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Lecture 4

BIOL 1201 Lecture Notes - Lecture 4: Protein Kinase A, Adenylyl Cyclase, Inhibitory Postsynaptic Potential


Department
Biological Sciences
Course Code
BIOL 1201
Professor
Bill Wischusen
Lecture
4

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1
Learning Objectives Number 4
Ocean
See the review sheet on Moodle
Cell Signaling
What are first messengers?
An extracellular molecule that elicits a cellular response when it binds to a
receptor protein.
o Ex) hormones, neurotransmitters
What are two examples of second messengers?
Cyclic adenosine monophosphate (cAMP)
o Synthesized from ATP
o Activates an enzyme called protein kinase
Calcium
o Muscle contractions
What is signal amplification and how does it benefit the cell?
Start with one molecule and end up with 108
Amplify faint signal
o A few first messenger molecules
o Many second messenger molecules
o Each step in the cascade recruits more molecules
Elicit a coordinated response
What is the role of phosphorylation cascades in signal amplification?
Activated kinase activates a different kinase, etc.
At each step more enzymes are recruited.
Where are the receptors involved in cell signaling located?
May involve a receptor in the plasma membrane
o An integral membrane protein
May involve an intracellular receptor
o Steroid hormone receptor
Signal amplification in the breakdown of glycogen
Epinephrine (adrenaline) activates a GPCR in liver cells (e.g., Fig. 11.10,
11.16, 45.7 and 45.9)
Calcium activates a phosphorylation cascade in skeletal muscle (after the
Protein Kinase A step in Fig 11.16, activating the enzyme phosphorylase kinase)
The relationship of adenylyl cyclase and cAMP
How is adenylyl cyclase activity regulated?
The enzyme is modulated (positively and negatively) by different G proteins
Intracellular cAMP levels go up or down depends on the summation of all
the positive and negative signals on that cell.
What reactions do protein kinases and phosphatases catalyze?
Protein kinases: phosphorylation reactions
o Covalent transfer of phosphate group form ATP to specific amino
acids of target protein.
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2
o Acts as an on or off switch for the target protein.
Phosphatase would remove a phosphate group.
What is the role of cAMP-dependent Protein Kinase (Protein Kinase A) in cell
signaling?
Protein kinase catalyze phosphorylation reactions
What are the subunits of Protein Kinase A and what do they do?
2 regulatory subunits: R
2 catalytic subunits: C
What is the subunit composition of the active enzyme?
In the cytoplasm cAMP binds to the R subunit
R2C2 dissociates into an R dimer and 2 C monomers.
C monomers catalyze phosphorylation reactions
Of the inactive enzyme?
R2C2
What is G protein cycle? Be sure you know all of the steps in the cycle and can
reproduce it from memory.
Switch protein either on or off.
Activated by occupied (activated) receptor
GDP is bound to the alpha subunit of the inactive protein heterotrimer
Inactive (off): alpha-GDPbetagamma
Active (on): alpha-GTP + betagamma
o Alpha subunit with GTP bound and a beta gamma dimer.
Cell Signaling: Activating of G proteins
- An activated receptor interacts with the G protein
- GTP replaces GDP on the alpha subunit
- The G protein dissociates into alpha-GTP and a betagamma dimer
- The G protein is now on.
- Activated G protein subunits interact with their target enzymes or ion
channels.
Cell Signaling: Inactivating G proteins
- Turned off when an intrinsic GTPase activity of the alpha subunit
hydrolyzes GTP to GDP
- alpha-GDP and the betagamma dimer reassociate.
o If GTP were not hydrolyzed to GDP, it wouldnt stop interacting with
the target.
Summary of G protein
- Membrane receptor is activated by an extracellular signal
- G protein transfers the extracellular signal into the intracellular
compartment by interacting with a target
- Target enzyme produces an intracellular second messenger.
What are examples of the types of G proteins?
Gs: stimulatory G protein stimulates adenylyl cyclase
Gi: inhibitory G protein inhibits adenylyl cyclase
Golf: olfactory G protein
Gt: transducin, involved in vision
What are the subunits of G proteins?
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3
Alpha, beta, gamma
What would happen if the G protein cycle is disrupted by mutations?
Loss of function of mutations: prevent formation of stable mRNA or protein
Gain of function mutations activation of receptor in the absence of an
agonist
Inhibitory and stimulatory signals and receptors
What would happen if the G protein cycle is disrupted by mutations? (see Potential
Targets for Disease on Moodle)
See the G protein cycle sheet and the work sheet on Moodle
figures 11.5, 11.6, 11.7, 11.9, 11.10, 11.11, 11.12, 11.15 and 11.16.
Hormones
What is homeostasis?
The steady-state physiological condition of the body.
Constancy of the interior environment of an organism
What are examples of homeostasis?
pH, temperature, ion concentrations, etc.
What is meant by antagonistic?
Hormones that work against each other
What is feedback regulation? (Fig. 45.17)
End product of a pathway inhibits an
enzyme in the pathway.
What is the endocrine system?
A hormone cascade pathway. In
response to the stimulus, the hypothalamus
secretes a releasing hormone that targets
the anterior pituitary. The anterior pituitary
responds by secreting a second tropic
hormone, which travels through the
bloodstream to an endocrine gland. In
response to this tropic hormone, the
endocrine gland secretes a hormone that
travels to target cells, where it induces a
response. In the example of thyroid hormone
regulation, thyroid hormone exerts negative
feedback on the hypothalamus and anterior
pituitary. This feedback inhibits release of
TRH and TSH, preventing overreaction to
the stimulus (such as low temperature in the
case of a human infant.)
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