Cell communication- signal transduction

Done by: Batool al Masri

Corrected by: sajedah nizar

بسم ميحرلا نمحرلا هللا

**مالحظة : هذه المحاضرة غٌر موجودة فً الكتاب المقرر ولذلك االعتماد على السالٌدات

وشرح الدكتور بشكل مباشر.

Cell Communication – Signal

Transduction

External signal is

received and converted

to another form

to elicit a response

كما في الهاتف الخليت تستقبل اإلشارة

وتحىلها إلً إشارة أخري

Signal Transduction & G Protein-coupled Receptors

Topics

• Signal Trans.: From Extracellular Signal

to Cellular Response

• Cell-Surface Receptors & Signal

Transduction Proteins

• G Protein-coupled Receptors (GPCRs):

Structure and Mechanism

• GPCRs That Regulate Ion Channels

• GPCRs That Regulate Adenylyl Cyclase

• GPCRs That Regulate Cytosolic Calcium

Learning Objectives 1. Learn the general properties of signaling molecules (ligands),

cell-surface receptors, & intracellular signal transduction components.

2. Learn the G protein cycle of reactions involved in GPCR signaling.

3. Learn the epinephrine receptor signal trans pathway used for control of glycogen degradation.

4. Learn about the GPCR-stimulated IP3/DAG signaling pathway.

General principles of signal transduction - What do we mean by signal transduction??

the overall process of converting extracellular signals into intracellular responses.

- The key players in signal transduction are 1- Signaling molecule (ligands) 2- Receptor 3- Signal transduction proteins and second messenger 4- Effector proteins

In general, the ligand will bind to the receptor then the signaling will happen. ** after the signaling the ligand will be removed from the receptor

- How do cells respond to the signals?? 1- Fast ……... by changing the activity of existing enzymes. 2- Slower ………. By changing the levers of expression of enzymes

and cell component by gene regulation in these steps:

Modification of cellular metabolism, fxn, movement

You will understand this

picture better after

studying the few pages

that are coming

Don’t worry this is just an

introduction

Modification of gene expression, development

** hormones, growth factor, drugs and neurotransmitter respond depends on the receptor and signal transduction system. Like we said before the communication between cells requires:

1- Ligand (the signaling molecule) 2- Receptor protein

The receptor to which the receptor binds and it may be: on the plasma membrane (membrane receptor) - hydrophilic ligand within the cell (intracellular receptor) - hydrophobic ligand

Will bind to the receptor

from the outside

c ii r n l w ii l r c ii w ri c l

l n w hn

Structure and function of receptors • Globular proteins acting as a cell’s ‘letter boxes’ (it means it

resembles mail box and the act of receiving letters)

• Located mostly in the cell membrane The structure of receptors

is globular protein NOT

fibrous .

• Receive messages from chemical messengers coming from other

cells

• Transmit a message into the cell leading to a cellular effect

• Different receptors specific for different chemical messengers

• Each cell has a range of receptors in the cell membrane making it

responsive to different chemical messengers

Note: we have peripheral proteins (on the surface) and integral

proteins (through the membrane).

Mechanism

• Receptors contain a binding site (hollow or cleft in the receptor

surface) that is recognised by the chemical messenger

• Binding of the messenger involves intermolecular bonds

• Binding results in an induced fit of the receptor protein

• Change in receptor shape results in a ‘domino’ effect

• Domino effect is known as Signal Transduction, leading to a

chemical signal being received inside the cell

• Chemical messenger does not enter the cell. It departs the receptor

unchanged and is not permanently bound (it means after the

signalling the ligand will leave the receptor and the receptor will

continue the remaining steps)

Overall process of receptor/ messenger interaction: - What kind of binding interaction we need in this process??

Implies a fine balance:

- strong enough to hold the messenger sufficiently long

for signal transduction to take place

- weak enough to allow the messenger to depart (leave)

M

M

E R R

M

E R

Signal transduction

ligand binding

receptor by the

same way that

substrate binding

enzyme in which

both of them

induced fit

This is called induced fit (change the size and shape of the

receptor). So the receptor changes its structure to receive the

ligand molecule.

Drugs depend on this principle, which the drug bind to the receptor

permanently blocking any other signal or molecule

Drug design - designing molecules with stronger binding interactions

results in drugs that block the binding site – antagonists

** note: the signal will not change it self so for example the drug should do

an inhibition to prevent the accumulation of acetylcholine because it should

only work for a short period of time then stops.

** if there was no degradation for acetylcholine there will be an excess and

that will produce tremor (exaggeration in movement).

- What type of bonding forces we need here??

weak interactions

• Ionic • H-bonding • van der Waals

- how does the binding site change the shape?

Substrate binding

• Bonding forces

• Induced fit - Binding site alters shape to maximise intermolecular

bonding

-In the site of binding there will

be relationship between the

amino acid and the binding

molecule

- every receptor contains a certain

type of amino acids.

