Cell structure/function - Week 2 (g-proteins and 2nd messengers)

What are the characteristics of a 2nd messenger molecule?

small, hydrophilic, free to diffuse/spread out

What are 2 examples of 2nd messengers?

Ca2+ and cyclic AMP

What do 2nd messengers act as?

Allosteric regulators (they bind to and regulate other proteins)

What is an effector?

An enzyme that acts as a catalyst for synthesis of a second messenger

Describe the 5 general steps for g-coupled receptors

ligand + recpetor → complex formed → activates transducer → activates effector → makes 2nd messenger

When is glucose released from liver cells

When blood sugar is low (hypoglycemia), and during fight/flight response

How is excess glucose in the liver stored?

As glycogen, in branched polymers

Why is glucose in the liver stored in polymers?

To decrease osmolarity

How does adrenaline stimulate the release of glucose in the liver?

The epinephrin ligand binds to specific receptors in liver plasma membrane, triggers intercellular signal, specific response occurs: breakdown of glycogen and release of glucose

What is glycogen

the branched polymer that stores glucose in the liver

What is the function of glycogen phosphorylase?

An enzyme which breaks down glycogen into individual glucose molecules (breaks off monomers)

What is glycogen synthase?

an enzyme which packs glucose into polymers (add glucose monomers to storage)

In order for glucose to be released, what needs to be inhibited/activated?

Glycogen synthase (hint: packs monomers) must be inhibited, glycogen phosphorylase (hint: frees monomers) much be activated

What is adenylyl an example of? Why?

An effector because it is an enzyme that catalyzes the production of cAMP

What is the functions of cAMP

A 2nd messenger that acts as an allosteric activator for protein kinase A

What is the name of the transducer that serves as an allosteric activator for adenylyl cyclase?

heterotrimeric G-protein

What enzyme is cAMP an allosteric activator for?

pka (protein kinase A)

What happens when cAMP binds to PKA (protein kinase A)?

cAMP binds to the 2 regulatory subunits in PKA, this releases the 2 catalytic subunits to be active

Kinase

An enzyme that transfers a phosphate from ATP to a target protein (phosphorylates a protein)

What is a phosphorylated protein?

A protein with a phosphorus group covalently bound to it’s amino acid.

What is phosphotase

A HIGHLY REGULATED enzyme that removes phosphate group from a protein

What happens to a target protein when it becomes covalently bound to phosphate?

It changes shape

How does protein phosphorylation act as a reversible process?

Kinase transfers a phosphate to a protein, phosphatase removes the phosphate

What kinds of function are changed by protein phosphorylation?

enzyme activity, binding partners

What is PKA made up of?

2 R (regulatory) subunits and 2 C (catalytic) subunits

How many cAMP can bind to each R subunit in PKA?

2

What does the “molecular switch mechanism” refer to? Why?

protein phosphorylation/dephosphorylation, its a reversible process

Why do signaling pathways need to coordinate multiple things at a time?

in order for one thing to be effective, it’s counterpart must be inhibited.

Is PKA a specific kinase?

No, it can phosphorylate many proteins

What is the benefit of amplification

Strong responses

Do secondary effectors cause amplification?

No, they each activate one enzyme

What does PDE (phosphodiesterase) do?

degrades cAMP

What is the effect of one primary signal?

Several coordinated signals

Does phosphorylation turn proteins on or off?

Phosphorylation can turn proteins on or off. Phosphorylation changes protein shape and activity but this differs for different proteins.

Characteristics of branch points

not selective, coordinate events

Are cellular responses specific to only one signal? Give example

Not specific to one signal. Adrenaline and glucagon both cause glucose to be released from liver

Why is the G-protein considered heterotrimeric?

made up of 3 different proteins

steps 1-6 of a g-protein

1) hormone binds to activate recpetor

2) Galpha binds to subunit

3) active receptor cause a conformational change in Galpha so GDP dissociates

4)GTP binds to Galpha causing it to dissociate from Gbetagamma and receptor

5) hormon leaves (deactivate receptor); Galpha activates effector

6) hydrolosis (GTP—> GDP) cause Galpha dissociate from effector and reassociate with Gbetagamma

What is one benefit of a multistep cascade of events?

amplification

Is Galpha active or inactive when bound to GDP?

inactive

Is Galpha active or inactive when bound to GTP?

active

Where does Galpha GDP bind to

Gbetagamma

Where does Galpha GTP bind to?

An effector

What are the 3 protein subunits of a heterotrimeric G-protein

alpha, beta and gamma

How does GTP turn into GDP to deactivate Galpha?

hydrolysis

Why is GTP hydrolysis considered a timer?

It takes about 30 seconds to occur meaning a Galpha is active for about 30 seconds before GTP—>GDP and it is deactivated

What is the branch point in the glucose release example?

PKA (can activate or inactive through phosphorylation)

How many types of g-proteins can each g-protein coupled receptor activate?

1 (sometimes 2)

What happens when different alpha subunits regulate different effectors?

They can therefore regulate different second messengers

How can g-proteins act as a branch point?

Galpha activates effector, Gbetagamma can sometimes regulate ion channels after Galpha dissociates

How many G-proteins can one g-protein coupled receptor activate?

Hundreds

Where are g-proteins bound to?

The membrane

What are the two advantages of having a multi-part g-protein?

potential branch point and amplification control