPs stimulate G-protein signaling by increasing the amount of active G-protein.

GEFs (Guanine nucleotide exchange factors) increase Kd-GDP.

GIPs (GTPase inhibiting proteins) decrease kcat-GTP.

Front 32

Which two regulators inhibit G-protein signaling?

Back 32

GAPs and GDIs inhibit G-protein signaling by decreasing the amount of active G-protein.
GAPs (GTPase activating proteins) increase kcat-GTP.

GDIs (Guanine nucleotide dissociation inhibitors) decrease Kd-GDP.

Front 33

How are polypeptide hormones formed?

Back 33

Polypeptide hormones are formed from pre-pro-hormones. Pre-pro-hormones are encoded by genes, transcribed into mRNA, translated into pre-pro-hormones, and processed to give the secreted and active form.

Front 34

What are the steps by which pre-pro-hormones are targeted to the secretory pathway? Which organelles are involved?

Back 34

1. An N-terminal signal peptide targets hormone precursors to the rough endoplasmic reticulum.
2. Uptake into the ER lumen occurs during translation.
3. Precursors progress through the Golgi network and are delivered to the plasma membrane in secretory vesicles.
4. Processing to mature forms begins during these transitions.

Front 35

When does a pre-pro-hormone become a pro-hormone?

Back 35

The pre-pro-hormone becomes a pro-hormone upon cleaving of the N-terminal signal peptide. The signal peptide is cleaved upon entry into the lumen of the ER.

Front 36

How is a pro-hormone converted to a mature hormone?

Back 36

Pairs of basic residues (i.e. Lys-Lys, Lys-Arg, Arg-Lys, or Arg-Arg) mark the cleavage sites. Pro-hormone converting enzymes (PCs) are serine proteases that cleave on the carboxyl side of the dibasic amino acid sites. Processing begins in the trans-Golgi network; may continue in the secretory vesicles. Final steps can occur in the extracellular fluid after secretion has occurred.

Front 37

What are the strategic features of the pre-pro-hormone mechanism.

Back 37

The strategic features of the pre-pro-hormone mechanism are:
1. Ensures transport through the secretory pathway.
2. Delays activation.

Front 38

What are enkephalin precursors?

Back 38

Enkephalin precursors are single genes that encode multiple copies of a single hormone. Enkephalins are pentapeptides with opioid activity, alleviating pain and promoting eurphoric effects. The two types or Met-ENK and Leu-ENK. The enkephalin precursor encodes 6 Met-ENK and 1 Leu-ENK.

Front 39

What are opiomelanocortin precursors?

Back 39

Opiomelanocortin precursors are single genes that encodes several different hormones.

Front 40

What is Pro-opiomelanocortin (POMC)?

Back 40

POMC is the precursor of six (*possibly seven) peptides that are hormones with diverse biological activities.

Front 41

How is POMC processing regulated?

Back 41

POMC is synthesized in specialized pituitary, brain, and skin cells in humans. Regulation of Pre-POMC production is under control of different hormones in each
cell type. Different mature hormones are secreted by these cell types depending on which prohormone converting enzymes are expressed.

Front 42

What is the endocrine action of insulin?

Back 42

In the liver, insulin inhibits glucose production.

In muscle and adipose tissue, insulin stimulates glucose uptake and utilization.

Front 43

What are the processing steps of the insulin precursor?

Back 43

Insulin is produced by the β-cells of the pancreas.

1. A 19 residue N-terminal signal peptide directs pre-pro-insulin to the ER.
2. Cleavage of the signal peptide upon entry of the ER marks conversion to pro-insulin.
3. Pro-insulin transits through the Golgi to secretory vesicles.
4. C-peptide is removed in the secretory vesicles. (C-Peptide is a ~30 a.a. peptide that connects the COOH end of the B chain to the NH2 end of the A chain)

Front 44

Pre-pro-parathyroid hormone (pre-pro-PTH, below) is synthesized in chief cells of the parathyroid gland in response to low extracellular Ca2+ and/or elevated extracellular phosphate. The precursor is targeted to the secretory pathway so that mature PTH is secreted into the blood stream.
a. What region of the polypeptide transits through the golgi apparatus? b. What region of the polypeptide encompasses the secreted hormone?

Back 44

a. Pro-PTH is amino acids 25-115. Nascent pre-pro-PTH is targeted to the secretory pathway by the N-terminal signal peptide. This peptide is cleaved off the polypeptide as it enters the lumen of the ER for transit to the Golgi apparatus.
b. Secreted PTH is amino acids 31-115. Conversion of Pro-PTH to mature PTH occurs in the golgi apparatus by the action of pro- hormone converting enzymes that cleave after pairs of basic amino acids (dibasic amino acid sites).

Front 45

Pre-pro-glucagon is synthesized in the α-cells of the pancreas and in L-cells of the intestinal mucosa. α-cells secrete only glucagon while L-cells secrete only t-GLP-1 and GLP-2.
To account for this cell-type specific pattern of hormone secretion, which of the two indicated pro-hormone converting (PC1 and PC2) enzymes and other protease (Arg protease) must be expressed in α-cells and L-cells?

Back 45

α-cells express PC2; L-cells express PC1 and the arginine protease

Front 46

An antibody that recognizes the C-peptide region of pro-insulin is available. What species does this antibody detect in plasma?

Back 46

It will recognize two circulating species:
1. C-peptide released from the processing reaction that converts pro-insulin to mature insulin
2. Pro-insulin that might not have been properly processed. (Circulating pro-insulin would
be high under circumstances where there is a deficiency in PC1.)