External signals are converted to internal responses: 1. Cells sense and respond to the environment

Prokaryotes ( like bacteria ) : chemicals

Humans:

light - rods & cones of the eye

sound – hair cells of inner ear

chemicals in food – nose & tongue( means for smell and

taste ) 2. Cells communicate with each other

Direct contact

Chemical signals

General principles:

Signals act over different ranges.

Signals have different chemical natures.

Cells respond to sets of signals.

Receptors relay signals via intracellular signaling cascades

The same signal can induce a different response in different

cells. (it means the signal that will go to the heart will respond

differently than the one that will go to the muscle)

Example:

Intermolecular bonds not optimum length for maximum binding strength

Intermolecular bond lengths optimised

Epinephrine in the heart will increase the heart rate and cardiac

output, while in the liver it will stimulates glycogen breakdown in

muscle.

Each protein in a signaling pathway Amplifies the signal by activating

multiple copies of the next component in the pathway.

1º messenger

2º messengers

Effector

Enzymes

Target

Enzymes

Cells detect signals and respond

EXTRACELLULAR FLUID

Receptor

Signal molecule

Relay molecules in a signal transduction pathway

Plasma membrane

CYTOPLASM

Activation

of cellular

response

Reception 1 Transduction 2 response 3

Cascade effect

Here we have 3 stages

1 msg

Ligand

receptor

Gene modification

Effector enzyme

Target enzyme 2 msg

- primary signal activates an enzyme activity, processes 100

substrates /second

- Primary enzyme activates 100 target enzymes

- Each of the 100 enzymes activates an additional 100 downstream

target enzymes

- Each of the 10,000 downstream targets activates 100 control factors

so rapidly have1,000,000 active control factors.

After the amplification there will be divergence to multiple targets:

1- Regulation of metabolic Pathway

Glucose-1-phosphate

(108

molecules)

Glycogen

Active glycogen phosphorylase (106

)

Inactive glycogen

phosphorylase

Active phosphorylase kinase (105

)

Inactive phosphorylase

kinase

Inactive protein kinase A

Active protein kinase A (104

)

ATP Cyclic AMP (10

4

)

Active adenylyl cyclase (102

)

Inactive adenylyl

Inactive G

protein Active G protein (102

molecules)

Binding of epinephrine to G-protein-linked receptor

(1 molecule)

Transduction

Response

Reception

A s

ignal cascade a

mplif

ication

1 e

pin

ep

hrin

e -

----

-- 1

08

glu

cose -

1-

pho

sp

hate

Cascade : شالل , زي ما الشالل كل مرة بٌاخد معاه صخور أكتر فً السٌل نفس الشً عملٌة ال ٌعنً

transduction . it amplifies the signal .

2- Regulation of gene expression

3- Changes in cytoskeleton

- What are the main types of receptors??

1- Ion channel receptor

2- G- protein – coupled receptor

a G-protein (bound to GTP) assists in transmitting the signal

3- Kinase – linked receptor

receptor is an enzyme that is activated by the ligand

4- Intracellular receptors

Cell-surface receptors -large &/or

hydrophilic ligands

Examples for cell- surface receptors:

1)) ion- channel- linked

2)) trimeric

G-protein coupled receptor

3 )) enzyme linked ( tyrosine kinase )

Ion channel receptor- the control of ion channels • Receptor protein is part of an ion channel protein complex

• Receptor binds a messenger leading to an induced fit

• Ion channel is opened or closed

• Ion channels are specific for specific ions (Na+, Ca2+, Cl-, K+)

• Ions flow across cell membrane down concentration gradient

• Polarises or depolarises nerve membranes

• Activates or deactivates enzyme catalysed reactions within cell

Examples on ion channel receptor:

1- Muscle contraction

2- Nerve cell communication

Remember the Na+/K+ ATPase (Na+/K+ pump)

[Na+] inside ~10mM; outside ~150mM

[K+] inside ~100mM; outside ~5mM

cell has membrane potential ~ -60mV

Potassium concentration can affect heart more than sodium . And it

has importance in heart

contraction

- What are the four basic mechanism for cellular communication?

1- Direct contact

molecules on the surface of one cell

are recognized by receptors on the adjacent cell

2- Paracrine signaling

signal released from a cell has an effect on

neighboring cells

example: nitric oxide, histamine, prostaglandins

note : when prostaglandins is produced by cell an

have effect on the same cell we name that signaling

(( autocrine signaling )) but when affect

neighboring cell it is (( paracrine signaling ))

3- Endocrine signaling

hormones released from a cell affect other cells throughout the body.

Example: estrogen, thyroxine, GH and epinephrine.

** endocrine glands and cells release hormones

to the blood stream to travel along distance

away from the synthesis to act on another

organ or an other tissue.

