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Have you ever had a sudden fright?

Perceived a threat, you’re all geared up for fight or flight

Heart pounding, face flushed, hair standing up on end. 

It’s partly from a hormone called epinephrine


Epinephrine’s got another name: adrenaline

Secreted from adrenal glands, it’s the stuff in EpiPens

A shot can save your life, if you have an allergy

Involving sensitivity to nuts or stings from bees.


Epinephrine’s polar so it can’t diffuse 

Through the lipid bilayer, so instead it induces 

A second messenger, it’s cyclic AMP

Which stimulates massive cellular activity.


Let’s learn how liver cells react to an epinephrine dose. 

What they do is break down glycogen to glucose.

Which streams into your blood to give you energy,

Required to respond to threats | as you fight or flee.


How does epinephrine get your liver cells to do this function?

It’s three steps, first reception  then transduction,

Then cellular response, an enzyme activation. 

We’re learning ‘bout the process of cell communication.



Cell communication: 

Works through phases three,

Reception of the ligand

They’re complementary,


Second messenger released

Transduction is a beast!

Now amplified response until the stimulus has ceased!

Today’s focus is G protein coupled receptors.

We’ll leave the other systems, for another lecture.

Epinephrine’s the ligand that with the receptor binds. 

It’s a hormone — the hydrophilic kind. 


Ligands bind to receptors only if their shape matches.

It’s specific based on complementary attachment 

Epinephrine goes everywhere but only affects,

Cells with receptors with which it can connect


Once epinephrine binds at the the receptor binding site

The receptor changes shape on its cytoplasmic side

Where, just inside the membrane, you’ll find the G protein. 

Which until epinephrine binds is dormant asleep


In its dormant form the G protein’s bound to GDP,

But epinephrine binding makes it bind with GTP

Then the G protein drifts in the membrane space 

’Til it bumps into Adenylyl cyclase


A membrane-bound enzyme whose signaling function’s 

Activating a second messenger setting up transduction. 

Transduction converts a message into another kind.

Message 1 was the ligand, which came along to bind


With the receptor which in response woke up the G protein

Which gets adenylyl cyclase to take an ATP

And remove two phosphates making cyclic AMP

That’s the second messenger, it takes the message deep


Into the cytoplasm where it unleashes a chain of

Enzyme action: a phosphorylation cascade. 

The message was received, it was then transduced

Next we’ll see how the cellular response gets produced



A phosphorylation cascade involves a chain 

Of enzymes |  known as kinases

These kinases  |  get phosphorylated

Which means “gaining a phosphate” which activates them


And what do kinases do when they’re charged up with a phosphate?

They find the chain’s next kinase and phosphorylate it.

This chain of activation is like falling dominoes,

‘Cause that’s how a phosphorylation cascade goes.


And each kinase carries out multiple activations, 

Causing massive signal amplification 

The signal grows in strength |  with every kinase action,

Building in intensity a chain reaction


One epinephrine binds causing many cyclic AMPs

Each of which activates many kinase proteins,

By the chain’s last kinase: tens of thousands activated,

A cellular tsunami as the chain is terminated. 


Glycogen phosphorylase is the enzyme at the end. 

It makes glucose by hydrolyzing glycogen.

So into the blood from the liver, glucose gets secreted,

And the threat that started this story can be fled from or defeated. 


It’s adaptive how your cells are poised to spring into action

Once they hear epinephrine’s alarm there’s a reaction,

In all eukaryotes from amoebas to acacias.

G protein coupled receptors used in cell communication!




Back to the ligand at the receptor it only stays

For a moment before it diffuses away

So when the threat is gone, the cascade gets shut down

With no adrenal secretion, the receptor’s unbound. 


G protein drops the phosphate, now bound to GDP

It goes back to sleep, and stops its activity

Bound to GDP it no longer stimulates

Adenylyl cyclase, which no longer creates


Cyclic AMP so the second message stops,  

Kinase phosphorylation quickly drops. 

As other enzymes, protein phosphatases,

Clip off phosphates, and turn off kinases


Glycogen phosphorylase stops hydrolyzing

Glycogen so blood glucose normalizes

Liver cells return to their resting state.

I love how G proteins let cells communicate!


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