Examples of endocrine gland : حسب التسلسل من أعلى

hypothalamus , pituitary gland ( both in brain )

, thyroid , suprarenal gland (cortex , medulla//above the kidney )

pancreas, testis. ovaries , ,

4- Synaptic signaling

nerve cells release the signal (neurotransmitter)

which binds to receptors on nearby cells

** here we have presynaptic, synaptic junction (in it we have

neurotransmitters that is released by releasing Ca)

these neurotransmitter then reach the receptor and bind to it …

the post synaptic neuron then response…and finally the

neurotransmitter will be separated from it's receotor .( the process

that we called "degradation") .

** in the synaptic signaling we have something called

Reuptake mechanism ٌعنً إعادة إستخدام الناقل العصبً الحقًا بعد ما ٌنفصل عن الرٌسبتور بعد

example: acetylcholine produced by nerve cells will do the)التنبٌه

signaling then when it finishes its function, degradation of

acetylcholine to acetyl coA and choline take aplace… then

acetylcholine synthesis starts again)

There are 3 subclasses of membrane receptors:

1. channel linked receptors – ion channel that opens in

response to a ligand

2. enzymatic receptors – receptor is an enzyme that is

activated by the ligand

3. G protein-coupled receptor – a G-protein (bound to

GTP) assists in transmitting the signal

cell’s response to a signal

often involves activating or inactivating proteins.

Phosphorylation is a common way to change the activity of a protein.

1- protein kinase ((–metabolism مهم فً الـ))

an enzyme that adds a phosphate to a protein, activate

2- phosphatase

an enzyme that removes a phosphate from a protein, deactivate

Proteins that participate in intracellular signal

transduction fall into two main classes:

GTPase switch proteins.

protein kinases/phosphatases

Kinases use ATP to phosphorylate amino acid side-chains in

target proteins. Kinases typically are specific for tyrosine or

serine/threonine (notice the three of them contain OH group) sites.

Phosphatases hydrolyze phosphates off of these residues.

Kinases and phosphatases act together to switch the function of a

target protein on or off.

- There are about 600 kinases and 100 phosphatases encoded in the

human genome. Activation of many cell-surface receptors leads

directly or indirectly to changes in kinase or phosphatase activity.

(memorize the numbers)

- Note that some receptors are themselves kinases (e.g., the insulin

receptor).

Insulin - it resembles hemoglobin

- It consists of 2 alpha (outside) and 2 beta (inside)

- When the insulin binds to the receptor it starts the dimerization and

activation of beta chains.

- Insulin is an anabolic hormone, it is responsible for glycogen and

protein formation , amino acid transfer and adipose tissue

formation((like lipid )) .

- It is the opposite of epinephrine and cortisone

-

- insulin synthesis glycogen from glucose using glycogen synthase

Growth hormone: - Tetrameric complex constructed in presence of growth hormone.

- Over production of growth hormone leads to Gigantism.

- growth-hormone deficiency leads to Dwarfism.

يجب حفظ الخطوات )مهم(

Epinephrine هاد عكس الــ

إلً كان ٌعمل تنشٌط إلنزٌم ال :

glycogen phosphase ,-

the enzyme responsible

of degradation 0f

glycogen to glucose-

intracellular receptors - Chemical messengers must cross cell membrane

- Chemical messengers must be hydrophobic

- Example-steroids and steroid receptors

Kinase active

site

opened by

induced fit

GH

O

H O

H

O

H

H

O

kinases

GH receptors

(no kinase

activity)

GH binding

&

dimerisation

O

P O

P O

P

P

O

ATP ADP

Activation

and phosphorylat

ion

O

H

Binding of

kinases

O

H

O

H

H

O

Z

i

CO2H

H2N

DNA binding

region

(‘zinc fingers’)

Steroid

binding

region

Zinc fingers contain

Cys residues (SH)

Allow S-Zn

interactions that

lead to a change in

the jelly shape of

the receptor

Steroid receptors

-have a nonpolar, lipid-soluble structure

-can cross the plasma membrane to a steroid receptor

-usually affect regulation of gene expression

An inhibitor blocks the receptor from binding to DNA until the hormone is

present.

Intracellular receptor Mechanism

7. Protein synthesis activated or inhibited

1. Messenger crosses

membrane

2. Binds to receptor

3. Receptor dimerization

5. Complex binds to

DNA

6. Transcription switched on

or off

A steroid receptor has 3 functional domains:

1. hormone-binding domain

2. DNA binding domain

3. domain that interacts with coactivators to affect gene expression

يجب حفظ الخطوات )مهم(

Here , we can see that the receptor exist inside the cell in the

cytoplasm but it still bounded to an inhibitor until a hormone pass

through the plasma membrane and bind to it .

When the hormone bind to the receptor the inhibitor released and

the result is a (hormone-receptor complex ) that move then to the

nucleus to bind DNA at specific binding site .

it has an effect on the RNA to either increase protein synthesis (

transcription went ON ) or decrease protein synthesis

(transcription went OFF) .

((َوَسٌَفتُح هللاُ لك بابًا من شدَّة الٌأس كنَت تظنهُ لم ٌُخلق بِمفتاح((

كل التوفٌق ^^